In article <3290B6CA.D80@umich.edu>, malicky@umich.edu says...

>I have some questions on the design/performance trade-offs
>in lenses, particularly zooms:
>
>1) I've heard there are more design compromises to make with
>fast lenses than slow ones, so that a fast lens may perform
>poorer than a slow one (performance of both measured between
>f4 and f8). (In the comparison, lets say we're comparing
>lenses from the same manufacturer, made with similar glass,
>quality control, etc.) If so, is it a significant
>difference? The question I'm trying to answer for myself
>is, except for the brightness of the viewfinder and
>potential to use higher shutter speeds, am I giving up some
>picture quality (in addition to low weight and cost) if I
>use the faster lens?

If you throw enough money at the design problems raised by
increasing lens speed, some of the problems can be lessened...
Since all lenses are compromise designs (at least, the ones
we can buy...;-), faster and slower versions of lenses in the
same lens line may have about the same performance level at
the same apertures, with the cost for the faster lenses
being in higher price, larger size, and greater weight.
Sometimes there are performance differences, but they can
go either way....

>2) What factors are related to whether a zoom is constant or
>variable aperture? I.e., will a constant aperture zoom
>(say, a 35-70/2.8) give up something significant (e.g.,
>sharpness, weight) to a variable one (35-70/2.8-4.0) (as
>above, lenses made with the same design and manufacturing
>standards).

Constant-aperture zooms are generally larger and heavier
(and faster overall) than variable-aperture zooms, which
can be equally sharp (or unsharp...;-).

>3) Some zooms stay in focus as the lens is zoomed, others do
>not (what is the name for this?) Is there any significance
>to this other than user convenience... i.e., why aren't they
>all made that way?

"Varifocal" design can also contribute to making a variable
FL lens cheaper/lighter/smaller - and many zoom lenses are
actually slightly varifocal...
Hope This Helps