Filter factor

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

Converting between filter factors and stops

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) 1/3
1.4 70% 1/2
1.5 67% (2/3) 2/3
2 50% (1/2) 1
2.5 40% (2/5) 11⁄3
3 33% (1/3) 12⁄3
4 25% (1/4) 2
8 12.5% (1/8) 3
16 6.25% (1/16) 4

Calculating exposure increase

The number of f-stops required to correct the exposure with a given filter may also be calculated using the formula:

       Filter factor = 2X

where the exponent "X" is the number of f-stop increases required.

An example: A deep red filter with a filter factor of 8

       8 = 23

The normal exposure will be increased by three stops with this filter. 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

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 http://www.hoyafilter.com/download/HOYA-Filter-Catalog-2015.pdf page 49 2.5
Orange 21 3
Red 25 A, 090, R2 8
Deep Red 29 F, 091 20
Green 58 X1 http://www.hoyafilter.com/download/HOYA-Filter-Catalog-2015.pdf page 49 4
Infra-Red 89B R72 16
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

References

Notes
    Further reading
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