Is there a rule of thumb for aperture opening and exposing time when using filters?
It really doesn’t matter which aperture that you use – the aperture is used more to control the length of exposure than anything else. If you were making a 16×20 print, then since the lens is a long way from the paper you need to open it up…maybe f2.8 or f4. If you are making a small print, such as a 4×5 then stop the lens down further to maybe f16 or smaller and make and an adjustment to the timer. If you do not adjust the aperture accordingly for the print size, then you will either have excessively long or very short exposure times. Very short exposure times are a problem, as they don’t allow any time for burning or dodging. Very long exposure times are just generally a pain, as well as the possibility of reciprocity failure. So generally for an 8×10 or 11×14 print use f8 to f5.6 as a lens setting.
