Originally posted by RioRico Ah, UV light and film. The basic photo-sensitive emulsions created during the evolution of photography are ONLY sensitive to actinic (UV-violet-blue) light. Dyes must be added to emulsions for the film or plate to 'see' other colors. With B&W, and color transparency and print films, this sensitivity to UV remains. Hence UV filters, especially outdoors. We don't want to tickle those silver nitrate particles into forming a latent image of background noise, now do we?
Your assumption "only this excess UV light will be reduced" is incorrect; assuming the filter meets specs, ALL light/EMF below 420nm (or whatever the precise cutoff point is) will be cut. Look at the other end of the spectrum -- a 920nm or 1000nm UV filter removes ALL visible light, no matter its luminosity. I use such filters on a Sony NightShot P&S; scanning a viewscape of varying luminosity with NightShot OFF, the said landscape just doesn't show on the LCD screen.
Digital sensors put filters in front of each pixel, and the blue and green filters effectively blocks UV, so such sensors don't need added UV filtering. The red filters pass UV, which is why most digicams have an IR-blocking "hot filter" in front of the sensor. Digital and emulsions are overly-sensitive at opposite ends of the visible spectrum.
[Damn, I'm falling asleep again. Hope I didn't write anything stupid here.]
You did not write anything stupid. But anyway, nearly every photographic lens already fullfills the purpose of a UV filter, as standard glass + coatings have a low transmission in the UV range. The "optical window" of standard glass only starts around 350nm, which is also the cutt-off range of those UV filters that actually work! (nice test here:
Filters - UV or not UV? - photo.net)
You need special "super-achromatic" or Apochromatic lenses with extended transmission to really make use of the UV part of the light.
Also, in traditional BW emulsions (silver halide), the silver halide obviously will absorb UV photons, but it will not form an image! This is simply due to the fact, that a most lenses will not focus UV light. What you get is an unfocused "UV noise" overlaying the whole image, which can be detected as unsharpness and loss of contrast. But as I wrote above: as lenses already absorb most UV, this "UV noise" is completelöy negligible, except for severe UV conditions.
The only point in your post I questioned was the "overexposure" thing. There is no overexposure caused by UV light. I have never used UV filters on my lenses in film days, except at high altitude in the snow or as a lens protection at thwe sea shore, my images never turned out overexpsoed by UV radiation - and I have never heared or read any indication for that being the case. And I guess, what I wrote above, pretty much explains, why there can't be UV-caused overexposure.
Ben