Quote Originally Posted by Mark Feldstein View Post
That's the way I'd do it too although I usually work at shorter distances. You want to make sure your camera is plumb, level and square to the target too. I usually use a couple of string lines to set equal distance from the edges of the print. This is also a good method for squaring the camera to art work you're photographing for reproduction. Having a bubble level in the accessory shoe on your camera is really helpful too.

I take multiple shots at close range, medium and infinity keeping notes as to where the camera is set and bracket the exposures 1/2 stop either side.
Look at the negs with a strong loupe not the finished prints.
It's actually a pretty meaningful real-world test, a pro told me about doing it that way back in the 80s. 20 feet is roughly 100x the focal length of the lens, so it's near enough infinity to tell you something. It's also the only test I've ever used. Try it on a modern kit zoom from a DSLR - you'll be appalled!