1. Low light.
One typical advantage of full frame cameras is better low light capability. Some would say that you can use flash in low light. Of course you can. But low light capability can complement flash use, not only increasing the potential range of your flash but also making it easier to mix the flash and ambient for natural-looking results.
|Nikon D600 with flash. ISO 12,800.|
Moreover there are times when using a flash is difficult. In the shot below, the background was cavernous, and the black ceilings would have been difficult to bounce from. At the same time we were constantly moving, therefore a long exposure would have resulted in a blurry shot. Thanks to the D3's high ISO capability, I could use a shutter speed as high as 1/160 (f/2.8, ISO 12,800) for a sharp shot.
|Nikon D3 with Tokina 11-16 at 16mm, f/2.8, 12,800 ISO.|
|Nikon D7100 at 12,800 ISO (with flash). No noise reduction yet.|
All factors being equal, a full frame camera will be sharper than an APS-C camera. I know there are skeptics out there so let's look at DXO scores for the Sigma 35 1.4, one of the sharpest lenses ever made.
On a 6mp Nikon D70, the Sigma 35 1.4 has a resolution of 6 perceptual megapixels (i.e. the lens outresolves the sensor). On the higher resolution sensor of the 12mp Nikon D300S, the Sigma 35 1.4 has a resolution of 10 pmp, a nearly proportionate increase of total resolution. On the 16mp Nikon D7000, however, the Sigma 35 1.4 only has a resolution of 11 pmp.
Although there is a significant increase in sensor resolution from the D300S to the D7000, the increase in total system resolution is marginal. It appears the lens has reached its limit.
Or has it? If we compare the Sigma 35 1.4 on the D7000 with the Nikon D4 which also has 16mp, the system resolution jumps to 14pmp. This alone should tell you that all factors being equal, full frame cameras will give a sharper image than APS-C. However, that's not the end of the story. If you go still higher to a 24mp Nikon D3x, the total system resolution leaps to 20 pmp. In fact, you may have heard that on the D800, the Sigma 35 1.4 reaches as high as 23 pmp. So in fact, the Sigma has far greater potential resolution than the D7000 result would imply.
The explanation is simple: an APS-C camera has smaller pixels than a full frame camera, and a lens has to have a higher resolution in order to fully utilize the resolution of a sensor with smaller pixels. Below a certain pixel size, a lens has to be extremely sharp to match the sensor resolution. The Sigma 35 1.4 is already one of the sharpest lenses out there but even the Sigma cannot fully use the 16mp resolution of the D7000. This is why I think that if DXO tests the Sigma 35 1.4 on the 24mp D7100 or D5200, the increase in resolution will be marginal. Hopefully, in the future, manufacturers step up and design higher resolution lenses for APS-C (there are after all many high resolution Micro 4/3 lenses). For now, though, there aren't any. See the list of sharpest lenses for DX here.
Of course there are other factors that affect sharpness (duh!). Nonetheless, if we stick to the issue of whether a full frame camera is sharper, the answer is that indeed, a full frame camera has a higher potential for sharpness than an APS-C camera.
3. Depth of field.
|85 1.8G on Nikon D600|
A full frame camera using a lens with the same field of view as an APS-C camera will appear to have a shallower depth of field, all other factors being equal (same distance to subject, same distance to background, same aperture). However, one counterargument is that you CAN achieve a very shallow DOF on an APS-C camera as well, such as by using a longer focal length, moving closer to the subject, etc.
|28-105 @ 105mm f/4.5 on Nikon D7100|
On the other hand, at wider focal lengths, it is not easy to achieve a shallow DOF on APS-C, this gives wide-aperture wide angle full frame shots a distinctive appearance.
|28 1.8G on Nikon D600|
|Sigma 50 1.4 on Nikon D600|
4. Cost vs. versatility.
|24-70 on full frame (Nikon D3)|
|24-70 on APS-C (Nikon D90)|
5. Wider angles vs. reach.
The conventional thinking is that full frame is better for wider angles because at the same focal length, you'll have a wider field of view, while APS-C is better for longer focal lengths because at the same focal length, you'll have a longer equivalent focal length.
I think this is not a big factor. There are wide angle lenses specifically designed for APS-C that will allow a wide field of view. For example, the Sigma 8-16 can have a field of view as wide as a 12mm full frame lens. That's just the same as the widest full frame ultrawide, the Sigma 12-24.
|Sigma 10-20 @ 12mm on Fuji S5, an APS-C DSLR|
As you can see, the choice between full frame or APS-C really depends on what kinds of photos you take. Personally, I like having the versatility of having both a full frame and APS-C camera. When I want shallower DOF, such as on a standard zoom, I use full frame. When I want a deeper DOF, such as an ultrawide or fast telephoto zoom, I use an APS-C camera.
Full Frame DSLR FAQ
Comparison of DOF between APS-C and FF in Depth of Field of Kit Lenses