One of the most fundamental yet difficult tasks for beginning photographers is proper adjustment of the focusing reticle of the traditional fundus camera. It is an essential element for capturing consistently sharp retinal photographs. The focusing reticle is the pattern of etched black lines, usually a cross-hair pattern, seen through the fundus camera eyepiece. The reticle is part of an aerial image focusing system like that used in microscopes. An aerial image system is brighter than one that uses a ground glass screen like an SLR camera. In fundus photography the aerial image is important. We need as bright a view as possible to keep the viewing illumination low enough for the patient to tolerate while still being able to see well enough to align and focus the instrument. Yet because the image is focused in “space” rather than on a ground glass, the reticle must match the same plane of focus as the fundus image.
The principle behind this adjustment seems simple enough, but there are some challenges associated with it. Setting the reticle adjusts for simple spherical refractive errors in the observer’s eye when their accommodation is relaxed at distance. You can think of this process as calibration of the optical system prior to focusing the fundus camera. The focus of your eye needs to be set at the same aerial focal plane as the camera. Simply put, both the crosshairs and the fundus need to be in focus at the same time. Set the reticle first and then adjust the focus of the camera.
In order to properly focus the fundus camera on a consistent basis, the photographer should relax their accommodation at distance to avoid accommodative shift during photography. The reticle is then adjusted by turning the eyepiece until the cross hairs are sharp. The barrel of the eyepiece is marked in diopters of correction. Since we are in the eyecare business, many of us know what our refractive error is, and you may be tempted to use that number as your reticle setting. Unfortunately, the diopter numbers may not be accurately marked on the eyepiece and can vary by manufacturer or instrument. So they can’t be relied on when switching from instrument to instrument. The reticle must be set correctly for each instrument.
You also can’t just set the calibration once and be done. A disadvantage to the aerial image is that your eye may change focus due to accommodation. Keeping the cross hair sharp requires constant awareness since your accommodation can change throughout the day due to fatigue or stress. Young photographers may struggle with keeping the eyepiece set properly because they typically have a greater ability to accommodate to near. Early in my career, I often noticed that my eyepiece setting would change as the day went on. It would also change during the week, Mondays were different than Fridays and accommodation also changed with stress levels. Pay constant attention to the cross hairs and adjust the reticle if your accommodation drifts.
If you normally wear glasses or contact lenses, it is usually best to wear them while taking fundus photos rather than rely on the camera eyepiece to correct for your refractive error, especially if you have any astigmatism in your dominant (shooting) eye.
A popular and commonly taught technique for setting the eyepiece reticle involves adjusting the crosshairs at least three successive times, noting the diopter setting each time, and then using the average of these numbers. This technique sounds like a good idea, but it can actually promote unnecessary accommodation and inaccurate settings. Each time the photographer looks at the numbers marked on the eyepiece, they accommodate to near, then immediately try to relax at distance before looking through the viewfinder again. Repeating these steps multiple times induces accommodative “gymnastics” and subsequent fatigue that can lead to improper settings when accommodation inevitably drifts during a photographic session.
For this technique to work properly, someone other than the photographer should note and record the settings, so the photographer can keep accommodation relaxed at distance the entire time.
A better strategy is to ignore the eyepiece numbers altogether, but pay constant attention to the crosshairs and image of the retina. As long as the crosshairs and the aerial image of the fundus both appear sharp at capture, the focus will be correct in a system that is properly calibrated for parfocality.
For more on the basics of using the fundus camera visit the Fundus Photography page.