Back in the film days, the owner of an entry-level 35mm SLR could use the same film and the same lenses as the owner of a pro 35mm SLR, and thus entry-level and pro models weren’t divided so much by image quality as by construction, features and add-ons. The pro models were beefier, more rugged, could shoot faster, had better viewfinders and metering systems (and AF systems, in the case of AF SLRs), and offered a wider variety of accessories.
Digital complicated things a bit, in large part because with a DSLR, the camera is also your film. In the film era, no one had to buy a new camera just because a new, better emulsion was introduced. But with digital, as sensors improve, so do image quality and high-ISO performance, and the user—especially the working pro user—needs to upgrade his or her camera more frequently to remain competitive.
Full-frame DSLRs, even going back a generation or two, produce better overall image quality than APS-C DSLRs because their larger image sensors can collect more light at a given shutter speed and ƒ-stop. Their larger sensor area’s ability to collect more light also accounts for the full-frame DSLR’s better performance at higher ISOs.
Until recently, almost all full-frame DSLRs were high-end pro cameras. The Sony A850 of a few years ago and the arrival of Canon’s EOS 6D and Nikon’s D600 brought us "entry-level" full-frame DSLRs, with prices not far from those of high-end APS-C models. These lower-cost full-frame models deliver terrific image quality, as you’d expect from current-generation full-frame image sensors. But they lack some features that pro models—and even mid-range APS-C DSLRs—provide. Let’s look into this, and also consider what separates the higher-end APS-C DSLRs from the entry-level cameras.
Everything begins with image quality. From an overall image-quality standpoint, bigger is better, and newer is better. In DxOMark.com’s RAW sensor ratings (which mainly consider noise), the four highest overall scorers are full-frame models introduced in 2012, followed by a $40,000 medium-format camera, another 2012 full-frame DSLR, another medium-format model, then yet another 2012 full-frame camera. APS-C doesn’t appear until positions 12 and 13 (a 2012 model and a 2013 model, respectively).
In DxOMark.com‘s ratings for high-ISO performance, the top 20 positions are held by full-frame cameras. In the ratings for dynamic range, four full-frame models are followed by three APS-Cs, another full-frame, then two more APS-C models. In color bit depth, the top two places are held by medium-format models, followed by two full-frame DSLRs, another medium-format and five more full-frames; the highest-placed APS-C camera in color bit depth is a 2013 model in position 14. (See DxOMark.com for complete sensor ratings of most current DSLRs and many earlier ones.)
There’s more to image quality than the factors DxOMark.com considers, of course, but you get the idea. Full-frame sensors are better (especially for high-ISO work), and newer sensors are better in both full-frame and APS-C. In terms of pro vs. amateur, an entry-level full-frame DSLR rates third overall, and an entry-level APS-C model, 12th overall. Two mid-level full-frame models rank 1 and 2 overall, and the highest-ranking all-out pro model ranks 6th.