35mm Focal Length and Zoom Factor
One question asked by many people, transitioning from consumer cameras into 35mm adapters, is “what is the zoom factor of a XXmm lens?” The short answer is none, the long answer is that it depends on the lens.
The reason for this is that we are talking about two different things. When manufactures list “Zoom” they are saying that, at 6x zoom the object appears 6 times closer than it really is. When the focal length is listed on a lens, say 50mm, it refers to the distance of the lens from the film or image sensor.
When you increase the focal length of a lens you narrow the field of view, the narrower the field the less of the object is in view, giving the appearance of zooming. This is why lenses with small numbers (15mm, 28mm) are called wide angle lenses and lenses with larger numbers (200mm, 400mm) are called telephoto.
A good starting point is to think of 50mm lenses as what your eyes see, the higher the number the closer object will appear, the lower the number the more of the scene you will see.
They always say a picture is worth a thousand words, and in this case that couldn't be more true, please take a look at these two sites for illustrated descriptions of this.
