Slit Lamp Biomicrography
Exposure

Photographic exposure is controlled by three separate adjustments. One is the flash power setting on the camera power supply. Another exposure control is an aperture on the elbow of the camera attachment which can vary on the Zeiss Photoslitlamp from f/45 to f/14. The third exposure control is the beam width. A wide beam is effective in higlighting deep endothelial pigmentation. A narrow beam is useful and showing intrastromal defects and topographic features.

Other factors that affect the exposure of the photograph are the tissue being illuminated, the sensitivity of the film or ISO setting on the digital camera, and the magnification of the microscope. If the tissue is very opaque and highly reflective, the aperture needs to be smaller or reduce the flash power appropriately. Less light will be required to photograph this subject as opposed to a clear cornea. Blue irides will reflect more light than brown irides and therefore exposure is affected by iris color. Some slitlamps contain a rotating filter wheel. If a photoslitlamp does not have an aperture diaphragm it will have a colorless gray filter called a neutral density filter. This filter is an exposure adjustment control that lowers the amount of flash hitting the eye by one stop (0.3ND = 1 stop) Bracketing exposures above and below the manufacturer’s published tables, will allow you to get properly exposed results with the narrow exposure ranges that slide film allows. ( Plus or minus ½ stop) Not all patients with corneal lesions will sit quietly through a rigorous photo session that requires multiple aperture settings for each type of illumination or magnification change. I recommend producing an exposure table based on photographing a blue and brown iris with multiple settings and various beam widths. Record the parameters for each exposure and base subsequent clinical photographs on the settings used for the well exposed slides.

Film sensitivity is carefully chosen to allow the dimmest form of indirect lighting to be properly exposed. Most photobiomicroscopy is done with a film sensitivity of ISO 200. There are situations where a faster film emulsion such as ISO 400 is required. Films with a low ISO number have lower sensitivity but can render fine details better because the emulsion contains a finer grain. (ISO stands for International Standards Organization which assigns the tested emulsions their proven sensitivity classification numbers).

Digital Single-Lens Reflex (SLR) cameras are rapidly replacing 35mm film cameras in slit lamp biomicrography. The sensitivity of the sensor can simply be adjusted on the camera, selecting an ISO. Because higher ISO settings on digital cameras create "noise" in the image which may impair its ability to render fine detail, the same settings of ISO 200 or ISO 400 should be used with digital SLR systems as well.

Microscope magnification will have an affect on the exposure of the film when taking a picture. The higher the magnification the more light needed. If a photograph is taken of the patient's eye at 6X magnification and a small lesion is photographed at 40X magnification the exposure difference will be significantly changed and the photographer will have to either open the aperture diaphragm further or increase the flash power to compensate for the illumination decrease. Some biomicroscopes will show no change in exposure vs. magnification.