Crop Factor

The sensor on many DSLR's is smaller than a frame of 35mm film. This results in the edges of a photo being cropped or cut on such cameras, from what they would be on a film camera. This is great for long range work, because it gives you more apparent magnification for a given lens. For instance a 300mm lens mounted on my camera is the equivalent of a 450mm mounted on a film camera. Conversely it lowers the effectiveness of a wide angle lens, when mounted on a DSLR. When I first bought my camera there weren't many options for a wide lens, but the market quickly responded and there are many good wide lenses available today.