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# 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 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) 1 1/3 3 33% (1/3) 1 2/3 4 25% (1/4) 2 8 12.5% (1/8) 3

## Calculating Exposure Increase by Formula

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.

## Filter factors for common filters

The table below gives approximate filter factors for a variety of common photographic filters.

Filter purpose Wratten number Other designation Filter factor
Ultraviolet UV(0) 1
Skylight 1A or 1B KR1.5 1
Contrast filters
Yellow 8 K2 2
Yellow-green 11 X0 4
Orange 21 G 3
Red 25 25A 8
Green 58 X1 6
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 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 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 1.5
5500 K to 3200 K 85B KR15 1.5
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
Polarizers
Linear polariser PL or LP 3 to 4
Circular polariser PL-Cir or CP 3 to 4

## References

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