On Fri, 11 Dec 1998 23:59:59 GMT, fabgo@powersim.no wrote:

>I know that zoom lenses are always slower than primes, but I am having some
>trouble understanding why. Why is it easy to create a 50/1.8 but hard to do a
>28-70/2.8. First I thought that each lens element would prevent a
>such-and-such amount of light from reaching the film, but now I know that the
>maximum aperture is only calculated as a ratio between the focal length and
>the actual diameter of the diaphragm. So how come zoom lenses are so slow?
>Where does all the light go?

Probably too smazzed by three days of listening to the %$#&
Re-Bublican non-sense at the impeachment hearings to give a
reasonable answer, but, here goes an attempt...;-)
If you were given a bunch of reasonably useful FL's of
single-element lenses, you could probably, in a reasonably
short time, come up yourself with a passable 50 to 100mm
or so single FL lens just by experimenting with different
combinations of a few of the elements. It probably wouldn't
be too hot around f2 or so, but by f4-5.6 maybe it wouldn't
be too bad... Now try to experimentally assemble even
a bad zoom. Kinda more difficult, huh...? Now try to
make it good at just one FL. Then all FL's within its range.
Then at all stops and all focus distances... Try to reduce
the distortion. Now try to also make it fast and still good...

>I am also wondering how some zooms manage to keep a constant aperture, while
>others vary.

The zooms with constant-aperture often are faster
at the short end of their range, but have a shifting
diaphragm that keeps the effective lens speed
constant as the lens is zoomed.

Oh, an' letz vote out them-thar 'Bublicans come
the year 2000...! ;-)