Excerpt from Scott Kelby's Digital Photography book, Chapter 3, The Truth About Lenses—Zoom Lenses and Full-Frame Lenses
You have probably realized by now that most digital cameras (including digital SLRs) have a crop factor. That means the millimeter number shown on a lens produces a different field of view on a digital camera than you were accustomed to with a traditional 35mm camera.
Suppose you mount a traditional 85mm lens on a digital camera: it is not really 85 mm. On a Nikon, the lens has a crop factor of one and a half, so an 85mm lens gives you the field of view of a 127mm lens. On a Canon camera, the lens has a crop factor of 1.6. That makes an 85mm lens behave more like a 135mm lens. This drives photographers who switched from film to digital completely crazy. In their view, an 85mm lens should simply be 85 mm, because that is how it always was.
But there is a new star on the scene: full-frame cameras. And on a full-frame camera, 85 mm is simply 85 mm again. No crop factor and no multiplication. The lens is finally what it says it is. Buuut there is a catch! (Isn't there always?) If you put a lens intended for a regular digital camera (which includes most digital lenses) on a full-frame camera, it magnifies. (In reality, it crops the photograph to the magnified dimensions.)
What does that mean for you and me? If you buy a full-frame digital camera, you will not get the benefits of a full-frame camera (as far as lenses are concerned) unless you buy lenses that are specifically made for full-frame cameras. At the high end, however, there are more expensive lenses that work perfectly with full-frame cameras. And they do not crop the photographs.
