Focal Length Multiplier and Crop Factor
Whether a camera lens is seen as wide-angle or tele is dependent on its Angle of View (AoV); for a particular image size AoV is dependent on focal length. Due to their widespread use in the past, people are familiar with the focal length terms of 35mm SLR lenses, e.g. 28mm is a Wide and 400mm a Telephoto lens. Although some professional DSLR cameras use full frame image sensors, most sensors on digital cameras are smaller than the 35mm format. To enable comparison of lenses for digital applications the 35mm equivalent is often used, i.e. the focal length of a 35mm camera lens with the same AoV as the subject digital camera lens. When using 35mm lenses on DSLRs one should account for the actual imager size. A sensor smaller than 24 x 36 mm results in a smaller AoV and hence in a longer apparent focal length than indicated on the lens. The illustration shows the image area for a 200mm lens used with a 35mm SLR, a Pentax *ist Ds and an Olympus E-300. The image is cropped by a (Crop) Factor of 1.5 and 2.0 respectively. On a 35mm film SLR, these same images would require a 300mm or 400mm lens resp.
Hence the term Focal Length Multiplier