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 factorProportion 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

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

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 purposeWratten numberOther designationFilter factor
UltravioletUV(0)1
Skylight1A or 1BKR1.51
Contrast filters
Yellow8Y, K2, Y48, 0222
Yellow-green11X02.5
Orange213
Red25A, 090, R28
Deep Red29F, 09120
Green58X14
Infra-Red89BR7216
Infra-Red7009220-40
Colour conversion (blue)
3200 K to 5500 K80AKB154
3400 K to 5500 K80BKB123
3800 K to 5500 K80CKB62
Warming filters
3400 K to 3200 K81AA21.4
3500 K to 3200 K81B1.4
3600 K to 3200 K81CKR31.4
3700 K to 3200 K81D1.5
3850 K to 3200 K81EFKR61.5
Cooling filters
3000 K to 3200 K82AB2, KB1.51.3
2900 K to 3200 K82B1.5
2800 K to 3200 K82CKB31.5
Colour conversion (orange)
5500 K to 3400 K85 / 85AKR121.6
5500 K to 3200 K85BKR151.6
5500 K to 3800 K85C1.3
Neutral density
0.396NDx22
0.696NDx44
0.996NDx88
3.096NDx10001000
Polarizers
Linear polarizerPL or LP3 to 4
Circular polarizerPL-Cir, C-PL, CP3 to 4

See also

References

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