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As I was saying, they can cause really nasty results if not used properly. When I was beginning in photography, my boss asked me to photograph a coat of arms that had a lot of shiny gold-paint. She told me to polarize the light and use a polariod filter. It worked far too well, and the gold came out looking like brown excrement! Reshoot. I did once test the effect of polarized lights and a polaroid filter on a face. The result was super-saturated colour - extremely unnatural and unattractive. Might be of some use in a science fiction movie, of people after the blast with radiation burned faces.
Removing all the reflections from glass or water may allow you to see clearly what is behind the surface, but it also can look very unnatural.
If you are shooting chromes under well controlled conditions, filters like those mentioned can allow one to shoot a Type B chrome film balanced for 3200K lights under 3400K lights and the like. If shooting under unknown light conditions, these along with a colour meter can give highly accurate slides. There are other filters like the 85B that allow one to shot tungsten-type film in daylight with a reasonable degree of accuracy. They are called "colour conversion" filters and are made for specific types of slide film, while the others are called "colour compensating" filters since they adjust for small variations in colour temperatures.
As I indicated above, they are of use only when shooting slides. They might be of use depending upon your workflow, with colour negative film, but only if you are doing your own printing. They would cause a royal screw-up with prints sent to a one-hour lab.
I really have not met anyone who used them with negatives, but expect that there may be some. They would be primarily of use when shooting under artificial light, they extract a penalty by cutting light transmission by at least 1.0EV. That is the last thing you need when shooting under low light conditions. They would require less filtration in the enlarger, but it would be a meaningless gain.
They are of absolutely no value whatever in digital photography, since digital cameras have all the filters built in, plus automatic and manual white balance capability for the most part. Opening a Nikon RAW camera file in Photoshop CS2 gives you precise colour balancing capability way beyond anything a photographer with a full set of filters and a very expensive colourmeter ever dreamed of. I not only can colour balance the overall cast of the image, but fine tune shadow, mid-tones and highlights in Lab Colour mode. Using RAW as my format in mixed lighting situations, I can specifically colour balance individual areas of the image to compensate for the light that is lighting that particular local.
Back when filters were generally necessary, Kodak published an excellent book on it. At one time, they were the absolutely best source of both filters and information about them. If you live near a used book store, you might happen across a photography textbook from the middle of the 20th century, that would also have information about them. There is less and less, since colour negatives greatly reduced the need for them and digital removed the need for all except the polarizer.
I did google for information and found a site with a lot of what you are probably looking for.
As per protection, if you are photographing sand-blasting or something equally abrasive, a coated flat piece of high-quality optical glass is really the only choice. Camera stores buy UV filters of much lesser quality for extremely low prices and then try to intimidate naive buyers into buying one for each of their expensive lenses. This has been going on for as long as I can remember. Of course the profit percentage is immense, allowing the salesguy to seemingly cut prices beyond the competition, making it up on the filter. It simply does not make sense to pay $500 for a lens then dull it down to the quality of a $0.50 filter.
Yes you could get hit by a flying object that would strike hard enough to break the filter but not the lens. I suppose it would give a feeling of some relief as the medical technicians are trying to remove camera parts from your skull on the way to the emergency room. Your skull will crush before your lens does.
Contemporary lenses are extra-ordinarily robust. When multi-coating first came into being, decades back, the Nikon tech-rep did a demonstration for us. Back then people smoked in public, including the tech-rep. When he finished the cigarette, he butted it out on the front element of an extremely expensive lens! He then reached in his back pocket and hauled out an old piece of cloth, spit on the lens and cleaned the burn residue off.
I did carry optical flats, but almost never used them. I worked as a photojournalist and industrial photographer for a couple of decades - daily work - and never lost a lens except at the point of a snub-nosed .38 revolver. Insurance paid for the lens - and the camera. A filter would not have made any difference at that point, but a flack jacket in my colour would have been nice.
larry!
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