Originally posted by MMVIII Just some cents from this side:
if I followed the discussion properly, than I see a dividing line not at the question if there is something like a mathematical concept of equalizing photorecording parameters especially when changing some parameters, like sensor dimensions. There is no doubt that this is possible, necessary, useful and to a certain part done automatically, at least by me and I guess most people having come from film era to digital.
I have the impression that the question is more, how relevant and determinating this concept is for the actual creative process of photographing.
And here I am more on the side if the photographers who say, fine, but in the end I look through the viewfinder and compose my image.
I think there's this assumption that people who accept equivalence are constantly 'using it' in day-day shooting - that's not
exactly the case, unless someone is actually shooting more than one format at the same exact time, like at a wedding, using two bodies, etc.
It's probably more accurate (for most shooters) to say this: We don't use it or worry about it
during shooting, just like you, but we
benefit from decisions made in
a purchase process in which equivalence was used.
For example, I can't really express how useful my 50mm f/1.8 is (for me) on FF. It gives me so much power, so much versatility - it's as if Pentax came out with a sharp, small, 33mm f/1.1 - for $110.
My $300 Tamron 28-75 f/2.8 is very powerful in a similar way - it would be about a 18-50 f/1.8 on my aps-c systems. It's a versatile, very sharp, low-light monster, and it's light and inexpensive.
Now, if I were a birder shooting long telephoto, or if I shot a lot of low-light venues in which I needed to maintain a certain DOF (like shooting performers moving in various distances from the front of a stage,) I might not benefit from FF as much or it may be very cost-prohibitive to 'gain' anything - and using equivalence would give me those hints before I plunked down the cash.
So in other words - equivalence is probably more useful for most people during the purchase process, in system planning. Once you do that, you just
benefit and don't worry about the math or any conversions during your shooting.
.