Sometimes it is necessary to know the diagonal dimension of a particular film format. This is especially true when trying to determine if a lens has a large enough image circle in order to adequately cover a certain film format. Modern lens manufacturers publish specifications that will tell you the maximum format the lens is designed for. But, with older lenses this information is sometimes not available. If you can determine the image circle of the lens, then you can determine which film sizes will work with the lens.
For example, say you are contemplating buying a lens and you know the image circle is 325mm. Looking at the table below you will immediately see that 325mm is the exact diagonal of a sheet of 8×10 film. This lens would probably work. But, you would have absolutely no movement available when using the camera. This lens would adequately cover 4×10 and would have generous coverage for 5×7 and 4×5 formats.
Here are a few common film sizes and their diagonal dimension;
If you need to calculate the film diagonal of any format, here is the formula.
A = Film Height
B = Film Width
C = Film Diagonal
Keep in mind that the actual image area is slightly less than the film dimension due to the hold-down guides that keep the film held in place in the holder. This calculated diagonal measurement is plenty close enough to determine adequate image circle of any lens.
Hope you find this information helpful!
JB
