In article <4judc3$r21@nlcnews.nlc.state.ne.us>, steinke@nrcdec.nrc.state.ne.us says...

>I don't fully understand the purpose of using filters on lenses. What
>makes a good filter good, what is a good all around filter and how do
>they make a photo better?

For lens protection, a UV is generally used, since there are few
"just plain glass" filters available for the purpose, and in practice,
since most lenses absorb most UV anyway, it is a nearly colorless
cover for the lens. A "good" one is one that introduces virtually no problems - which is to say, it is optically as non-existent as possible (flat, colorless, without scratches, with multicoating [perhaps...], and with a thin metal rim that will not vignette on wide-angles, deform easily, or strip threads easily. Other than for protection purposes, filters have colors, light polarizers, or other effects built into them (for soft-focus, star effects, etc.). My
feeling about filters is that if you can tell what filter was used,
its use was probably unwise, since it has added an unnatural look to
the photo (but, then, I always was old-fashioned....;-) - so I go
along with your first statement, much of the time ;-). When using
color slide film, various color correction filters are useful for
restoring (or nearly restoring) a daylight-normal color balance
to the images when shooting under tungsten, flourescent, sodium-vapor,
etc. light, which would be rendered very off-color without the filter.
In B & W, when there is some blue in the sky, a dark yellow filter can produce a slightly darker sky tone in the print, with orange and red filters providing greater sky tone darkening. Polarizers, and most colored filters have a "filter factor", and require additional
exposure when used (and camera TTL exposure meters do not generally
compensate well for the color filters, contrary to what most camera instruction books say - but that is another story.....).
Hope This Helps