When I go on the 1d, 5D forum, they hands down say the 7D IQ is NOWHERE near the IQ of the 5dmII. When I come on the 7D forum, everyone says the difference is not very much????
What's the deal?
This is a difficult question to answer concisely, but I'll have a go.
To make a meaningful comparison, you obviously have to compare like with like usage, but what does "like" mean? One possible interpretation is taking as near as possible the same photograph from a creative point of view - same subject from the same distance in the same light, same subject framing, same motion blur, same depth of field and of course same exposure. So:
To get the same subject framing, the focal length used on the 5DII must be 1.6x the focal length on the 7D. This is well known of course.
Same motion blur is easy, just use the same shutter speed.
Given the preceding constraints, to get the same depth of field on the 5DII means using a 1.6x larger f-number. (This is actually the same entrance pupil diameter on the longer lens.) So if we use f/2 on the 7D, we will need f/3.2 on the 5DII.
Finally the same exposure. Since we used f/3.2 on the 5DII we have to compensate for this by increasing the ISO speed by 1 1/3 stops.
All the above is inevitable once you decide to take the same shot, and the consequence of the increased ISO speed is that you lose everything you thought you were going to gain from the larger sensor. In this situation, the 7D image quality is essentially the same. Score 1 to the 7D.
However, there is another kind of like with like. Take the very best shot you can with the 7D, vs the very best shot with the 5DII. Perhaps for a certain shot you don't need the same depth of field, because the 5DII gives you enough without having to use a smaller aperture. Perhaps you are shooting a static subject from a tripod (many landscape shots, for example), so you can freely use a slower shutter speed. In these situations you don't need to increase the ISO speed so you can get the full benefit of the "1 to 1.5 stops noise advantage" often quoted for the 5DII. Score 1 to the 5DII.
A similar situation applies to lenses - a 7D photograph has to be enlarged 1.6x as much which is a disadvantage in relation to lens sharpness; on the other hand the 7D crops away the corners which are never as good as the centre (and often much worse) so the 7D has the advantage. In practice, it partly depends on how the lens is optimised - for maximum performance in the centre, or for even performance across the frame. It does vary.
One last point - if you are looking for more reach, such as for wildlife photography, you will reach a point where the only option you have is to crop the image. (You are "focal length limited".) In this situation the advantage is squarely with the camera which has the higher pixel density - the 7D.
Hopefully that gives some insight into how some can claim the 5D is much better, and others can claim the 7D is at least equally good. They are both right, depending on the circumstances.
