On 27 Sep 1998 19:49:43 GMT, "John R" wrote:

>I bought a used Vivitar 24mm f2.8 (K mount) in mint condition cheap. I
>wanted a wide angle and figured the lens was probably okay to experement
>with, but nothing spectacular. I put a skylight filter and on vacation I
>went. After returning and processing the slides, I examined them under my
>microscope and found that the quality was not bad but nothing spectacular.
>I then went shooting without the filter and found the lens performing equal
>to my best lenses - very crisp.
>
>I wasn't aware that filters made that much difference in sharpness. Yes, I
>am aware that adding more flat glass surfaces in the light path does no
>good other than adjusting color and lens protection. Has anyone experienced
>this with their wide angles? What about telephotos.
>
>For now, I'm keeping the filter off the 24mm but, the filter stays on my
>400mm for fear of scratching the big front element.

If the filter was at least average quality (Hoya, Tiffen, Vivitar), and not
defective, you should be unable to detect ANY image sharpness
degradation anywhere in the frame resulting from using the filter
(particularly with short FL lenses...), even with careful testing.
I suspect a defective filter, and I suggest trying another one...
(metal-rimmed Hoya's are good...).