Tech Tip: Using White Light to Focus for Fluorescence

One challenge you can run into with fluorescence photography is that the fluorescence excited by the flashlight you use to find and view subjects may not be bright enough for the camera’s autofocus system. The camera focus software likes strong edges in the image to aid in focus lock and you don’t always get those in fluorescence, whether due to brightness or pattern.

You can make the task easier by using a white spotter light to illuminate the scene. Your first thought is probably – ‘Won’t that overwhelm the fluorescence and ruin the shot?’ The answer is that it can, but that it does not have to as long as you take the same approach that we describe in the article on fluorescence photography without darkness. That article talks about fluorescence photography in daytime, but it works just as well if the ‘ambient’ light comes from your spotter light and not the sun. The key is to use as short a shutter speed as your camera/flash sync will allow, which is generally something like 1/200 or 1/250 sec. (If your camera can go to super-short exposures, don’t go any shorter than 1/500 sec or you will start cutting off some of the fluorescence excited by the flash.) If your aiming light is a tight bright spot then you may have to reduce the intensity by adding a diffuser or pointing the light a bit away from the target area and working with the spill light, but you can make this work.

The photos below illustrate this method. My photo subjects were a leaf (very weak red fluorescence from chlorophyll) and a lens cap. I used a Light & Motion Sola NIGHTSEA at low power with the remote phosphor in place so it was flooding the image area with enough white light for the camera to focus easily. The picture on the left was taken with an exposure time of 1/60 sec (the ‘normal’ speed for flash sync) and the one on the right at 1/200 sec. (Other camera settings – ISO 400, f5.6 for both images.) You can see that the white light has a strong effect on the left photo and no effect on the one on the right.

Click image for larger view

This technique works perfectly well anywhere you do fluorescence imaging, whether above or below water. However and wherever you do this, get out there, love the dark, and love the glow!