Unique Original Articles » Describe filter factor

Plenty of photographers use filters of various kinds on their camera lenses. Filters can correct the color of a scene, create special effects within a scene and correct improperly balanced light in a scene. Because certain filters reduce the amount of light reaching the camera's film or digital sensor, manufacturers assign these filters a filter factor. The filter factor assists the photographer in determining the proper exposure setting.

What a Filter Does
Filters do a wide variety of tasks. Color correcting filters such as 80B and FL-D (florescent daylight) help correct the color balance of a scene. For example, photographs shot under tungsten light have an excessive amount of orange and yellow, but mounting an 80B filter, which is blue in color, on the lens helps correct the excessive orange and yellow. Other filters such as polarizing filters help reduce unwanted reflections on non-metallic surfaces and at the same time increase color and saturation in a scene. Most filters that are not completely opaque in appearance have a filter factor.

Factor in the Factor
When a photographer uses a filter that is not totally clear, he must take into account the filter factor of the filter in use. The filter factor tells the photographer she must increase the exposure of the photograph by a factor of “x” in order to still get a correctly exposed image. In practical use this is how the factor works. You're photographing a scene and the meter reading calls for an exposure of 1/125th of a second at f/5.6 using an ISO setting of 100. You want to put a polarizing filter on that has a filter factor of “2”. The filter factor tells you that you need to give the photo twice as much exposure (an increase of one stop) as you would without using the filter. Knowing this, you can do one of three things--lower the shutter speed to 1/60th of a second, increase the aperture to f/4.0 (increasing the aperture means making the size of the opening larger which decreases the aperture number) or decrease the ISO setting to 50.

Using Multiple Filters
If you have to use two filters to create the scene and each filter has a factor of “2”, you must increase the exposure according to each filter's factor. Using the same example, the photographer now has to increase the exposure by four times the original unfiltered light reading. This increase equates to two additional stops of exposure. The photographer can either lower the shutter speed from 1/125th to 1/30th of a second, increase the aperture from f/5.6 to f/2.8 or lower the ISO from ISO 100 to ISO 25.
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