Originally posted by jsherman999 Which equates to more DOF control when you realize that you can simply stop down to match the aps-c DOF for that FOV if you wish with no penalty - you can't always open up a lens wider to do the opposite.
Consider: A typical 50mm lens shot at f/4 is blisteringly sharp at the plane of focus, probably at it's peak MTF in the center. Most 35mm lenses are at f/4 as well. But 35mm at f/4 on aps-c, depending on the subject distance, maybe doesn't really isolate the subject very well. f/2.4 gives you that nicer isolation, but you miss that f/4 acuity, that pop. 50mm @ f/4 on FF gives you that peak sharpness with the nicer f/2.4 DOF. and it's just an option. You can stop down even past f/4 if you want more DOF. With the extra 1+ stops (at least) of noise control from the bigger sensor, shutter speed isn't a problem when you stop down because you can just reclaim that stop with an ISO bump with no additional noise. If you need to, if light is a problem in the shot.
Consider II: A subject is far enough away that you can shoot your 50mm f/1.8 at f/2 on FF and still get the whole subject in focus, within the DOF. To get the same 'look' on aps-c, you'd need to be shooting a 35mm f/1.3 lens - wide-open. Think for a moment 1) how much a 35mm f/1.3 would cost, 2) how big it would be, and 3) how a 35mm f/1.3 would probably perform with regards to acuity and CA - wide-open at f/1.3, on a high-MP aps-c sensor.
.
Pal's point (and I think I agree to a certain extent) is that APS-C gives you one stop more depth of field for a given exposure over full frame. This is definitely true. If you
want that extra stop of depth of field on full frame you will need to stop down and push your iso up. I think you overestimate the difference in noise between APS-C and full frame sensors as to how easy it is to go up a stop. I shoot to iso 12,800 on my K5. Would I be comfortable going to 25,000 on full frame? I kind of doubt it.
Narrow depth of field is easily achievable on APS-C, it just really tends to be there more at longer focal lengths (55mm seems about perfect to me on APS-C).
Anyway, I understand that your photography style depends on narrow depth of field, but there are many (macro, landscape, even journalism) where narrow depth of field is not particularly beneficial and sometimes is detrimental to the end image. Most photogs that shoot with full frame have an APS-C back up.
There is a Sigma 30mm f1.4 out there that is pretty decent, as mentioned earlier in this thread. It works quite well as a portrait lens as even wide open it has good center sharpness and is very reasonably priced.