What does it mean by "the reach of crop" and extending your focal length? I may simply have no idea what I'm talking about but the entire idea of comparing a Full Frame sensor to a cropped APS-C sized sensor by talking about extending zoom confuses me. This comparison is usually at the top of the list on many reviews as an APS-C advantage.
Speaking in Canon terms, the same EF lens on a APS-C would produce an image that appears 1.6 times closer compared to that same lens on a FF camera. This zoom effect caused by the sensor being smaller then the projected image from the lens. My personal opinion is this should only be compared when the cameras megapixel count is equal or the photographer needs to use the entire frame regardless of resolution.
I accepted this 1.6x fact with little thought until so many comments and reviews are comparing the new 21MP 5DmkII with cameras like the 10MP 40D by using the focal length as a deciding factor. I simply don't believe that I should buy a 40D (or 50D) if I want my 200mm to have the same angle of view of a 320mm.
A 17mm EF lens will project an image with a 17mm angle of view but the smaller APS-C sensor will only see and record the center of this circle. You're not really zooming, you're seeing less.
The more I thought about this the more I needed to know the actual difference. Cropping the full size image from a 5DmkII by 62.5% (equal to the 1.6 sensor crop) results in a file almost equal to the full resolution of the 40D that crops the lens. If 10MP is all you need, you could essentially capture the same photo with the same EF lens from the same location on both camera bodies.
If telephoto lenses are a priority and you can't get closer to your subject, then the resolution of the 50D at 1.6x would produce a technically larger (and closer) image but I'd bet that a cropped 10-12MP 5DmkII would still product a cleaner print then a full 15MP 50D file.
One disadvantage of using the full image circle of a FF sensor would be any lens vignetting that a APS-C sensor simply ignores on EF lenses. Lenses preform better towards the center. EF-S lenses will not work on FF cameras because they project a much smaller image circle. These lenses do not benefit from vignetting cropping.
Am I completely wrong on this? Deciding between FF and APS-C sensors with vastly different resolutions should not be judged based on the illusion of an increased 1.6x focal length. If the FF and APS-C sensors being compared were both 15MP and produced an equally clean image, then maybe the crop could be considered an advantage.