On Fri, 14 Aug 1998 12:43:20 GMT, johnchap@erols.com wrote:

>Fast glass is heavier and more costly. What you are buying is the
>option to shoot wide open at faster shutter speeds when and if you need
>to. Normally, to increase DOF and the fact that optimum lens
>performance is about 2 stops down, one does not shoot wide open.
>Unfortunately, on the slower less expensive lenses, 2 stops down becomes
>prohibitively slower.

Hate to be a nudge, but...
The "two stops down..." rule of thumb just doesn't hold water...
(most good lenses peak around f8 in the center, regardless
of speed - and the corners often don't optimize until around
f11-16, especially with super-wides, and very especially with
WA zooms). I would use any (including Nikon's f2.8) super-wide
zoom at smaller than usual stops (which is quite practical, since
wides can be used at slower than normal shutter speeds).

>Additionally, in certain cases like the Tokina 20-35 lenses being discussed,
>other factors come into play. Besides being simply faster, the new 20-35/2.8
>lens is built to professional standards both in ruggedness and optically, as
>direct competition to, say, the Nikon 20-35/2.8. It then becomes a personal
>choice whether these fast high quality lenses are worth the extra cost and
>weight. This choice is not unlike the choice makes when one is choosing, for
>example, between the excellent Tokina 17mm and the Nikon 18mm lenses. Is the
>Nikkor worth 2-3 times the cost of the Tokina?

Um, if you want sharp pictures, yes! ;-) I tried the much-touted
Tokina 17mm f3.5 ATX (in a sample that its former owner really
liked...), and was surprised how bad it was (decent in the center
stopped well down, but never really good in the corners at any stop).
The 18mm f3.5 Nikkor, though, was quite good in the center and
decent in the corners wide open, and excellent to the corners by
f5.6 or so - a performabnce level improvement that is well worth
the difference in price for some. (And, then, there's that resale
value...;-)