Filter Factor
The exposure increase, in stops or as a multiplier, needed to compensate for the light a filter absorbs.
Formula / rule
Stops = log₂(factor)
Measured in
stops (EV)
In depth
A factor of 2× costs one stop, 4× costs two, 8× costs three. Through-the-lens metering handles most filters automatically, but very dense neutral density filters defeat both the meter and autofocus, so the working method is to meter and focus without the filter, then add the compensation manually. Polarisers cost between one and two stops depending on rotation.
Examples in practice
A ten-stop ND turns a 1/125 s exposure into eight seconds — the same 1024× factor printed on the filter ring.