Hi--

>One of my lens has a range of 28mm to 80mm (I don't know what you call
>that; 50mm?). The other lens has a range of 75mm to 300mm (what would you
>call this; 200mm?). (I would like to know the generic number you call these
>lenses and why.)

I'd call them "28 to 80mm, and 75 to 300mm zooms".

>Lens one zoomed to 28mm, the f-stop can be maxed to f3.5 and if zoomed to
>80mm the f-stop can only max to f5.6. The second lens at 75mm maxes at
>f4.0 and at 300mm f5.6.
>My question is in either case, at 28mm should I use f5.6 or f8 for maximum
>subject sharpness? And at 80mm, f8 and f11 are maximum sharpness results?
>Or, is f5.6 and f8 the best selection for this lens no matter if zoomed to
>28mm or 80mm?

With most lenses (at the center, anyway - with cheaper zooms and very wide
angle lenses, good edges and corners may need smaller apertures to peak),
best performance is at (true) f8 or so (marked f11 on most variable aperture zooms is actually f8). With your lenses, peak overall performance may happen nearer f11, or even f16, if not first-line lenses.

>In other words go two f-stops away from the maximum opening at the zoom set

No - an innacurate rule of thumb that should go away (though, in this case,
it appears, by chance, to apply [but would not to most faster lenses]).

>OR each lens has a fixed maximum sharpness f-stop regardless of the range
>of zoom.

Zooms vary in performance with aperture, distance, focal-length selected,
and design/manufacturing quality of the particular lens, so predictions other
than the general ones above, are not possible.

>So the first lens can max to f3.5 and the second lens can max to f4.0 IF
>they are zoomed all the way out. If they are zoomed all the way in they
>both max at f5.6.
>Rick

David Ruether