The HOYA color correction filters, BLUE COOLING and UMBER WARMING, are designed to adjust the white balance of the image.
The UMBER WARMING series corrects blue cast caused by cool lighting conditions, especially in shadowy areas, by adding a red/orange tint to the image. Typical conditions are overcast days or shady conditions. Since UMBER WARMING filters add a warmth to pale skin tones, they are especially ideal for portrait shots.
The HOYA color correction filters were designed for shooting with film cameras but also find use in digital photography.
Technology
The UMBER WARMING series features W2, W4, W10 and W12 gradation, which refer respectively to a mired shift of +20, +40, +100 and +120.
In other words, as seen in the transmission graph, the UMBER WARMING series reduces blueish and greenish light entering the lens, therefore indirectly enhancing yellow and red colors. The higher the mired value, the less blue and green light enters the lens and the stronger the warming effect.
It is possible to combine various UMBER WARMING filters to achieve different values, which allows photographers to achieve almost any desired color correction.
Mired
Contracted from the term micro reciprocal degree, the mired is a unit of measurement used to express color temperature. It is given by the formula: M = 1,000,000K/T, whereas M is the mired numerical value, T the color temperature and K denotes kelvins. Since the formula is a reciprocal, a high mired value means warmer light (= lower Kelvin value), while a low mired value means cooler light (= higher Kelvin value).
The mired system has the advantage over the Kelvin system in that the Kelvin scale is not linear. Therefore, working with mired values provide more accurate and controllable readings than Kelvin.
Example:
The actual cloudy setting is 6500K, so the mired value is 1,000,000 divided by 6500 = 153.8. Your desired color temperature is 5500K, where the mired value is 1,000,000 divided by 5500 = 182.
The difference is approx. -28 mired value. This means, if we want to correct the color to the film setting, we need to add +28 mired value.
When checking the table below, you will find that the W2 filter with +20 mired value is probably the most fitting filter to correct the color.
| Mired Shift | Filter Type | New Filter | Filter Factor | F-stops |
|---|---|---|---|---|
| 200 | ( W10 + W10 ) | 4.0 | 2 | |
| 140 | ( W10 + W4 ) | 2.8 | 1 1/2 | |
| 130 | 85B | 2.1 | 1 | |
| 120 | -285 | W12 | 2.0 | 1 |
| 110 | 85 (205) | 2.0 | 1 | |
| 100 | W10 | 2.0 | 1 | |
| 80 | 85C (442) | ( W4 + W4 ) | 1.8 | 4/5 |
| 60 | -206 | ( W4 + W2 ) | 1.7 | 3/4 |
| 50 | 81C | 1.5 | 3/5 | |
| 40 | -443 | W4 | 1.4 | 1/2 |
| 30 | 81B (223) | 1.4 | 1/2 | |
| 20 | -444 | W2 | 1.2 | 1/4 |
| 10 | 81A | 1.2 | 1/4 |
