Essential Filters: Warming Filters
Number three out of three on my personal hit parade of essential filters are the warming filters. As the name suggests, these useful items work to remove the bluish cast that can result from cloudy, overcast lighting or shade. Snow has an exceptional ability to pick up this blue cast so it's a good idea to keep a warming filter handy during winter.
Technically, a warming filter lowers the color temperature of your subject. Yes, I said lowers. Color temperature can be a bit confusing since it seems rather backwards, but the orange/amber range is actually cool, and the blue range is warm. The term "color temperature" is inherited from physics where it refers to a measurement of the radiated effect of an object as it is heated. Think of a gas barbecue grill or camp stove: the temperature is the hottest when the color is blue, not orange.
Warming filters come in various strengths and even slightly different colors from different manufacturers. The standard ones are part of the 81-series, a designation from the Kodak Wratten system. A straight 81 has the least effect, followed by an 81A, 81A, 81C, and 81EF; a few companies also make an 81D. There is also an 85-series that are much stronger and not as generally useful. Tiffen also makes an 812 filter that is similar, but has a slightly more coral tone as opposed to the amber/straw color of standard 81 filters. Nikon's entry to the warming filter market is the A2 which is slightly less straw colored than the standard 81B and represents my favorite warming option.
As mentioned last week, there are also warming polarizers that combine a standard warming filter with a polarizer. This can be convenient, but my advice would be to get the essential single-effect ones first before you go out and purchase one of the combo-effect ones.
In contrast to both Graduated Neutral Density filters and polarizers (and the above explanation on color temperature for that matter), the use of a warming filter is not complicated. Simply attach it to the front of your lens and shoot away. Beware of overdoing it though if you are using one of the modern saturated slide films.
A mild warming filter can also work well for portraits as it can give people a "healthy" complexion.
To some extent, the effect of a warming filter can be replicated in Photoshop afterwards, but I'd rather get it right in-camera.