Time was the 35mm camera was almost a standard. For years, a serious amateur would buy a 35mm SLR and a fixed focal length lens, say a 50mm lens. When the change came to digital, light-sensitive chips were very expensive to manufacture, and so these chips were usually made only a fraction of the size of a 35mm piece of film. For example, the original Canon Rebel, which had a light sensitive chip smaller than a 35mm piece of film was said to have a "crop factor" of 1.5. That is, to understand how much of the world could be recorded on that chip with your 50mm lens, you would have to multiply the lenses focal length by one-and-a-half times. 50 x 1.5 = 75. The lens hadn't changed, of course; only the size of the recording medium was smaller (kind of like you were exposing that 35mm piece of film and then trimming it down to the size of the chip). So, if the D7000 has a chip that is smaller than a 35mm piece of film, and a "crop factor" of greater than 1, you will have to multiply the focal length of your lens to get an approximation of your field of view with this camera. The 50mm lens has not changed, but the size of the recording medium has.
Clear enough? There is enough nuance in the optics of this to choke a cat (and you'll probably read some of that below). But in general, that's the deal.
Ben Marks
