The HOYA color correction filters, BLUE COOLING and UMBER WARMING, are designed to adjust the white balance of the image.
The BLUE COOLING filters raise the overall color temperature value and are used to correct reddish or orange hues when shooting outdoors, or indoors under tungsten or fluorescent lighting.
The HOYA color correction filters were designed for shooting with film cameras but also find use in digital photography.
Technology
The BLUE COOLING series features C2, C4, C8 and C12 gradation, which refer respectively to a mired shift of -20, -40, -80 and -120.
In other words, as seen in the transmission graph, the BLUE COOLING series reduces orange and reddish light entering the lens, therefore indirectly enhancing blue colors. The lower the mired value, the less orange and red light enters the lens and the stronger the cooling effect.
It is possible to combine various BLUE COOLING 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 light condition is 4800K, with a mired value of 1,000,000 divided by 4800 = 208.3. Your desired color temperature is 5500K, where the mired value is 1,000,000 divided by 5500 = 182.
The difference is approx. 26 mired value. This means, if we want to correct the color to the film setting, we need to reduce the mired value by -26.
When checking the table below, you will find that the C2 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 |
|---|---|---|---|---|
| -20 | 82A (218) | C2 | 1.2 | 1/4 |
| -30 | 82B (203) | 1.4 | 1/2 | |
| -40 | C4 | 1.4 | 1/2 | |
| -50 | 82C | 1.5 | 3/5 | |
| -60 | ( C4 + C2 ) | 1.9 | 1 | |
| -80 | -202 | C8 | 2.0 | 1 |
| -90 | 80C | 1.9 | 1 | |
| -100 | ( C8 + C2 ) | 2.4 | 1 1/4 | |
| -110 | 80B (281) | 2.0 | 1 | |
| -120 | C12 | 3.0 | 1 2/3 | |
| -130 | 80A | 2.4 | 1 1/4 | |
| -140 | -201 | ( C12 + C2 ) | 3.6 | 1 4/5 |
| -160 | ( C12 + C4 ) | 4.8 | 2 1/4 |
