Figuring Out Your Lens: Flange Focal Distance

Has someone given you a lens for your camera, but it doesn’t fit and you’re left wondering what mount it is (so you can buy an adapter)? Here’s a quick and easy way to figure out where the Film Plane of your lens should fall in order to help you in making a more educated guess…

First, to get a clue about where it should be and what size the lens allows the image to be, you can use your hand. Go outside and open the lens to it’s widest aperture setting, then set it to infinity. Point the front of the lens at something far away. Hold the lens in your one hand and hold your other hand up a few centimeters away from the back of the lens. You’ll see an image, quite literally in the palm of your hand. Now move your hand back and forth away from the lens until you see the image come perfectly into focus. That’s where your film plane would lie. The distance from the back of the lens to the film plane is called the flange focal distance.

If you want to get a little more exact, you can set the lens up on a table and use a piece of paper and a ruler to get more exact measurements. Once you get close to the proper measurement, go to wikipedia and find out which mounts are similar to the results you found: http://en.wikipedia.org/wiki/List_of_lens_mounts

BTW: If the image takes up most of your hand, it’s probably a Medium Format lens. If the image is smaller than your pinky nail, then it may be for a super-8 or a 1/3″ CCD camera.

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