Anterior Segment and Fundus Photography
External Ocular Photography
Of the several types of ophthalmic photography, external eye photography is the least reliant on sophisticated instrumentation. Readily available components (ie, camera body, lens, flash) can be configured into an apparatus that can be used for virtually all photodocumentation of the external eye and adnexa. Based on the simple system, the external camera can be used for photography of one eye alone, both eyes together, and full face views, in addition to intraoperative photography and even nonmedical imaging (Gibson, 1973).
In a major market change, traditional film-based cameras are being supplanted by digital technology. Most prosumer digital cameras currently lend themselves to the precise composition and magnification standards required for medical photography. These cameras with their digital sensors are rivaling film for color, contrast, sensitivity, and resolution. The potential benefits of electronic imaging are significant. High-quality digital single lens reflex (DSLR) cameras have become increasingly affordable and are beginning to replace standard film-based 35 mm SLRs. When selecting a camera, it should feature interchangeable lenses that can be used in full manual mode rather than as an exclusively automatic instrument. If a practice uses electronic health records, images from a digital camera can be directly imported or film-based images can be scanned and converted to digital files.
The highest magnification generally considered useful for external eye photography assignments is 1:1, or life size on film, combined with a sufficiently long focal length to provide natural perspective and adequate space for subject lighting. Several lens and accessory combinations provide these parameters, including a macro lens plus bellows, a macro lens plus teleconverter, and a macro lens with integral extended focusing mount. Of these choices, the last 2 are the most practical.
The combination of a 50 mm or 60 mm f/2.8 or f/3.5 macro lens and a 2X teleconverter can be focused from infinity to a 1:1 magnification ratio without the need for supplemental close-up devices. Longer focal length (100-105 mm) macro lenses are available with extended helical focusing mounts, which allow them to focus to the same magnification, again without requiring additional components. The difference between the 2 examples above is the wide-open viewing aperture; the latter lenses allow a bright f/2.5 or f/2.8 view, while the former configuration only transmits light through a rather dim aperture of f/5.6 or f/7. The brighter view is especially useful when attempting to photograph a dark iris.
The next component, the light source, should be selected from the variety of small electronic flash units that are commonly available. A manual flash is perfectly adequate for this application. However, a camera body equipped with off-the-film (OTF) or through-the-lens (TTL) flash control circuitry may provide for better exposure; an automatic flash dedicated to that exposure system is also acceptable. It is important to note that many newer digital systems use external flash units that are proprietary for use of TTL technology.
The point source quality of illumination from the standard electronic flash yields high-quality photographs in any of the 3 standard magnifications (ie, one eye, both eyes, full face). Because the cornea acts like a convex mirror, the reflection of the electronic flash appears in the photograph. When a single eye is photographed, the light source must be small so that its reflection does not obscure any required detail.
A ring flash also has been used successfully for photography of strabismus cases in which the standard view is both eyes together.1 In that application, its reflection on the cornea is acceptably small. It cannot be used for successful photography of a single eye because its proximity to the eye creates a very large reflection on the corneal surface that may obscure detail. If only one camera setup is to be used for all external ocular photographic assignments, then one with a point source type of electronic flash is more universally applicable.
The electronic flash is attached to the front of the macro lens by means of a special rotating flash bracket. One of the most useful such brackets has set screw adjustments that allow the flash to be positioned in 5 different positions within a 180° arc, enabling the light source to be positioned with precision. Such a device is available from Adolph Gasser, Incorporated (5733 Geary Boulevard, San Francisco, Calif). This bracket is also useful when camera orientation must be changed between horizontal and vertical.
This camera, lens, and flash combination can be used as a portable instrument in the clinic, in patient rooms, or even in the operating room. However, a higher degree of focusing accuracy and compositional standardization can be achieved by mounting the camera on a focusing stand that also has provision for patient head support. Sometimes, such stands can be adapted from pieces of obsolete clinical equipment (eg, keratometers). If such dedicated equipment is not available, try to stabilize the patient’s head in another way. An examination chair with headrest can be a useful expedient for external ocular photography. Alternatively, ask the patient to sit against a wall, and then to press the head back so it touches the wall.
A number of digital sensors from major manufacturers and color transparency films are suitable for external ocular photography. For finest granularity, a film of medium speed (ISO 50, 64, or 100) is preferred. On a DSLR, the sensor will have different gain settings that are represented by the familiar ISO ratings from the film realm. Use the lowest setting to reduce any digital noise. Keeping these ISO settings on your digital sensor below 400 will ensure the best quality image. The DSLR will come equipped with presets for different light sources (eg, flash, fluorescent, tungsten, daylight, auto). Setting this to auto will allow the camera to judge the lighting conditions milliseconds before the exposure is made. When attempting to obtain the most lifelike recording of skin, the flash output may require modification with an ultraviolet absorbing filter (eg, Kodak Wratten 2E).
As with any other clinical photographic equipment, a series of tests should be undertaken to determine the proper flash settings and exposure parameters before using the equipment on actual patients.
If a manual flash is used, the exposure test should include a series of bracketed exposures (at half-stop increments) at the 3 customary magnifications: 1:1 (one eye), 1:4 (both eyes together), and 1:10 (full face).
If an automatic flash is used with OTF or TTL metering, the goal should be a rather small f-stop (ie, f/22) for single eye views, where depth of field would otherwise be quite shallow.
For photography of one eye at 1:1 magnification, the angle of the flash is set to aim at the eye from the superior temporal or temporal position; these positions are chosen because they avoid the problem of the nose obstructing the beam. Adjust the location and angle of the flash so that the anticipated location of its reflection misses the major pathology. This can be predicted easily; when the patient assumes primary gaze, the reflection always is located between the corneal apex and the limbus in the same quadrant as the flash position relative to the lens. Thus, if the flash is set directly above the lens, then the reflection appears in the superior cornea. Preset the lens magnification to its 1:1 setting, and set the aperture to the predetermined setting. Instruct the patient to look in the direction that presents the proper aspect of the eye to be photographed.
Although the conventional anterior-posterior (A-P) or primary gaze photograph is often appropriate, additional photographs in which the camera position or the patient’s direction of gaze is changed often may result in a more informative image.
Next, in a smooth continuous movement, move the camera toward the subject, while carefully assessing the composition and focus in the viewfinder. When holding the camera, a modified sharpshooter stance and grip are required. Hold the right side of the camera body with the right hand so the index finger is able to reach the shutter release button. The left hand should cradle the lens from below. The elbows should be tucked in, and the feet should be spread slightly. Focusing is accomplished by leaning in and/or bending at the waist.
Small specular highlights often provide useful clues about focus; they are smallest when the camera is focused on the plane where they are located. At the instant when the subject comes into critical focus, take the photograph. With practice, this technique allows fast and accurate focusing.
By presetting the magnification ratio and then focusing by moving the entire camera assembly, the same magnification for all images is ensured. Scientific validity is compromised if the lens is focused by adjusting its focusing ring because this also changes the image magnification. This technique and equipment is also applicable to ophthalmic surgical photography. By quickly and fluidly leaning in, making the photograph, and leaning out, the interruption to the procedure is limited.
When a photograph of both eyes together is needed, use a 1:4 reproduction ratio. This allows the center of the image to be located at the bridge of the patient’s nose, with a complete view of both eyes and both temporal orbits. Careful camera work results in a symmetrically composed photograph. Place the plane of sharpest focus at the level of the corneas; again, a small specular reflection provides a reliable indicator for use in focusing. Place the flash directly above the lens to provide symmetrical illumination for both eyes.
Although the primary gaze (A-P) position is usually appropriate, a different camera position occasionally might be required. For example, in the case of thyroid ophthalmopathy, proptosis can be recorded most effectively by shooting down from a position above the patient’s head.
The photographic motility series or views of the 9 cardinal gazes require a standardized series of photographs in which only the direction of the patient’s gaze changes from photograph to photograph.2 These directions are ordered precisely and include straight ahead, straight up, up and right, straight right, down and right, straight down, down and left, straight left, and up and left. Standardize the order of imaging; if a patient has severely affected eye movements, it is easier to identify individual positions of gaze through the order of photographs. If a muscle imbalance is to be documented, have an assistant gently raise the patient’s upper eyelids when photographing the 3 views that include a downward component.
An assistant also can give the patient a target for proper positioning of the various gazes. Because patients often have a tendency to turn their heads in the direction of eye movements, they should be reminded to maintain a fixed head position and to move only their eyes during the photographic session.
Because facial photographs are uniquely recognizable, ask the patient to sign a photographic release form prior to photography. If the photographs are published later, the necessary legal document is already available.
Photography of the full face requires a magnification ratio of 1:10. Because this size allows a small area of background to appear in the photograph, carefully choose the background color. To avoid any unusual color casts on the subject resulting from reflected light or from the psychological effects of juxtaposition, a matte finish neutral gray color should be used. If neither a formal photographic studio room nor the space for a roll of seamless background paper is available, one wall of a room in the clinic may be finished with neutral gray paint of between 18-36% reflectance. If space permits, seat the patient several feet in front of the wall to avoid recording any wall texture or sharply defined shadows.
Hold the camera parallel to the frontal plane of the face. Ensure that the patient maintains this parallel position; many patients incline their heads when posing, perhaps in an attempt to camouflage a double chin. The camera must be held on the same horizontal level as the center of the face because undesired psychological effects and skewed facial geometry may result if the camera is directed at the patient from either a superior or an inferior position.
On rare occasions, images of hands, feet, skin, and other body parts may be needed to document pathology that is linked to ophthalmic findings. The simple system is equally useful for these assignments. Keep a few pieces of gray illustration board available; these can provide suitable backgrounds for views of the hands or feet.
Slit Lamp Biomicrography
One of the most challenging forms of ophthalmic photography is the recording of the anterior structures of the eye with the use of a photographic slit lamp biomicroscope. This instrument, with its built-in variable magnification and binocular eyepiece head, functions as a compound microscope. It extends the reach of photographic recording from the external camera’s 1:1 magnification up to nearly 10 times life size. The flexibility of the biomicroscope’s light source, located on a rotating arm, is also an important part of the instrument’s ability to function as a complete photomacrographic apparatus.
The challenge of slit lamp biomicrography stems from the instrument’s flexibility. The user must choose from a wide range of magnification and lighting choices to best observe and record the lesion of interest. The actual photographic recording is straightforward. By bracketing exposures, successful images from a photographic point of view usually can be obtained. What is more problematic for the neophyte biomicrographer is learning how to use the instrument to record the impression of the clinical examination and to reveal what is often a visually subtle condition.
A special problem, and one which is often crucial in understanding how a technically good photograph may not precisely match an expectation, stems from the contrast between the dynamics of the clinical slit lamp examination and the nature of still photography. During slit lamp biomicroscopy, fluid and continuous movements are used to scan the patient’s eye. By moving the slit back and forth, adjusting the width and focus of the illumination, and observing the play of light on the subject, a continuous stream of images are synthesized intellectually into a final composite impression. The challenge in biomicrography is to distill this process of scientific investigation into a few static photographs, each of which must reveal maximum information about the subject.3
The observation and recording of opaque ocular tissue is simple to achieve, since the light may be adjusted to reveal either overall color or morphology, or it may be angled in such a way as to emphasize surface texture. More challenging is the recording of nominally transparent structures, such as the cornea and lens.4 In some ways, this task is similar to the photography of glassware; frontal, side, reflected, or even silhouette lighting often can reveal dramatically different aspects of the same subject.
Although considerable variation exists among the several photographic slit lamp biomicroscopes currently available, all are derived from the clinical slit lamp biomicroscope and share certain basic features of that instrument, including optical design, illumination systems, and the rotating arms upon which those 2 components are mounted.
The optical part of the biomicroscope consists of the objective lens assembly and the eyepiece head. The objectives can be either individual lenses of varying powers (that are rotated into position) or a zoom lens system. Typically, the range of magnification extends from slightly less than life size on film up to about 9 power.
After being formed by the objective, the image is projected to the eyepiece head that is comprised of a binocular fixture with 2 eyepiece lenses. Both the interpupillary distance of the binoculars and the individual focus of each eyepiece are adjustable. Because the photo slit lamp biomicroscope uses an aerial image focusing system, the eyepieces must be focused critically to ensure acceptably sharp photographs. An eyepiece reticle is provided to indicate the film plane; both the image and the reticle must be seen sharply to ensure a properly focused image.
Some form of beam splitter or movable mirror system is used to divert the image to the 35 mm camera body or DSLR. On most biomicroscopes, the camera records a view identical to that in the eyepiece. If the eyepiece reticle is interchangeable, it must be placed on the same side of the binoculars as the camera to ensure parallax-free recording.
The second major component of the biomicroscope is the illumination system. Through a system of condensers and a knife-edge aperture diaphragm, the modeling lamp (and, in photographic instruments, the electronic flash that shares the same optical pathway) is projected onto the patient’s eye as highly collimated and focused rays. When the slit aperture is adjusted to project a very thin beam, the play of light clearly differentiates the component parts of such structures as the cornea and lens. The optical section effect produced by the thin slit beam shows the observer a view suggestive of a histopathologic specimen viewed with darkfield illumination; the structures highlighted by the beam appear bright against a dark background.
In addition to its thin slit configuration, the light also may be broadened until a wide area of the eye is illuminated with focal rays. For still wider coverage, especially at low magnification, some instruments provide a diffusion cap that is placed over the exit prism to diffuse and broaden the beam.
Unlike clinical slit lamp biomicroscopes, the photographic version also is equipped with some means of providing general diffuse illumination from a second light source. This supplemental lighting may be a complete fixture with modeling lamp and separate flash tube, or the lighting may come from the main flash tube via a fiberoptic cable.
The film for biomicrography is selected according to the efficiency of the optical system and the amount of available flash power; color transparency film or a DSLR rated at ISO 200 most commonly is used.
When producing slit lamp biomicrographs, the sequence should begin with a general overview of the eye and then proceed with increasingly emphatic use of magnification and lighting effects. The following illumination techniques are variously applicable to a variety of anterior segment conditions, although not all are useful for every patient.
Dual Diffuse Illumination
Low magnification survey photographs in which the eye is illuminated diffusely have value by demonstrating the general status of the eye and its surrounding tissue. Open the slit beam to its maximum width, place the diffusion cap over the slit prism, and angle the illuminator approximately 45° to one side of the microscope; this angle may be changed to prevent the light reflection from obscuring the primary lesion of interest. Turn on the background illuminator, or, if the instrument is so equipped, activate the fiberoptic background light. Select the power setting and/or aperture control as demonstrated by practice or by prior testing. Unless the eyelids are the subjects of interest, retract them fully to provide an unobstructed view of the cornea and surrounding sclera and conjunctiva.
The first survey photograph should show the eye in the A-P position. Subsequent diffusely illuminated photographs at low-to-moderate magnifications may feature a different direction of gaze so that the lesion of interest is centered in the view. To satisfy the requirements of maximal optical resolution, the lesion not only must be centered but also must be the closest object to the objective lens; thus, changes in the patient’s direction of gaze are mandated, along with repositioning of the microscope head. The microscope is focused on the reflection of the light source on the cornea or on the plane of the lesion.
These low-power, diffusely illuminated photographs are unable to resolve small or faint lesions. However, they can demonstrate the relative transparency of the cornea, the general size and morphology of the lesion, and the degree of involvement of adjacent tissues.
Thin Slit Illumination With Background Light
When configured to project a thin slit beam, the slit lamp biomicroscope is uniquely able to emphasize topographic changes and to reveal changes in the structure of transparent media. When the thin beam strikes an opaque lesion at about a 45° angle, the resulting undulations provide visual clues about the 3-dimensional nature of the lesion and its surface texture. When directed at a transparent structure (eg, cornea), the beam similarly reveals structural information. If the surface is elevated, the slit beam appears to deflect toward the light source, while a depression in the surface diverts it away from the incident beam.
When observed and photographed at high magnification, 3 layers of the cornea are visible as alterations in the brightness of the beam. The corneal epithelium is a fine bright line closest to the light source, the stroma is a relatively thick band of medium reflectivity, and the endothelium appears as a very thin bright line furthest away from the incident light. In their normal state, neither the Bowman nor the Descemet membranes are visible in thin slit illumination.
When a thin slit beam is directed to the front of the eye, it appears first on the cornea, apparently disappears as it traverses the minimally reflective aqueous, and then reappears on the iris plane or the front surface of the lens. The gap between the 2 slit images can be useful in demonstrating the depth of the anterior chamber. If there is diminished space between the iris and the cornea, or if local adhesions (synechiae) have formed between the 2 structures, then thin slit illumination is likewise useful in delineating those changes.
To perform this type of biomicrography, use the slit width adjustment to almost close the knife-edge diaphragm; then, the resulting thin vertical beam should be projected onto the lesion of interest. The patient’s gaze should be directed to the point at which the lesion is centered in the eyepiece.
When properly adjusted and focused, the middle of the thin slit beam should intersect the center of the cross-hair reticle, and it must remain centered when the illuminator is swung in an arc around the biomicroscope. If it cannot be brought to this point and remain sharply focused, then the instrument is misaligned and must be repaired by a service technician.
Broad Tangential Beam
The slit beam aperture control is continuously variable from fine slit to very wide beam, and photographs may be made at any point in that continuum. A particularly useful configuration for illumination of either transparent or opaque tissues is the broad slit beam, angled far to the side so the light just grazes the surface.
When an opaque lesion is photographed, such lighting creates highlights and shadows and thus imparts strong visual clues about its 3-dimensional nature. The most pronounced chiaroscuro effect of light and dark is achieved by removing the diffusion cap so that the raw beam illuminates the subject. At higher magnifications, the tiny specular reflections and crisply delineated shadows produced by this raw lighting are absolutely necessary to fully resolve fine detail.5, 6
Broad tangential lighting is also valuable when applied to the photography of corneal lesions, although its effect is quite different than when opaque subjects are recorded. When contrasted with thin slit illumination that reveals structural detail and topography only in one very thin slice, tangential illumination does not provide information about the full thickness of the cornea, and the observer is not able to identify the specific layer that contains the lesion of interest solely from this illumination technique. However, broad tangential illumination is able uniquely to reveal changes in corneal transparency over a much wider area than that of a thin slit beam.
When photographing with broad beam tangential illumination, the background illumination source must be turned off because its diffuse quality would otherwise lessen local contrast by filling in the shadows. While focusing critically on the area of interest, rotate the main light to the side, positioning it so that the best illumination is achieved. If possible, large or distracting reflections from the light source should be positioned away from the lesion of regard. This may necessitate a slight adjustment in the direction of the patient’s gaze. Close attention to the overall brightness of the image may suggest changing the aperture or power setting; some corneal lesions are highly reflective, while others are quite faint. Bracketing the exposures increases the probability of success.
After front and side lighting, a third type of illumination provided by the biomicroscope is retroillumination. Although one obviously cannot move the slit lamp light source behind the cornea or lens, the beam may be directed eccentrically at a structure behind the subject of interest, so that the light reflects back through it. Thus, the object of interest may be observed in a silhouettelike fashion, in which various opacities appear contrasted against a bright background.
When the slit lamp biomicroscope is configured in the usual fashion, both the slit beam and the microscope are focused to the same plane. However, all slit lamp biomicroscopes allow the slit beam to be decentered laterally away from the point of its prime focus. By manipulating this control, the microscope can allow continued focus and observation of the lesion, while the lighting is diverted and reflected from posterior structures for special effects.
Reflective structures that lend themselves to this bounce lighting effect include the iris and retina. Either may be used for retroillumination of the cornea, while the retina provides an excellent reflector for photography of the lens or iris.
To use the iris for this effect, first view the corneal lesion with direct thin beam illumination. Next, without moving the biomicroscope, widen the slit beam, displacing it laterally. The desired effect is achieved when the appearance of the lesion reverses; from brightly lit against a darker background to a silhouette seen against a light background. Interesting lighting effects can result by situating the lesion midway between the reflected beam and the unilluminated adjacent area. Placing the edge of the pupil behind the lesion also can yield striking visual effects.
For retroillumination views of the iris or lens, the retina is the only structure behind them capable of acting as a reflector. To direct light to the retina, it must pass first through the pupil. This is achieved by simply bringing the illuminator to a position just adjacent to the microscope, creating a semicoaxial illumination. However, the retina is not uniformly reflective. The brightest area of the retina is the optic nerve head, and the brightest reflection (or red reflex) is achieved when the illuminating beam strikes the nerve. To achieve this, position the slit lamp illuminator to the temporal side of the biomicroscope.
Retroillumination views of the iris usually are designed to demonstrate defects in its structure caused by pigment migration and atrophy. The resulting absence of pigment in various areas (transillumination defects) can be illustrated dramatically by directing the beam through the pupil to reflect from the retina. If the patient’s pupil is undilated, then its physiologic response to the light results in automatic constriction (thereby preventing sufficient light to reach the retina). Similarly, if the patient is fully dilated, the iris structure retracts into itself, causing the atrophic areas to disappear.
The best moments to produce retroillumination photographs of the iris occur a few minutes after dilating drops are instilled, when the pupil just begins to dilate. The slit beam is first shortened and configured into either a small square or circle, depending on the capability of the instrument. The dilating drops are administered to the patient while seated at the biomicroscope. Direct the beam at the pupil, and closely watch the effects of the dilating process. Begin to photograph as it begins to dilate. At one point during this process, the incident beam just fits through the pupil; with enough backlighting from the retina, the defects appear as orange lights visible through the darker background iris.
The retina also is used as the reflector for photographs of the lens, but, in this application, the pupil first must be dilated maximally. The slit beam is adjusted to a moderately thin width, and then the slit lamp illuminator is positioned nearly coaxial to the microscope. While moving the illuminator from side to side, a reddish orange reflection is seen emanating through the pupil. The relative positions of microscope and light source are fixed when the brightest reflection is obtained. Next, the light must be trimmed or fine-tuned. To do so, decenter the slit laterally so that the beam is brought to a position just inside the pupil edge. The beam is then shortened and slightly widened.
The purpose of this trimming exercise is to obtain the brightest reflection possible, without having any light spill over onto the iris (thereby lessening image contrast) and also without having too large a reflection on the lens (thereby obscuring needed detail). On some slit lamps, a combination of slit width and height adjustments may be used to form the beam into a half-moon shape that can be neatly tucked into the curve of the pupil border.
Because the lens has considerable thickness, it is usually impossible to record more than just a shallow plane when retroillumination is used. Consequently, produce several photographs, focused on different levels of the lens opacities, to completely illustrate the pathology. If no opacities are present, an adequate retroillumination recording of the lens may be achieved by focusing on the pupil border.
Although the broad tangential beam is capable of revealing a relatively large amount of cornea in a single photograph, it cannot be used to illuminate the entire corneal surface due to the curvature of the eye. If pathology is widespread geographically, then sclerotic scatter illumination may be a useful addition to other lighting techniques.
The regular orientation of lamellar fibers that constitute the bulk of the corneal stroma can serve as a fiber optic. When a strong light is applied to one end of a fiber, it is conducted (through internal reflection) to the other end. Under such illumination, the length of the unblemished fiber is dark, and the light only appears at the distal end. Any local change of texture or uniformity of these fibers results in the light appearing at that location.
To observe the cornea under sclerotic scatter illumination, first focus carefully on the most anterior portion of the cornea (the apex). The background illuminator is turned off, and the slit beam should be moderately wide (ie, 1-2 mm). Then, the beam is decentered laterally so that it strikes the limbus. If it cannot be sufficiently decentered, the top of the slit beam prism may need to be removed and replaced off-axis. After careful adjustment of the beam, the limbal area on the opposite part of the eye glows; all lesions involving the corneal stroma also glow. Any noninvolved portions of the cornea remain dark, as well as the iris and pupil.
Because this technique can portray faint lesions, care must be exercised to place the incident beam so that little light spills onto either the cornea or iris. The choice of either the 3-o’clock or 9-o’clock limbus is decided by the location of the lesion of interest. To ensure that no extraneous light from the incident beam contaminates the area of regard, the beam should be directed to the limbus farthest from the lesion.
To begin goniography, note the exact location and nature of the lesion to be photographed, because the lens must be placed on the eye with some precision so that the mirror to be used is in the proper location. Furthermore, the orientation of the slit beam may need to be changed from its typical vertical alignment to a horizontal or an angled configuration to send maximal illumination into the mirror. The various settings of the biomicroscope should be preset as much as possible, including the power setting or aperture, and the magnification changer should be set to low power. The plane surface of the goniolens must be cleaned carefully, since any fingerprints, dust, or scratches severely degrade the final image. To this end, purchasing a goniolens for exclusive photographic use may be advisable.
Next, instill a drop of topical anesthetic onto the patient’s eye. Fill the small concave end of the lens with 2% or 2.5% methylcellulose solution. Any tiny bubbles suspended in this solution will degrade the image through flare. To prevent this, store the tightly-capped bottle upside down. When ready to use, squeeze some fluid onto a tissue until no bubbles appear. Immediately transfer the stream into the cup of the lens.
Position the patient’s head firmly against the headrest of the slit lamp, and ask the patient to look up. Pull down the lower lid, and place the lower edge of the lens against the eye at the lower lid margin. Next, press the lens forward into full contact with the eye. Direct the patient into primary gaze. Quickly inspect the lens for bubbles; if these are present, they sometimes may be forced out by spinning the lens while it is pressed gently onto the eye. However, if this maneuver is unsuccessful, the only recourse is to remove the lens, clean out the residual methylcellulose, and begin again.
Use the thumb and forefinger to keep the lens on the eye. Only moderate pressure is required. A special support device, designed for ophthalmic laser surgery, may be used to provide an elbow rest for lengthy sessions.
Once the lens is placed securely, rotate it to bring the appropriate mirror to a position of 180° opposite the lesion. Of the 3 mirrors in the Goldmann lens, the small half-moon mirror, set at 59°, is used to visualize the filtration angle. The middle size mirror, set at 67°, also may be used to view the angle, as well as the peripheral retina and anterior vitreous. The largest mirror is set at a 73° tilt and allows observation of more posterior details in the retina and vitreous. Direct observation of the posterior retina may be achieved by looking through the clear center region of the lens.
After the goniolens has been set into its approximate position on the eye, direct the microscope and a moderately wide slit beam into the chosen mirror. Locate the pathology, and then adjust the magnification to provide the best coverage. To obtain the highest possible photographic contrast, trim the slit beam’s configuration so that no extraneous light spills onto adjoining structures. Location of the subject in the frame, avoidance of reflections, and clarity of view all can be modified by slight changes in lens yaw or pitch; the view through the eyepiece confirms the optimal position.
Strive to work quickly once the goniolens is placed on the eye. With prolonged contact, the cornea may become edematous, resulting in deterioration of the view. Efficiency is demanded when considering patient comfort.
When the photographic series is finished, remove the lens by firmly twisting and tilting; this maneuver breaks the suction that sometimes occurs between the lens and cornea. Irrigate the excess methylcellulose from the eye with a stream of sterile saline solution, and wipe the lids and cheek.
Because the depth of field is limited in slit lamp biomicrography, special techniques designed to present a flat plane of tissue to the microscope are valuable for photography.
To observe and photograph the superior tarsal conjunctiva, the eyelid must be everted. Lid eversion is initiated by having the patient look down. Next, the unwrapped end of a cotton-tipped applicator stick is applied about halfway across the first skin fold above the lid margin. The eyelashes are grasped and used to pull the lid up and over the stick; the lid should fold and reveal the inner conjunctival lining. While pressing back against the lashes, remove the stick, and make the images. Remind the patient to continue looking down because this minimizes any discomfort.
The direction and pressure of holding the lid can be changed so that a reasonably flat plane is presented for observation and recording. Both dual diffuse, thin slit beam and broad tangential illumination may be used to reveal the texture and morphology of any conjunctival lesions. Because the tissue is moist, reflections from the light source are observed commonly, but, by suitably angling the incident beam, they usually can be suppressed.
The lower tarsal plate can be imaged by first directing the patient to look down. Retract the lower lid firmly. While maintaining its retracted position, ask the patient to look up. The lower conjunctival surface unfolds and presents itself to the biomicroscope. By changing the direction of pressure on the lower lid, the tarsal conjunctiva can be formed into a nearly flat surface for maximum image sharpness.
Rose bengal is applied by first asking the patient to lean the head back while looking down. The upper lid is retracted, and a small drop of the dye is placed onto the superior bulbar conjunctiva. The patient is instructed to blink several times. Excess dye may be irrigated with sterile saline solution or gently swabbed from the lower cul-de-sac with a cotton-tipped applicator. The lesions revealed by this solution can be recorded with either dual diffuse or broad tangential illumination.
A common form of topical sodium fluorescein consists of paper strips impregnated with dried solution that may be activated with a drop of sterile saline solution. This is placed onto the bulbar conjunctiva, and, after the patient blinks several times, the excess immediately is irrigated out by a minimal stream of saline solution. The lesions of interest are recorded by blue light, achieved by either dialing in the appropriate filter or placing the special cobalt blue filter cap onto the slit lamp prism. The background light must not be illuminated because it would otherwise obscure the fluorescent pattern.
Be prepared to take the photographs immediately after instillation of the fluorescein because the dye diffuses into surrounding unaffected tissues in a short time and no longer is able to sharply delineate the lesions of interest.
A variation of this technique is the Seidel test, designed to demonstrate leakage from a corneal wound. Immediately following application of the dye (note that excess solution is not irrigated away in this application), blue-light photographs are made that may reveal a dark stream of aqueous through the brightly fluorescent concentrated dye.
Color retinal fundus photography
For over a century, many researchers, clinicians, and photographers have worked to perfect the technique and application of retinal photography. The earliest fundus photographs were produced in 1886 by Jackman and Webster. The next century saw steady improvements in instrumentation, including enhanced optics, the replacement of constant illumination with flash (and back again to continuous when considering fundus imaging with scanning laser technology), and the revolutionary changeover from film-based to electronic recording media.
Historically, the biggest impediment to direct observation of the fundus has been the highly refractive (and reflective) cornea. True coaxial illumination is required to illuminate the deep recesses of the globe without creating obscuring reflections, along with an optical system that includes an aspheric front element to examine the curved retinal surface. These components are intrinsic to modern fundus cameras.
From the standpoint of camera function, all conventional fundus cameras are similar.7 Both the viewing bulb and the electronic flash are directed along the same optical pathway (through a series of beam splitters, condensing lenses, and mirrors) until their rays emerge as an annulus of light from the front objective of the camera. The annulus is projected through the cornea, pupil, and lens to the interior of the eye, where it spreads out to the retina. The image-forming rays reflected from the field of interest in turn proceed back through the center of the annulus, through the eye and camera optics, and finally reach the eyepiece and camera body. Because the incident and reflected light paths do not overlap at the cornea, most reflections are avoided, while the small remaining ones are collected and absorbed by a tiny black dot found on the rear surface of the objective lens.
Although nonmydriatic fundus cameras use infrared illumination to focus on eyes that have not been dilated pharmacologically, conventional fundus cameras all require a pupil of at least 3.5 mm to about 7.0 mm (minimum pupil size varies with the size of the projected annulus). Because some common eye conditions prevent patients from being dilated maximally, the use of an instrument with minimum pupil requirements can be advantageous.
The view through the fundus camera is truly remarkable, providing a bright, erect view of the retina that is clearer than that produced by a direct ophthalmoscope. The standard magnification of the fundus camera is 2.5X, and its standard field of view is approximately 30°. By using either zoom optics or auxiliary lenses, some cameras allow the angle of view to be modified. It may be reduced to 15° (5.0X magnification) to record the optic nerve head alone or the center of the fovea, or it may be expanded to 60° (1.25X image magnification) for recording intraocular tumors or widespread retinal pathology, such as diabetic or hypertensive retinopathy. There is a compromise between field size and magnification of detail; therefore, the recording angle must be chosen to produce the image that shows the most important attributes of the pathology.
Focusing the eyepiece
To ensure that the observer is given the brightest possible view through the eyepiece, all fundus cameras use aerial image focusing. A cross-hair reticle is found in the camera eyepiece, and the photographer always must use it to produce sharply focused photographs. The reticle and film plane are exactly the same distance from the objective lens; thus, if the fundus image is focused to the plane of the reticle, the recorded image is likewise in focus.8 Novice fundus photographers sometimes forget that the reticle must be seen clearly at all times and that it provides a crucial reference point for focusing the fundus image.9 Virtually all problems with focusing the fundus camera can be attributed to lack of attention to this vital point.
It is important to understand, however, that the photographer’s own ability to accommodate can also work against him or her in acquiring the sharpest possible image. It may be necessary for the photographer to look away from the camera and focus on something far away and then go back to reset the reticle. The adoption of digital fundus imaging may relieve many issues involved with improper reticle use. In fact, it is likely that the newer generation of fundus cameras will lack a reticle and eyepiece altogether, opting instead for infrared video viewfinders. This has already been seen in many of the nonmydriatic fundus cameras on the market today; their ease of use can make anyone a decent photographer.
A simple procedure can be used to focus the cross-hair reticle. First, turn on the viewing lamp, and place a plain piece of paper in front of the objective lens. Next, rotate the eyepiece to the highest plus diopter setting (plus and minus settings are marked on the eyepiece). Keep both eyes open, and look through the eyepiece with the dominant eye. In one smooth motion, rotate the eyepiece toward the minus setting. The cross hair comes into view, first as wide blurry lines and then as fine sharp ones. At the instant when the cross hairs are at their thinnest, note the eyepiece setting.
To check accuracy, repeat the procedure several times, averaging the results. The eyepiece should not be turned back and forth in an attempt to sharpen the focus, because this stimulates the accommodation of the eye and makes accurate focusing impossible. The focused crosshair must remain visible whenever using the instrument, and critical focusing is accomplished by bringing the retinal image to the same plane of sharpness as the crosshair adjacent to it. Although some experienced photographers focus routinely while closing one eye, keeping both eyes open is less taxing physically.
Camera alignment techniques
In terms of design and operation, the patient’s eye must be considered an integral part of the fundus camera’s optical system. Consequently, the spacing and orientation between the camera’s objective lens and its projected light annulus to the patient’s eye must be accomplished with great precision. When aligned correctly, the annulus is collected by the cornea and projected in its entirety through the pupil, thus providing uniform illumination of the retina. Distinct changes occur in the observed lighting patterns if the alignment is too close, too far, or eccentric to the eye.
To obtain the correct alignment of the camera to the eye, the patient’s head first is positioned in the instrument’s head support bracket so that the chin rests in the chin cup and the forehead is pressed firmly against the support bar. The camera is moved laterally until it is positioned roughly in front of the eye, and then (if possible) the joystick that controls lateral movements is locked in its vertical position. Most fundus cameras are aligned correctly when about 50 mm separates the front objective lens from the cornea. The camera elevation control is used to center the camera lens vertically in front of the pupil. The next step is to maneuver the patient’s head (via the head support bracket) fore and aft while closely observing the patient’s cornea. The correct spacing reveals a ring-shaped reflection of the light source, sometimes replete with an image of the viewing light filament, centered on the cornea in front of the pupil.
At this time, look into the eyepiece. If the preceding steps were followed carefully, the retina should be in view, although the composition and focus may not be correct. Ask the patient to follow the external fixation light to obtain the desired field of view, and then coarsely focus the instrument.
The view through the camera eyepiece reveals the exact status of camera-to-eye alignment.10 When correct, the view exhibits deeply saturated color throughout. If the camera is much too close to the eye, the eyepiece shows a desaturated image with a brilliant bluish white reflex toward the center. This problem is solved by pulling back slightly on the joystick. If the incident beam is too close to the both eye and eccentric to the pupil, a bright orange crescent appears along part of the image periphery.
To correct this alignment error, move the camera in the direction opposite the location of the crescent, and pull back slightly on the joystick. If the camera is too far away from the eye, the image again appears to be desaturated, but, this time, a rather diffuse bluish gray ring is seen around its circumference. The solution is to push forward slightly on the joystick. The final, correct alignment for fundus photography produces an evenly illuminated image of deep color saturation in the eyepiece. Interestingly, moving the joystick forward or backward has absolutely no effect on image focus; only the focusing knob controls that function.
Many physicians and ophthalmic technicians get thrown off by this property, because they are used to using a slit lamp, which is focused by moving the joystick in and out. Because the aerial image is focused on, the patient could literally get up, walk around the room, and sit back down, and the retina would still be in perfect focus. Once focus is achieved on a patient, there is usually no need to refocus the camera until the next patient sits down.
Composing the view
Inherent to all forms of pictorial and technical photography is the process of selection. The desired image can be thought to exist within a much larger whole, surrounded by extraneous material. By selecting and refining a point of view, all surplus visual elements are removed, leaving just the desired image within the confines of the frame.
This exact process is used to create a fundus photograph. The recording angle of the fundus camera provides a frame into which the pertinent pathology must fit. The knowledgeable photographer angles the camera and directs the patient’s gaze so that only the subject of regard is recorded.
Because much retinal pathology occurs around the disc and macula, a standard for photographic composition has evolved. This basic field of the posterior pole is based on a 30° camera; in it, the fovea is centered in the field, and the entire disc is just inside one edge of the image. Thus, the entire macula, the entire optic disc, and the major vascular arcades are recorded in one view. Of course, if the camera records a wider angular field of view, then centering the fovea results in a greater percentage of peripheral retina recorded nasally, temporally, superiorly, and inferiorly.
Another protocol provides a standardized technique of creating a photographic montage that covers a much wider field of view. Referred to as the Airlie House Classification, the Seven Standard Fields, or the Diabetic Retinopathy Study Protocol, this convention describes 3 overlapping views along a horizontal meridian, and then 2 superior and 2 inferior overlapping views.
Photography of the retinal periphery is often challenging technically. One reason stems from the fact that the entrance pupil becomes elliptical as the patient’s gaze moves eccentrically. As the eye moves farther and farther away from its primary position, a point is reached where the pupil border bisects the image-forming rays, resulting in shadowing or vignetting that cannot be removed. Directing the imaging rays obliquely across the cornea and lens, resulting in severe astigmatic distortion, causes another problem. Extremely fine vertical or horizontal camera movements may partially overcome this distortion, and, on cameras so equipped, an astigmatic control device further neutralizes it.
Fine focusing the retinal image
The fundus camera is capable of only shallow depth of field, so the major concern in the focusing process is to determine upon what structure to focus. Because the fundus is illuminated evenly, there are no highlights or shadows to provide visual cues about relative elevations. Knowledge of the 3-dimensionality of the normal posterior pole and of various pathological conditions, coupled with the apparent differences in sharpness of select parts of the retina as seen in the eyepiece, is the only useful data.
The normal retinal surface has a uniform concavity except for a very small depression at the fovea and often a greater indentation at the optic disc. The small blood vessels surrounding the fovea and the slightly grainy texture of the retinal nerve fibers provide easily seen targets on which to focus.
Many retinal and choroidal diseases produce elevation in specific areas. The greatest elevations are produced by papilledema, causing a pronounced swelling of the optic nerve, and central serous choroidopathy, producing a domelike elevation of the retina.11 Retinal detachments can also provide the photographer with a challenging focal problem. When the camera is focused critically on the center of these lesions, the surrounding tissue appears somewhat blurry and such discrete objects as vessels are rendered less sharply. Customarily, the fundus camera is focused on the most anterior structure in the field, although additional photographs taken at levels between the top and the base can be useful.
As the patient’s eye is an intrinsic component of the camera’s optics, a decrease in the transparency of any part of the eye’s media is deleterious to the final image. The cornea, lens, and vitreous all must be clear to obtain sharp and saturated views of the retina. In some cases, compensation can be made for suboptimal clarity of these structures. If the cornea is edematous, topical proparacaine hydrochloride followed by topical glycerin may improve the view temporarily. If the patient has a cataract, careful movement of the camera within the pupil margin may reveal a part of the cataract that is less opaque, yielding an improved view of the retina. This is where a trained ophthalmic photographer earns his or her keep; the experienced photographer can produce quality images, where an inexperienced photographer may otherwise fail. Unfortunately, the view through a hazy vitreous usually cannot be improved by camera work.
Stereo fundus photography
Stereoscopic photography enables the viewer to understand the 3-dimensional spatial relationships of the subject. In fundus photography, this ability provides a more complete understanding of the nature of lesions by identifying whether they are deeper or shallower than other known anatomical reference points.
Standard pictorial stereophotography duplicates human stereoscopic vision by photographing the same subject from 2 slightly offset camera positions. The apparent depth is influenced by the distance between the 2 positions (stereo base), with greater distance enhancing and less space diminishing the effect. For close-up or macro views, the 2 camera positions must be offset and converge. The viewer is able to reconstruct the spatial configuration of the subject when viewing a pair of stereo photographs set in proper orientation.12
The stereo base used in fundus photography is limited by pupil size. In this application, convergence of the 2 points of view is especially important. In stereo fundus photography, corneal curvature provides this convergence. By laterally sliding the camera so that its point of view moves from one side of the corneal apex to the other, the same point in the fundus may be observed from 2 points of view. The 2 camera positions are parallel to each other, and the cornea provides the necessary convergence of the image-forming rays.
Two basic methods may be used to obtain stereo fundus photographs. The first uses specially designed stereo fundus cameras that produce simultaneous stereophotographs in split frame 35 mm format. Because their narrow field of view limits their use to photography of the optic nerve head, these cameras are not in widespread use. The other method produces sequential stereo slides, in which each half of the stereo pair is produced as a separate exposure. Although patient movement, blinking, and other artifacts at times ruin the stereo effect, any fundus camera can be made to produce sequential stereophotographs.
Pupil dilation limits how much stereo separation is possible. For well-matched stereo pairs, the minimum pupil size should be about twice the size of the illuminating annulus; a smaller pupil still allows stereo but with some vignetting.
To perform stereo fundus photography, first align the camera with the eye, compose the view, and critically focus the image. Then, move the joystick slowly to the left while watching the image through the eyepiece. The catoptric image or orange crescent appears on the left edge, but continue to move the joystick still farther to the left. The crescent sweeps to the center of the field and then disappears. At this point, make the exposure. Next, move the joystick to the right. The crescent reflex appears on the right side this time but again disappears with further movement, at which point the right half of the stereo pair is taken. To make sure that no blinks prevent proper stereo imaging, an extra stereo pair or two should be taken.
Beginning fundus photographers sometimes are dismayed to find that they have produced less than optimum photographs. Prior to the digital revolution in ophthalmic imaging, some time would elapse between the photography session and the availability of the finished slides; because of this, the photographer may have been unable to recall any reason why the photographs look the way they do.
In fact, fundus photographs tell the observer as much about camera technique as they do about the illustrated pathology. The digital age, with its instantaneous feedback, has allowed photographers to self-critique the image immediately after the image is taken. They can then decide on-the-fly if the image is within the acceptable limits of focus, exposure, and positioning. With digital imaging becoming standard, the photographer’s quality can only improve.
Ideally, the fundus photograph should be sharply focused, evenly illuminated, and deeply saturated in color. Only the patient’s retina should be visible; anything else is an artifact. Several of the more common artifacts and their remedies are as follows:
Orange crescent: Camera misaligned. Move camera in opposite direction.
Blue peripheral haze: Camera too far away from eye. Move camera forward.
Bright blue-white central reflection: Camera too close to eye. Move camera backward.
Pale vertical tan or white streaks: Lashes. Retract eyelid during photography.
Spots or streaks remaining in field regardless of subject position: Tears, dust spots, or dirt on objective lens. Clean.
Orange or red image with no detail: Closed eye or blink synchronized with flash. Retract eyelid.