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I sure am glad that I don't need to use this thread to understand aspects of crop factor, focal length, pixel density, field of view, aspect ratio, etc. With a few tidbits of correct information mixed in, there is a large collection of information that, even if presented with good intentions, is incorrect or, at best, confusing or misleading in this thread.
I wouldn't even think of trying to sort through all of it, and I'm not sure, at this point, trying to give a complete explanation would be helpful, but I'll just make a few quick points.
1. A 400 mm lens is a 400 mm lens regardless of what camera body is being used.
2. With a 1.6 crop factor camera, what you are seeing is only a portion of the full frame view. That portion is roughly 40% of the view that you would be seeing with the full frame camera and the same lens.
3. Regardless of whether you are using a full frame camera or a 1.6 crop Canon camera, the aspect ration is still the same--3:2.
4. Yes, wildlife photographers often find advantage in using the 7D, and the reason is that it has a high pixel density, which can be very useful in focal length-limited shooting situations. All of its 18 MP are concentrated into the smaller sensor that is only 40% the size of the full frame sensor. The theory is that, if you were using an 18 MP full frame sensor and if you needed to crop to the field of view of the 1.6 crop camera, you would be left with only about 7 MP in that cropped image from the full frame camera. Thus, even though a 400 mm lens is still a 400 mm lens regardless of the camera on which it is being used, if you are in a focal length-limited situation and you are going to need to crop, you can put as many pixels on your subject with a 400 mm lens and an 18 MP 1.6 crop camera as you would be able to put on the subject with a 640 mm lens and an 18 MP full frame camera.
5. Even though it is the pixel density of the 7D that gives it an advantage in these focal length-limited situations and not its 1.6 crop factor, since the Canon camera bodies with the highest pixel density currently are the 1.6 crop bodies, some occasionally carelessly refer to the crop factor as being what gives the advantage.
6. If Canon makes a full frame body in the future with 45 MP and it can still shoot at 8 frames per second, there will be no pixel density advantage to the using a 1.6 crop 7D. If, at the same time, Canon makes a new version of the 7D with 45 mp on a 1.6 crop sensor, it would regain a pixel density advantage.
Edited on Feb 13, 2013 at 12:15 AM · View previous versions