2.6. CROP FACTOR
If you were to put the same lens onto different camera bodies with different image sensor sizes —full-frame and smaller—you would notice that what you see through the viewfinder is a smaller physical area in the cameras with the smaller sensors, as seen in 2-9 and 2-10.
The term crop factor refers to the fact that the imaging area is physically smaller. Less of the image circle projected by the lens is used; therefore, it is in effect cropped. The image remains the same size at the film plane for a given lens and subject distance — it is in no way "magnified." This is also why a telephoto lens appears so much more powerful — the field or angle of view has been reduced. This is great for nature and sports photographers, as the net result is more-apparent telescopic magnification with no tradeoff of maximum f-stop loss. The opposite is, true, however for wide-angle shots, because there is a lessening of wide-angle viewing.
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