In photography, filter factor refers to the multiplicative amount of light a filter blocks.
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Converting between filter factors and stops [link]
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 |
| 16 | 6.25% (1/16) | 4 |
Calculating exposure increase [link]
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 [link]
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 | Y, K2, Y48, 022 | 2 |
| Yellow-green | 11 | X0 https://www.hoyafilter.com/products/hoya/cf-09.html | 4 |
| Orange | 21 | G | 3 |
| Light Red | 25 | 25A, 090, R2 | 5 |
| Red | 29 | 091 | 8 |
| Green | 58 | X1 https://www.hoyafilter.com/products/hoya/cf-09.html | 6 |
| 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 [link]
References [link]
- Notes
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This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009) |
- Further reading
- Hoya Corporation, Filters for imaging
- Cokin S.A., Cokin Creative Filter System
