« on: October 16, 2014, 01:33:09 PM »
I have both a 7D and a 5D3 and use and like both, but for different things. I use Raw (almost) exclusively. Simply stated, the trade-off is (1) higher pixel density (i.e., smaller pixels), (2) larger sensor and/or (3) high frame rate. At any given price point, choose any two of the above.
The 5D3 gives better image quality, especially in very low light. Its Auto-ISO max is set to 12800 whereas the 7D's is set to 3200. In particular, the 5D3 seems to have more DR. Furthermore, regardless of what Canon says, I don't believe that the 7D2 image quality will match or exceed the 5D3's (or 6D's) image quality.
The 7D uses smaller pixels than the 5D3. Thus, using a given lens focal length at a given distance from the subject, the 7D uses more pixels to make the image, unless the subject is so large that it "overfills" the 7D's field of view. A FF camera with the same pixel density as the 7D would have about 47MP. To maintain the same data frame rate as the 7D through the image processing circuitry would require dropping the FPS from 8 to about 3.2. Raising the frame rate back to 8 would requiring using better, faster and, therefore, more expensive circuitry.
Also, the smaller sensor of the 7D should have cost implications regardless of the number of pixels. The mirror and related components can be smaller and lighter and, in principle, cheaper and easier to produce.
So, what do I use and where? My default camera is the 5D3. The 7D is used almost entirely to photograph racing cars and motorcycles with a 100-400, 70-200 or 300 f/2.8 with or without TCs. When doing that, the 5D3 is usually also at hand, with a shorter lens attached. This is a role which the 5D3 can play but its predecessor, a 5D, couldn't. I suspect a 6D can't either. Anyway, unlike some, I find the extra "reach" to be real and advantageous. I suppose I could buy a 600 f/4 or Sigma 200-500 f/2.8 and a 1Dx , but then I'd have to carry them.