In photography, filter factor refers to the multiplicative amount of light a filter blocks.

Converting between filter factors and stops

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The table below illustrates the relationship between filter factor, the amount of light that is allowed through the filter, and the number of stops this corresponds to.

Filter factor Proportion of light transmitted (1/FF) Number of stops
1 95–100% 0
1.3 75% (3/4) 13
1.4 70% 12
1.5 67% (2/3) 23
2 50% (1/2) 1
2.5 40% (2/5) 113
3 33% (1/3) 123
4 25% (1/4) 2
8 12.5% (1/8) 3
16 6.25% (1/16) 4

Calculating exposure increase

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The number of f-stops of light reduction, given a filter factor, may be calculated using the formula:

 

Most calculators do not have a   function. An equivalent calculation is:

  

or

 

An example: A green filter with a filter factor of 4

 
 
 

The green filter factor of 4 yields a 2 f-stop light reduction.


The filter factor, given the exposure change in f-stops, may be calculated using the formula:

 

An example: A deep red filter with an f-stop change of 3 stops

 

A change of 3 f-stops is equivalent to a filter factor of 8.


As a consequence of this relationship, filter factors should be multiplied together when such filters are stacked, as opposed to stop adjustments, which should be added together.

Filter factors for common filters

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The table below gives approximate filter factors for a variety of common photographic filters. It is important to note that filter factors are highly dependent on the spectral response curve of the film being used. Thus, filter factors provided by the film manufacturer should be preferred over the ones documented below. Furthermore, note well that these factors are for daylight color temperature (5600K); when shooting under a different color temperature of ambient light, these values will most likely be incorrect.

Filter purpose Wratten number Other designation Filter factor
Ultraviolet UV(0) 1
Skylight 1A or 1B KR1.5 1
Contrast filters
Yellow 8 Y, K2, Y48, 022 2
Yellow-green 11 X0 2.5
Orange 21 3
Red 25 A, 090, R2 8
Deep Red 29 F, 091 20
Green 58 X1 4
Infra-Red 89B R72 16
Infra-Red 70 092 20-40
Colour conversion (blue)
3200 K to 5500 K 80A KB15 4
3400 K to 5500 K 80B KB12 3
3800 K to 5500 K 80C KB6 2
Warming filters
3400 K to 3200 K 81A A2 1.4
3500 K to 3200 K 81B 1.4
3600 K to 3200 K 81C KR3 1.4
3700 K to 3200 K 81D 1.5
3850 K to 3200 K 81EF KR6 1.5
Cooling filters
3000 K to 3200 K 82A B2, KB1.5 1.3
2900 K to 3200 K 82B 1.5
2800 K to 3200 K 82C KB3 1.5
Colour conversion (orange)
5500 K to 3400 K 85 / 85A KR12 1.6
5500 K to 3200 K 85B KR15 1.6
5500 K to 3800 K 85C 1.3
Neutral density
0.3 96 NDx2 2
0.6 96 NDx4 4
0.9 96 NDx8 8
3.0 96 NDx1000 1000
Polarizers
Linear polarizer PL or LP 3 to 4
Circular polarizer PL-Cir, C-PL, CP 3 to 4

See also

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References

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Notes
Further reading
  • Hoya Corporation, Filters for imaging
  • Cokin S.A., Cokin Creative Filter System