Imagine that you are standing looking at a landscape; this is a wide-angle view. Your eyes are lenses projecting an image on the back of the eyeball. The image is circular. This is the basic situation.
Lenses of differing focal lengths are used to "crop out" narrower and narrower portions of this image circle and bring it into focus at a larger size. If you shoot a picture at extreme wide angle, on a tripod, and then replace the WA lens with a telephoto and shoot that, then load up the two images side by side and crop the WA shot to the same field of view as the telephoto, then (sharpness issues aside) the two shots will be indistinguishable from each other.
So long focal-length lenses are image magnifiers just as telescopes and binoculars are.
The simplest description of how focal length is derived is to look at "view cameras"; these consist of a lens board, a film holder, a monorail connecting the two, and a light-proof bellows to fill the space between. With a view camera, when you use a 70mm lens you focus by moving the (optical center of the)lens to 70mm from the film plane (for infinity focus). With a 400mm lens, the lens will be 400mm from the film plane, etc etc. For truly long lenses you need to add extensions to the monorail in order to focus.
These are simple lenses.
Now on a point-and-shoot cam or a dSLR, the distance between lens and sensor is FIXED; it can't be varied. Therefore the lenses use internal adjustments of the optical system to effect their focusing. Zoom lenses are even more complex, where the movement of elements in relationship to each other creates a longer or shorter focal length. For lenses on these cameras, there's not any fixed relationship between focal length and the actual, physical length of the lens.
Canon, for example, has a very nice, compact, high-precision (and expensive) 70-300mm DO (diffractive optics) zoom that is no larger than my Tamron 28-75mm zoom lens. My Canon 70-200mm zoom, a truly excellent lens, is like 3x as long physically as that 70-300 mentioned above, and much heavier.
In general, the larger the lens physically and the shorter the zoom range of the lens, (we're speaking specifically of telephotos here) the higher is the possible quality of the optical system. All sorts of compromises have to be made to fit these extended zooms into smaller and smaller packages.
Whoot, I'm rambling and this my be way off-topic of what you wanted to hear...
R. |
