Originally posted by normhead Exactly... I don't know a single guy either from when I was in Photo Arts at Ryerson, to any of the working photographers I've known, who gave an owls hoot about the math or physics of lens performance. It's about getting your eye to the viewfinder and saying " I can work with this." That's a photographer. Analyzing charts for comparative depth of field etc. FOV etc. doesn't mean a thing, until you put your eye to the viewfinder and see what it looks like.
DoF is even more problematic. In the field I tend to bracket my f-stops.. shooting wide open, 5.6 11 and 22 or some other combination that makes sense based on the scene. It is so rare that the wide open shallow depth of field is the one I want to keep that I shudder whenever I hear about people wanting to pay big bucks to get it. I often find the images of those who seem to think shallow depth of field is really important, irritating. Quite simply stated. Shallow depth of field is more often a problem, not a solution.
But I digress, the point being, I often can't evaluate what I want from DoF until I'm looking through the lens. Math doesn't help with that.
I would tend to agree, the only thing I really ever use math for, is if I know a specific venue subject and shooting location, i routinely go through the approximate (distance >>> focal length) calculation of
Image size = subject size x Focal length / distance
or some recomposed form of it to consider what lens or range of lenses I want to carry.
Aside from that, you are correct the viewfinder is all important.
Other useful uses of math are in manual flash calculations, and occasionally a more detailed distance and magnification calculation for macro, but generally the viewfinder rules.
My present obsession with wildlife only shows one thing, and I think we are on the same page, regardless of the depth of field we have it is generally never enough, therefore why do people get such fast lenses.