Originally posted by RobA_Oz It seems that the main advantage of increased size for sensors is reduced noise at higher ISO settings. The main disadvantage, apart from cost, seems to be the reduced depth of field that arises from the increased diameter of the image circle. Of course, that's also an advantage in those situations where discrimination is required, to isolate a subject from its background. Generally speaking, though, wider field of view shots, where large depth of field is almost always desirable, are probably advantaged by smaller sensors, and telephoto portraits are probably advantaged by larger ones.
It also seems from the little reading I've done on the subject, that a very high lens resolving power can be a problem for larger sensors than smaller ones with the same pixel count, so, when I hear people talk about full-frame sensors taking "better" pictures than smaller ones, I wonder what they're really talking about. Are they comparing like with like, or are they just impressed by the pixel count?
FF can go to a higher fstop to get the same DOF with the same diffraction effect as APS-C. Only time smaller format can win in wide DOF contest is at f22 or whatever the limit is for the lens. Smaller system cannot decrease DOF in the manner that FF offers on the other end because they run into a lens restraint road block. The question is, which end of the aperture scale do you like to hang out at?
At base iso a good quality APS-C will look similar to FF. But, sensor only has one real iso setting and this is the base iso. Each iso higher is made by amplification and the FF pulls ahead slightly at each higher iso and will have better DR. You need to decide what iso and DR you will need and if it is worth the additional investment. Nice to have choices.