In article <5cal88$qk@mozo.cc.purdue.edu>, tujf@ecn.purdue.edu@ says...

>I have a dummy question. We talk about the optical quality of the lens.
>I, for example, try to get high quality lenses and try to protect them with
>UV filters. However, it seems to me that a filter is still a piece of optical
>object placed in between the object and the film. Do we have to worry about >the optical quality of the filters? Are they all created equal?
>Right now I have Cannon, Tiffen, and Toyo filters attached to three of my
>Nikon lenses. Is it worth the dollor to buy the L37C filter from Nikon?

I suspect it is far easier to make a piece of optical glass flat, than to
make it curved on both sides just right, and then to combine it with a lot
of other precisely-curved pieces of optical glass... I have seen bad filters, but they are relatively rare - I would guess that all of yours are fine in
terms of sharpness. I avoid mixing filter brands as much as possible, since they can be slightly different in color... Some filter brands have thick
rims which can slightly vignette wide-angle lenses. BTW, the least-good
filters may be fine on wide-angles - save the most-likely-to-be-best for
your longer, faster lenses (which will show the greatest ill effect from non-flat filters). Good brands are: Hoya, B & W, Heliopan, Nikkor (and
others, like Toyo...). I am not fond of Tiffen, but they are fine
for most uses. Hoya filters (metal-rim version, multi-coated or not)
are good enough for most critical uses.
Hope This Helps