Mired in photography is calculation, which is easier and practical method of finding correct color correction filter in photography. The term is derived from words - micro reciprocal degree.
This micro reciprocal degree is calculated by 1000000 / color temperature of light.
The color temperature of photo-flood light is 3400 degree K. So value number is:
1000000 / 3400 = 294
The color temperature of day light flash is 5500 degree K. So value number is:
1000000 / 5500 = 181
Now if we want to find the shift value, then it is:
294 - 181 = 113
In this case, we require +113 shift.
Filters are assigned with shift value number. In our example, if we use a filter with shift value +112, (Filter No. 85) then with daylight film, which is balanced for 5500 degree K, we will get
181 + 112 = 293
(which is closest value of 294)
This is how we can find the required color correction filter. The calculation has another use. We can combine more than one filter of different value, in such a way, that we get the closest required value number.
So in our example, we can also use combination of filter No. 85C, which has shift value +81 and filter No. 81C, which has shift value +35
Now when we combine these two filters, we get:
81 + 35 = 116 (closest value of 113)
This is how, mired in photography is useful for finding correct color correction filter. Simple arithmetical calculation is applicable with this value number, as it gives constant rating at any section of the scale of color temperature. Here is the explanation on how it works.
Color temperature of photo-flood light is 3400 degree K. Flash light is 5500 degree K. Now here, the difference is 2100 degree K, which will play a big role in color shift. However, when we come to open shade with overcast daylight, the color temperature can be between 10000 to 12000 degree K, and though there is difference of 2000 degree, the result remains almost same.
If these figures are converted to value number, then for 3400 and 5500, we have 113 shift value. In case of 10000 (value number 100) and 12000 (value number 83) the shift is only 17 shift value number. This is how mired calculation gives considerable figure of shift, which may be within tolerance for any color shift in a photograph. The choice is purely subjective and slight color correction can easily be done during post production and retouching of photograph in any photo editing software.
Most of modern digital cameras have fine tuning option for color correction and color cast. That is the reason why color correction filters are slowly going out dated in digital photography.
Read more about color temperature
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