It is mostly useful with slide film when shooting photographs in shade lit by open sky, using slide film. It will remove some of the bluish cast that pure skylight often transmits. With landscapes, it will also filter out some of the unseen ultra-violet that may give a bluish cast to distant haze. Since the filter covers the whole lens, the cast will be over the whole picture - which may well be what you want if the situation you are trying to correct is throughout the whole image area.
Colour compensating filters can however be used for effect - golden skin tones, warming a sunset with the 81 series, or using cooling filters to create an atmosphere of gloom on a cloudy day with the 82 series.
As with any filter and chromes, shoot both with and without. Often the uncorrected version may be preferable if you are wanting a natural look.
Realize too that you are adding two additional glass surfaces to the optical system. Filters such has the Hoya have the least impact upon sharpness with their multi-coating. Filters are specific solutions to specific photographic problems and are not universal improvers in spite of what some magazine writers might say.
With negative film, there is far more control in the darkroom for colour balancing - the skylight is a fairly subtle filter. If you are not doing your own printing, the lab will probably neutralize it. It all depends upon the taste of the person doing the printing.
With digital, it is of little use, since either a manual white balance or auto white balance will neutralize it. Of course the same is true of any colour compensating filter. In most digital cameras, these filters are in essence built in.
I would not think of using a skylight filter with anything other than slides and then rarely. I don't think I even own one.
Before using it on any sort of mission critical shoot, I would certainly test it with the film of choice under similar conditions to where it will be used. Realize that while a colour balancing filter such as the 85B has a precise colour definition, "skylight" is a somewhat generic term, and each company is pretty much free to interpret it however they feel. Since it does often reach into the UV region of the spectrum and different films react differently to those wave lengths, testing is essential.
larry!
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