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Posted by John Boyd on 18:06:01 02/02/08
In Reply to: Re: Focal length posted by joel rosenthal
Suppose I take a picture using a 1.5 crop camera and 500mm lens, then crop further by cutting in half (both ways). Am i then entitled to say I've used a 1.5 x 2 x 500 = 1500mm lens?
By your logic I am.
No photographer in their right mind would make such a claim when they do the crop themselves. Why does it make a difference if the camera auto-crops for them?
It shouldn't.
Here's what the real issues are.
When we crop, we also usually compensate by enlarging by the same factor. The issue is what happens when you enlarge the image like this, and this is not simply a matter of the crop factor. What really matters is the quality of the image you're trying to enlarge (by 1.3, or 1.5, or 1.6, or whatever).
Two things that matter a lot for the resulting image with digital cameras are the pixel density and noise level. These can vary greatly for cameras with the same crop factor. In fact, the Canon 40D over has about 10% more cropability than the 20D or 30D, and about 30% more than 10D, even though all have a crop factor of 1.5.
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