Originally posted by marcusBMG Tony Northrup got similar responses. I (and he) never said that the focal length or aperture of the lens changes. I (and he) are saying that if you want to think in terms of full frame equivalence then its both aperture and focal length that need to be multiplied by the crop factor. And the fact is that many do - only yesterday I pm'ed a fellow forum member who had written that a classic 135mm f2.8 would give 200mm!! at f2.8!! And the fact is that many don't appreciate the effect on DoF..
Several flaws in the argumentation:
1) Insisting that different formats should have the same output and/or collect the same amount of light. This is wrong according to the theory of exposure. If it was correct a Pentax 645 should be compared to a cell phone. The theory of exposure is based on exposure (surprise). DOF doesn't enter into it.
2) Using one format as benchmark; usually FF, and directly or indirectly indicating at whatever DOF you get on that format at whatever aperture is the correct one or desired one which any other format must comply with. This is cheating. By this method only the properties of the reference format is used as benchmark. Whatever properties the smaller format has is ignored.This is only a one-sided biased opinion. Obviously, in order for two lenses to be DOF equivalent they both have to do the same. Ie an equivalent FF lens must be able to do what the smaller format lens does as well DOF-wise to be "equal". Such lenses do not exist. DOF equivalency between formats is a pipe-dream. And why only wide open at a certain focus distance? Why not compare DOF range and maximum DOF as well?
3) Pretending that DOF is only depending on aperture and equivalent focal lenghts. It isn't. It is dependent on aperture, focal lenght, focusing distances, and subject magnification. Ie lenses claimed to be DOF equivalent are patently not. Not even wide open.
Some may be surprised if they put "equivalent" lenses into a DOF calculator. For comperable lenses (eg prime vs prime, zoom vs zoom) you'll find that the smaller format lens usually has thinner minimum DOF than the larger. This is because it has larger maximum magnification.
In reality you loose one stop with FF compared to APS cause you have to shoot at one stop longer shutterspeed with FF at the same ISO for the same DOF.
Defining aperture form DOF is equally sensible as defining shutterspeed from motion blur.
The claim that thin DOF is the holy goal of photography is nonsense. In fact, 99,999% of all photograph ever taken, and I'm not even exaggregating, has DOF achievable with APS.
Insising that thin DOF is sole purpose of high speed in a lens is equally misguided.
Last edited by Pål Jensen; 10-09-2014 at 06:12 AM.