Originally posted by Canada_Rockies 
Take note that a 4 inch lens (100 mm) is a normal lens on a 4x5. The focal length affects the field of view, agreed, but the fov is also affected by the size of the receiving medium. So people throw around the "Your 200 mm lens is now a 300 mm lens" when it is not. The statement should read "Your 200 mm lens, when used on an APS-C sensor digital camera, has the field of view that a 300 mm lens has on a 35mm camera's 24x36 mm film gate."
The camera makers kept emphasizing how great the crop factor was, and conveniently ignored the consequences at the wide end of things, where your 24 mm lens now was cropped down to the middle two thirds of the image, and now you need a 16mm lens to get the same scene into the image.
This is one of the reasons I warn people about the 4/3 system, with its 2x crop factor, you need a ridiculously expensive 7mm to get the same field of view as a 15mm on film.
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