One last note about crop sensors for now (I have not talked very much about EF-S lenses and I am not going to).
One of the catch phrases you hear with the crop sensors cameras is the “Equivalent Focal Length” or some variation. With the Canon sensors you get a crop factor of 1.6 (Nikon is 1.5) so you’ll hear a lot of folks saying things like “The 50mm lens on the 40D is the equivalent of an 80mm lens”. While there is some truth to this statement it is essentially a shortcut to explain what is actually going on.
True, the “Field Of View” is roughly equivalent to an 80mm lens in terms of how wide the camera can see but that’s about where the comparisons end. Because it is still a 50mm focal length cropped you get the characteristics of a 50mm lens.
I shot a couple of test pics to try and illustrate this point. The first photo was taken with a 50mm lens set to f4 on a Canon 40D (crop sensor). Next I made the exact same shot, from the same distance with a Canon 5D mkII (full frame) and a 70-200mm set to 80mm at f4. (Okay, I missed slightly and it was actually 78mm if you want to split hairs). Do these images look identical?
I shot two images of the same subject with my Canon 40D using my 17-85mm lens. I made the first shot zoomed all the way in at 85mm.
I then attempted to recreate the same shot zooming all the way out to 17mm. I didn’t change any other parameters (specifically distance from camera to subject). I then cropped in post to give me a similar framing, just as a crop sensor would do.
You can see a big difference in these two images in terms of Depth Of Field. Shooting a 17mm lens on a smaller sensor (crop factor of 5x in this case) is clearly not the “equivalent” of shooting with an 85mm lens.
Keep in mind that the results here are exaggerated by the 5x crop factor vs the 1.6x that we normally get. So while it is “similar” it is not “equivalent”. As I mentioned in the previous post however, I wouldn’t toss my 7D in the bin just yet.