Filters function by absorbing a portion of the light reflected from the subject to the camera. To compensate for this absorption and the loss of light, you may have to increase the exposure to compensate for the light absorbed by the filter. A numerical value is assigned called a filter factor or multiplying factor. This numerical factor is based on several variables that include the color sensitivity of the film, density of the filter, color of the filter, and color temperature of the light source. As these variables change, the filter factor also changes to produce the correct exposure consistently. Filters are often identified as 2 X yellow or 4 X orange. That implies that the filter factor is 2 and 4, respectively. Remember, the filter factor does not always remain constant when conditions change.

For example, a blue filter used with panchromatic film exposed with daylight requires a smaller filter factor than when the same film and filter are used with tungsten light. The reason for this is daylight has a higher content of blue light that is readily transmitted by the blue filter. Thus, with the same film and filter combination and with the same camera shutter speed and f/stop, more exposing light is available at the film plane with daylight as compared to tungsten light.

A filter that absorbs a great amount of illumination from a given light source is assigned a larger filter factor. A filter that absorbs a lower amount of illumination from the same light source is assigned a smaller filter factor.

To obtain the necessary light at the film plane for correct exposure with a filter, you must increase the original calculated exposure (without a filter). This increase in exposure is determined with a filter factor. When a filter has a factor greater than 1, an adjustment to the exposure must be made.

There are three general methods of using filter factors to determine the exposure increase required:

1. Divide the ISO speed by the filter factor, and use the product as the effective film speed.

Example: If the filter factor is 2 and the IS0 speed of the film is 100, the effective film speed is 50 (100 + 2 = 50).

Thus setting a film speed of 50 on your light meter produces the equivalent of 1 f/stop of additional exposure.

2. Determine the required exposure without the use of a filter; then multiply the unfiltered shutter speed by the filter factor.

Table 3-6. Filter Factor Equivalent Exposure Table

Example: The unfiltered exposure calls for l/60 second, and the filter factor is 3. The correct exposure is 1/20

second (1/60 x 3 = 1/20 second). However, most cameras do not have a 1/20 second shutter speed; therefore, use 1/15 second or the next slowest shutter speed.

3. When you are using an SLR camera with through the lens (TTL) metering, put the filter on the camera lens and adjust the exposure in the normal manner. However, certain dark blue, red, and orange filters may give faulty readings if used with TTL metering systems because the meter reads 18 percent gray. The camera light meter may not be sensitive to the color of light passed by the filter.

4. Consult a filter factor equivalent exposure table. (See table 3-6.)