One aspect of this comparison that has not been mentioned is the fact that crop-sensor cameras are generally faster for a given price point.

• The 7D shoots at up to 8 Frames/Sec, the 5D manages 3.9 Frames/Sec
• The 1Ds III manages 5 Frames/Sec, while the 1D III/IV manages 10 Frames/Sec

In sports photography, where continuous drive is often used, those extra frames could mean the difference between capturing a player right before hie hits/kicks a ball, and actually at the moment of contact.

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Regarding using telephoto lenses on a crop-sensor body, the critical measurement here is the pixel pitch, which is the thing that actually determines how much detail you will get from a lens.

Basically, if you have two different sensors with the same pixel pitch, the larger one effectively takes the exact same image, with some cropping.

As an example, I have a 30D and a 5D2. Both have 6.4µm pixels. Therefore, every exposure on the 5D2 effectively includes the entire area that a 30D exposure would capture, with the same resolution.

However, the 7D has 4.3µm pixels, so for a given focal length, the 7D will resolve 1.5X (1.488 to be exact) the detail.

This is all assuming an ideal lens. If your lens cannot resolve fine details, either camera will produce a blurry result. Also, small pixel sizes will be less forgiving of lens defects than larger pixels, since the smaller pixels require a larger lpm from the lens. A lens that is at the edge of it's resolving capabilities on a 5D2 may not see any improvement on a 7D, since the extra pixel resolution has no effect on the lens sharpness.

There is a nice breakdown of the Canon series camera pixel pitches on the-digital-picture.com. It's about 2/3 of the way down the page.

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There are other considerations - larger pixels generally give less ISO noise, though modern image processing is advancing faster then sensors are shrinking, so it is not as much of an issue as it could be.

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Note: I am writing about canon bodies because I am a canon user, and know them far better. However, most of the arguments are much more broadly applicable, basically to anything that uses a CCD/CMOS image sensor:

• The two critical factors in a camera's FPS are Number of pixels, and ADC speed.
• The critical factor in how much detail you get from a lens is pixel pitch (Smaller pixels gets more detail, until you reach the limitations of the lens).
• The largest influence in ISO performance is pixel size (larger pixels are less noisy). The readout electronics have much more influence here, though, so it is not an absolute determining factor as the two above are.

This is true across all brands.