35mm film has an image area of 24mm by 36mm. These are the exact dimensions of the area on the film to which an image is recorded.
Medium to low-end digital cameras sold today have sensor chips smaller than 24x36mm in size, since producing a 24x36mm image chip is still quite expensive to do. Similarly, APS film records to an area of film 16.7x30.2mm in size.
The upshot of this is that if you use such a digital or APS camera you�ll be taking photos which do not record the same image size as 35mm film. So it�s like taking a photo using 35mm film and then cropping out (snipping off) the edges. Imagine drawing a smaller rectangle within a given 35mm photo and then cutting it out - you�ve got a digital or APS photo.
This cropping factor is often confusingly referred to as a focal length multiplier. This is because the cropping makes, say, a 50mm lens on an APS camera behave rather like a 70mm lens on a 35mm camera. Not because the focal length has actually changed - it hasn�t - but because of this cropping of the image. The cropping factor is sometimes specified as a numeric value - 1.3x or 1.6x, say.
If you want to use your lens to take photos of things far away then this might actually be to your advantage. But if you want to use a wide-angle lens then this cropping factor can be a problem, since wide-angle lenses yield less dramatic results when you crop out the edges.
Some people object to the term cropping factor as well, arguing quite rightly that the issue is a matter of a change in format of the image recording area and using lenses designed for a different size format. This is true, but people are so used to equating a given 35mm film focal length with a given coverage area (or field of view) that I think the concept of a cropping factor is convenient and easily understood.
To give an example, let�s say you have a 100mm lens. When used on a 35mm film camera you get a certain coverage of the scene. But if you were to put the same lens on a digital camera with a 1.6x crop factor (ie: a smaller than full frame sensor) then you would not get the same view of the scene - you�d get less. The view you would see on your 1.6x digital camera would be the same as if you had a 160mm lens, were there such a thing, on your 35mm film camera.
