Pesky UV and IR
It took a while but I've gradually managed to put a UV or haze (1a) filter on every lens I have. It makes sense to simply leave them in place instead of constantly swapping filters back and forth and scrounging around for step-up and step-down rings. They certainly do no harm (only really bad, off, off brands ever detract from the lens's sharpness), and generally do some good.
First, there's simple protection. When you're shooting in dusty and windy conditions or by the shore where that fine salt haze gets on everything, it's nice to know your expensive glass is not exposed to the elements. Then there's the occasional inquisitive kid who (gasp!) just has to touch that big shiny piece of glass. Even worse, there was the time when my camera plus zoom lens (bigger gasp!) took a dive out of a moving car. It landed on the lens and when the smashed UV filter was removed the rest of the lens was unscathed (at least externally).
The main job of the UV-haze filters (like Uvo, Uvb, 1a, 1b, etc.) is to get a handle on the fact that film sees far into the ultraviolet --- where no mere eyes can follow. They really can't do much about the visible haze you see in distant scenes, they only cut out the extra haze you would be getting without them. Ultraviolet light registers as blue on color film, of course, and I generally find most films too bluish, especially near mid-day, if a filter is not used. This is my main reason for standardizing on their use. The difference between haze and UV filters is mostly that haze filters have a very slight touch of red while the heavier UV filters will show a trace of yellow. You can see the differences better by laying them out on a white sheet of paper
Both ultraviolet and infrared can be the cause of puzzling errors in color rendition. For years I wondered why I couldn't get a good kodachrome of clematis flowers. The deep blue usually came out as a dirty pink. At first I thought it was overexposed, but even exposure brackets failed to give good images. Then one day I shot an ektachrome and it looked fine. It turns out that kodachrome has a bit of sensitivity in the near infrared and the clematis bloom is a bright reflector of it. It was the red response that created a weak pink when it was added to the visible blue. This is rather unusual since IR sensitivity is hard to attain --- only a few B&W films have it and the only color film that sees it (Ektachrome Infrared) deliberately renders everything in "false color", where colors our eye can see are used to represent a world we'll never see. Just like UV and 1a filters, there is an IR filter fix for the clematis bloom problem, but the IR filters are extremely expensive and rarely needed.
Color errors from UV typically involve fluorescence. Here's a typical case; you use a flash, a strong emitter of ultraviolet, at a dinner party where there are white tablecloths. In the photo they may look bluish especially toward the back where the color isn't burned out (you also see this occasionally in white shirts}. The fluorescence comes from the whitener many manufacturers put in laundry detergents. Obviously the color of pale pastels could be seriously weakened by fluorescence. For a cheap and easy fix just tape a few pennies worth of a UV filtering gel or acetate such as Rosco #3114 on your flash unit. The UV filter on your camera won't help this time because the fluorescence has already happened before the light hits the lens.
When shooting a studio setup with tungsten lighting (hot lights) on type EPY, tungsten balanced film (usually with a large 4x5 camera) I often test the color balance in advance on the batch of film being used. Even with all that care I occasionally got a crazy shift of color into the blue -- either on the tests or the job. I was puzzled until I noticed that I had left UV filters on some, but not all lenses involved, thinking that there was not enough UV in the light to matter. WRO
again. It turns out that the heightened blue sensitivity of the tungsten film made it wildly hypersensitive to the trace of UV in the bulbs. Using the UV filter on all lenses solved that problem.
There has been a lot of talk lately about UV from quartz-halogen lights, even warnings about possible sunburn. This seems strange since tungsten lights are normally a poor source of UV. On the other hand quartz glass freely transmits all ultraviolet, unlike normal glass which cuts off at 310 nm (the visible spectrum of light goes from 400 nm, the blue end, to 700 nm the red), so what does get through is a cause of concern.
Fluorescent lights put out lots of UV. Since ultraviolet is the arch villain in the fading of dyes and colors it's not a surprise that they're banished from art museums and galleries. This is also why, when archival permanence is desired, it is mandatory to use UV absorbing glass or plastic in your pictures-for-display, rather than plain glass. It's usually called Museum Grade or Conservation Glass.
There's a good side to UV of course. Instead of banning and filtering out ultraviolet light, you can revel in it by creating whole sets of fluorescent objects that are lit by "black light" (UV of course). Lacking both shadows and specular highlights the colors are quite intense and luminous ----- but that's another story.
The positive aspect of infrared light is its legendary ability to penetrate haze. Both B&W and color can actually see beyond the point where the eye just records a flat gray haze. Also there are the weird, unearthly or nostalgic effects you get when foliage (which freely reflects infrared) turns white in the B&W shots, or into a world of surreal color on Infrared Ektachrome.
Are there times when not to use a filter? Well obviously if you need another filter like a polarizer or an orange for B&W it's both redundant and ill advised to leave your UV in place. Stacking filters may vignette (cut) the corners and reflections between the surfaces of anything less than multicoated filters can cause flare and "ghosts". I've also noticed that some specific lenses will show ghost images of lights when only one filter is used. This is why I usually take the haze filters off for night photos. Those streetlights really get bright in the long exposures.