Very simply stated, Anders, the longest dimension of a film's frame size rectangle is what the cone of light that passes thru a standard lens has to cover. The 35mm (or 135mm) film useable frame dimensions, where the image appears, are 24mm x 36mm. The diagonal of that rectangle is just a teeny bit over 43mm. For reasons I will not go into here, the 50mm lens (instead of 43mm) has long been considered the 'standard' lens size for 135mm film.
If you divide 50mm into 200mm, the magnification of the 200mm lens is going to be 4x. Or four times the 'normal' lens magnification, which we normally think of as 1X. The 4.65 figure quoted below came from a strict consideration of the film diagonal, 43mm. Divide 43 into 200 and you come up with 4.65 times. Simple, huh? <g> keith whaley Anders Hultman wrote: > > Steve Larson: > > >Just to add to Franks post regarding telephotos. "Normal" lenses are around > >50mm or 1 power, so a 200mm would be 4 power (actually 4.65, but we won`t > >go there). > > How is this "power" figure calculated, and what is it used for? > > anders > ------------------------- > http://anders.hultman.nu/
