Originally posted by Pål Jensen No problem with the relationship. It is conclusion people make form it that is misleading. Eg saying an 300/4 on FF is equivalent to a 200/2.8 on APS as the DOF wide open is identical is perhaps correct but irelevant for 99.99% of the cases. The shallow DOF when shooting eg. wildlife or even landscape is a problem, not a solution. However, the shutterspeed used at F:2.8 is a solution for most an not a problem. Hence, for most long lens shooters a 300/2.8 is the real life photography equivalent of a 200/2.8 on APS. The difference in image quality is the tradeoff you get from choosing APS. The cost and size + extra DOF at the same exposure is the benefit.
If you're in a situation where you're shutter-speed constrained - i.e. worried about exposure, getting adequate shutter speed when you want the
same DOF you would have had on aps-c - simply bump ISO in the FF shot one stop. It has the headroom over aps-c, and you won't be seeing any more noise.
In most telephoto cases - besides indoor concert shooting and the like - you are not usually that shutter-speed constrained, and have the freedom to choose your aperture to control DOF the way you like. At in many cases, the lenses available will not allow you on aps-c to match the DOF options you have on FF. And no, I'm not talking about the extremely rare f/22 and smaller cases you like to use...
Quote: Thin DOF as holy cow is totally irrelevant as 99,999% of all images do not display DOF thinner than what is possible with APS (in fact I've never seen such images in any books, magazines or art galleries). In addition, no one here can tell whether a picture is shot with APS or FF with regard to DOF; we have seen that in other threads.
Using a specialized usage as normative is totally misleading...
It's not 'specialized usage', it's day-to-day usage. A 50mm shot at f/2.8 on FF gives you what a 33mm lens shot at f/1.8 on aps-c would in FOV/DOF, while being sharper than (probably) any 35mm lens out there at f/1.8. That same 50mm at f/4, probably it's highest-resolving aperture, gives you a 33mm at f/2.5, still nice for subject isolation while having blistering sharpness on the focal plane. And if you do want something more extreme, shoot a 50 1.4 or 1.8 wide-open - better-corrected than any f/1.2 or f/1.0 lens out there wide-open.
My 180 f/2.8 wide-open on FF gives me what a 120mm f/1.8 would on aps-c - they don't make one of those, although there are Pentaxians out there willing to pay a pretty penny for the A* 135 f/1.8. I hate to say it, but I'm pretty certain my 180 wide-open is going to show more sharpness and less CA than that $$$ 135 1.8 wide-open.
Here's a few from that combo from a few hours ago:
180mm @ f/2.8