The Filter Factor

Make exposure compensations by slowing the shutter speed by the filter factor (filter factor times true shutter speed; or speed indication number divided by filter factor) or by opening the lens aperture by the f-stop increment. If you aren’t sure of the f-stop increment sequence, multiply the starting f-stop number by the f-number multiplier in this table (or refer to the f-stop document).

A filter factor of 2 (1 stop) applied to a base exposure of 1/250 second at f/4.5 yields:

1/125 second at f/4.5
1/250 second at f/6.3

(2 × 1/250 = 1/125; 250/2 = 125 – 1/125; 4.5 × 1.41 = 6.3).

Exposure compensations for combined filters can be assessed either by multiplying the filter factors together (for shutter speed adjustments) or by adding the f-stop increments (for aperture adjustments).

The f-stop increment for higher filter factor conditions can be evaluated by dividing the high filter factor value into two or more component parts and treating the parts as combined filters.

For example, a filter factor of 75 can be evaluated as combined filter factors of:
  • 3 and 25
  • 5 and 15
  • 10 and 7.5.

In all cases, the resulting f-stop increment is about 6.23 (1.58 + 4.64 = 6.22; 2.32 + 3.91 = 6.23; 3.32 + 2.91 = 6.23; trivial differences are due to rounding of the second decimal place).

Filter Factors: Neutral Density Filters

Filter Factors Formula

Click on the PDF icon to open the Filter Factor Table