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How do I calculate depth of field, in portraiture, when I don't know the subject distance, but I do know the subject's height in the frame? See: Pic related as an example of what I'm talking about.
I'm curious what the difference in depth of field is between something like a 56mm f/1.2 lens and a 90mm f/2 in such a situation, on APSC sensor. Obviously, the 90mm will create more background compression, which will blur the background despite actual depth of field, but I wonder if the depth of field would also be thinner with the longer lens, or if it would have a deeper depth of field and more background blur?
