External Eye Photography
Equipment

William C. Nyberg, RBP CRA
Scheie Eye Institute
University of Pennsylvania

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.

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.

Lens

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.