Calculating Filter Factors

[Large Format Dave]

Is there a test you can do to find out what filter factor (exposure compensations) is required for a coloured filter which does not have this info printed on its rim?

I have a red filter (brand unknown) with no information on it and I have read that it could be two or three stops extra exposure required. Is there any way to find out exactly how much is needed?

[BradS]

There is really no way to know "exactly". Some people meter through the filter and, usually that is a pretty good estimate. The problem with this approach is that the meter's spectral sensitivity may not match that of the film. This is especially the case with yellow and green filters...however, metering through the red filter should give you a pretty good place to start with a red filter.

There is also the matter of your personal vision or preference. What kind of effect are you trying to achieve....The best approach is to bracket your exposures and keep good notes. Compare the prints to your notes.

(EDIT) PS: welcome!

[Jerry Bodine]

What BradS said. Some filter manufacturers publish factors that depend on the color of subject illumination. For example, Wratten says the #25 red - one of several different reds available - has a factor of 8 in daylight (~5500K) and a factor of 6 for tungsten light (or early sun and late sun), because the red filter will lighten objects illuminated by warmer light. It will darken shadows, because they are generally illuminated by blue (sky) light. A factor of 8 may even be too low for shadows that are deeply cool color. If you apply the daylight factor for a warmly lit subject you'll have added assurance of holding shadow detail. Published factors should be considered as a guide to put you in the ballpark, but you certainly can temper them based on existing conditions.

So my point is: when you're taking notes as Brads suggests, try to record the type of light on your subject as best you can.