I have the Canon EF 24-70 f/2.8L USM (mark I). If I were buying today, I'd get the mark II. But I haven't bothered to upgrade my lens becuase it's not a lens I use very often.
If I'm taking something "artsy" and I want some background blur, etc. then f/2.8 doesn't generate a lot of out of focus blur in the 24-70 range... the 70-200, on the other hand, generates very pleasant blur (particularly at the long end). This means my 24-70 is mostly relegated to candid shots (and often indoors), but not portraits (if they were portraits, I'd grab my 85mm f/1.4).
I will make a general comment...
If you want a full frame camera because you were hoping for an improvement in image quality, then you're going to need to supply your camera with a diet of high-quality lenses.
What you get when you move to full frame is a couple of things.
#1 You typically do get better low-light performance (especially with a 5D IV ... it's a nice improvement in both ISO and dynamic range).
#2 When comparing an 80D (or any APS-C body) to a 5D IV (or any full-frame body) using the same focal length (e.g. just pick on 100mm) you'll find you get a wider angle of view with the full-frame camera.
For an APS-C camera at 100mm, the field of view is 12.8° in the wide direction. With a full-frame camera it's 20.4° using the same lens. The focal length is the focal length and all the math about focal lengths don't change. BUT... since the angle of view is wider, the photographer will typically change how they shoot ... by moving in closer to the subject so they get the equivalent composition (e.g. suppose it was a portrait). When they do this, the shorter camera to subject focus distance means that the depth of field was reduced and THIS means that the background blur will be increased.
If I take a subject at 16' away using a 100mm lens using a crop-frame camera, and then do the math to calculate the dimensional field of view (it's 3' 7.2" wide by 2' 4.8" or a diagonal of 4' 3.9") and I flip to a full-frame camera, I would get the SAME dimensions if I move closer... to 10' (notice that 10 x 1.6 = 16 ... so it literally is the difference of the crop factor).
But this difference will actually make quite a noticable change in depth of field. For example, at f/2.8 (using a circle-of-confusion size of 0.019mm) the depth of field at 10' would be 16' would be 9.8", but the depth of field at 10' would be 3.8" (that's quite a noticeable reduction).
This is why you'll hear people say the full-frame cameras are better for things such as portraits... those people are probably going for the increase in background blur.
Tim Campbell
5D III, 5D IV, 60Da
