Filter factor
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In photography, filter factor refers to the multiplicative amount of light a filter blocks.
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[edit] 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 |
[edit] 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.
[edit] 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 | |
[edit] References
- Hoya Corporation, Filters for imaging
- Cokin S.A., Cokin Creative Filter System
[edit] See also
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