When most filters are placed in front of a lens, or in front of an artificial light source, less light reaches the film or CCD sensor. This loss of light must be calculated to prevent erroneous underexposed readings, especially with transparency film or digital capture.

Many filters require an increase in exposure to compensate for the loss of light. Even diffusion filters which at first glance seem to cause no light loss at all, may still block about 2/10 stop worth of light, a significant amount with digital. To determine the amount of light loss caused by a particular filter, take a reading on a uniform, evenly-lit surface without the filter, then place the filter over the light sensor and take another reading to learn the difference. If the first reading is stored in the meter's memory, you may use the "Brightness Difference Function" and have the meter calculate the light loss for you.