External Eye Photography
William C. Nyberg, RBP, CRA, FOPS
Scheie Eye Institute
University of Pennsylvania
External photography is the term used by ophthalmic photographers to describe pictures of the eye made with conventional cameras. The pictures look familiar to a person uninitiated in the more esoteric images made with fundus or slit lamp cameras, even though both of these cameras can also photograph the "external" (rather than internal) eye.
The typical range of subjects for external photography is from a low power image of the face to the a high power picture of the individual eye. External photographs of patients faces are used by general ophthalmologists and ophthalmic-plastic surgeons to document skin types as well as lesions and morphological abnormalities of the skull, eye lids and surrounding areas. Neuro- ophthalmologists use pictures of the head and shoulders, face and eyes to document nerve anomalies. Perhaps the most challenging and difficult to accomplish external photos are those that document the anterior segment of the eye ball itself.
Not only are external photographs unparalleled for showing conditions of the face, skin, conjunctiva, sclera, and the eye lids, but are capable of telling spectacular stories of the cornea, iris, and lens, to work in concert with slit lamp and fundus images.
Oblique View of Skin Lesions
Ophthalmic photographers sometimes use very ordinary camera equipment to document large subjects such as a whole family with lid ptosis, or head and shoulders "portraits" of patients with a neurologically induced head tilt, or face and scalp infections of Herpes Zoster. Pictures of a face or two eyes together showing bilateral lid conditions or eye muscle imbalances can be accomplished with modest equipment. More typically, however, the techniques and equipment of close up photography and photomacrography are needed to get the magnifications necessary to show the individual eye.
Neurofibromas of the Lids
Digital single-lens reflex cameras are the overwhelming preference of medical photographers specializing in ophthalmology. Digital SLR's offer the enticements of excellent image quality, the availability of many superb camera systems, and the extreme practicality of many existing systems to view and handle the resultant pictures. Computer programs are readily available to view the images on computer screens, they can be incorporated into software like Microsoft PowerPoint® and projected onto screens for lecture presentation, they can be submitted for publication in books and journals, and photo finishers can easily make reflection prints should they be desired.
The single-lens reflex type of camera is a natural choice for an ophthalmic photographer doing external photography. These cameras let the photographer see the actual image that will be captured. This is not only appealing for low magnification photography, but is necessary for working at higher magnifications. Image quality is dependent upon the resolution capability of the camera, measured in pixels. Cameras that acquire 6 megapixels or more will capture images of sufficient quality for on-screen viewing, projection, and publication.
Another consideration when purchasing a digital SLR for external eye photography is the size of the sensor. A "full-sized" sensor is the same physical size as a frame of 35mm film, which was the gold standard in ophthalmic imaging for decades. A smaller sensor will have the effect of magnifying the image. While the actual sensor size is generally burried in the technical literature for the camera, manufacturers publish the "magnification factor" which provides more useful information. Factors of 1.6x and 1.3x are common. Full frame sensors are very expensive, and are not necessary for external photography.
The magnification factor should be considered when choosing a lens system for external photography. Using the same lens, a digital camera with a 1.6x factor will produce images that are 1.6 times as magnified as they would be with conventional 35mm film.
Manufacturers of digital single-lens reflex cameras offer various ways to accomplish higher than normal magnifications. Auxiliary close up lenses are inexpensive and can let normally available lenses be used at close distances. Extension tubes that fit between the lens and the camera body will allow even more magnification and better image quality than close up lenses. The principle of extending a lens beyond its' normal range to achieve a close focusing distances and high magnifications can also be accomplished by using a bellows device instead of extension tubes. The bellows allows more extension than is mechanically practical with extension tubes.
The greatest image quality is obtained by using special "macro" lenses. These specially formulated optics have been optimized by the lens designers to deliver their greatest quality at high magnifications. They are also much more convenient, as they typically allow the user to make up to "life size", 1x close up images without needing awkward tubes and bellows attachments. Tele converters can be used in conjunction with macro lenses to achieve higher than 1x magnifications.
Lens focal lengths of about 100 mm are the most commonly used. This length usually provides a comfortable working distance from the subject and a pleasant perspective rendition in the final picture. Other focal lengths should not be totally avoided however. In select situations, longer and shorter lenses can provide an ideal solution. But remember the cameras "magnification factor" mentioned above. A 100mm lens on a 1.6x digital camera is the equivalent of a 160mm lens on a 35mm film camera.
Chemosis of Conjunctiva
The use of conventional 35mm film in external eye photography is quickly becoming obsolete in favor of digital cameras which offer comprable image quality without the need for film processing or scanning for publication. However, many legacy systems are likely to exist for some time, and some photographers may simply prefer, for one reason or another, to work with film.
If film is used for external photography, daylight-balanced, color reversal (slide) films, with a sensitivity ("speed") of between ISO 25 and 100 are typically employed. These films usually respond to electronic flash with little or no color correction required. They offer superb color rendition and physical image structure (graininess and sharpness). Kodacrome film has been a favorite among plastic surgeons for its' particular rendition of flesh tones, but it is increasingly hard to find. The E-6 process ("Ektachrome") films are also excellent, and are more practical to have processed and duplicated. The differences in tone between the these two groups are subjective.
Ghost Vessels on Cornea
Accomplishing photomacrographic imaging of a live patient in a clinical setting is very challenging. The most immediate problem encountered is the natural movement of the patients head. High magnification photography results in very shallow depth of field however it is accomplished, and even small patient movements are enough to seriously disrupt imaging efforts. Verbal encouragement and chin/head rests are the most commonly attempted solution. But having the patient lean his head against a wall, or lie on an examining table and photographing from above will also work.
Some 35mm and digital slingle-lens reflex cameras have interchangeable focusing screens. Some of these screens are designed to work with high magnification images and dim lighting conditions, and are definitely worthwhile. Perhaps the most valuable aid to focusing an external camera on lesions in the anterior chamber of the eye is to do slit lamp photography before attempting external photography. The education obtained during the slit lamp exam will allow the photographer to approach the viewfinder of the single lens reflex camera with confidence, as he/she will know the exact location and appearance of the lesion. Focusing the camera should be done by setting a preselected magnification and then moving the entire camera/lens apparatus until the image in the viewfinder is clear. This technique maintains consistent magnification. The usual practice of rotating the focusing collar on the lens changes the magnification of the image. This is perfectly acceptable in normal photography, but not in scientific photography.
Displaced Intraocular Lens
Careful positioning of the patient and the eye is crucial to excellent photography. Images used for scientific documentation deserve to be made with great care and attention to detail. The usual conventions of medical photography for anatomical positioning should be observed. Protocols for camera viewpoint should be developed giving consideration to bone structure and iris position. Magnification protocols should consider telling the pathological story with a series of photos. Typically, intervals of a factor of 2 (each picture taken at twice the magnification of its predecessor) do the job nicely. Once these protocols are developed (usually in conjunction with the physicians who will use the final images), they should be followed religiously, giving the images more scientific utility over time.
The face is nicely rendered at about 1/8x (in which the image on the film is 1/8 as large as the subject). Two eyes together, or oblique views of one eye that include substantial lid area are made at 1/4x or 1/3x. Magnifications of 1/2x to 1x are needed to document the eye lids. 3/4x is a personal favorite. The globe is best shown at magnifications from 1x to 2x (the film image being twice as large as the subject). At 2x, a cornea will fill the frame from top to bottom. 1 1/2x will show the entire iris as well as enough sclera above the limbus to include surgical cicatrices. 1 1/4x will show the whole eye including each canthus. 1x pictures are useful for visual establishment as a base for higher magnification detail.
Cataract Through Dilated Pupil
Electronic flash is so practical that consideration of other sources, such as incandescent, for routine photography will not be considered here.
As with all photography, the actual rendition of the subject matter is done with light. Good lighting is crucial to the success of external photography. External eye photos are specialized medical photographs, and they are usually best made with a single, carefully placed light source, coming from superior to the patient anatomy. Single light sources typically yield pictures that are less confusing to view than those from multiple sources. Additional light sources should be used with extreme caution. A "white card fill" is useful when additional lighting is needed to lower the lighting ratio by filling in a shadow. It is much safer to use than an additional light, but is only rarely needed in ophthalmic external photography.
"Straight ahead" medical/ophthalmic face pictures are best done with the single light directly above the lens as this provides equal light to each side of the face. Lighting to reveal morphology is best accomplished with conventional methods. "45 degree, 45 degree" so called "architectural lighting" is a good place to begin. The light source is placed 45 degrees above and 45 degrees away from the front of the subject, and then adjusted to suit the individual situation.
Most external photographers use some system to mount the light source on the camera or on the lens. The lens option more frequently allows the light to be rotated around the lens axis. Whether mounted on lens or camera, flexibility in light placement is desirable. This allows individual lesions to be revealed to best advantage.
It is also desirable to have a light source that incorporates a modelling light. A modelling light is a continuously burning incandescent light that is designed to work in concert with the electronic flash tube so as to show the effect of the flash in the final image. Modelling lights allow the photographer to see exactly where highlights, specular reflections, and shadows will appear. All is not lost, however, if the photographer does not have the luxury of a modelling light. In fact, most of the commonly available, small, light weight, electronic flash sources do not have this feature. Excellent photographs can be made by experienced people using their knowledge and skill to place the light in its most advantageous position. A small flash light can be used to practice and visualize different lighting effects.