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Changes in colour temperature are much more noticeable at low values than at high values. For example, a change of 500K from 8,000K to 8,500K might produce a difference in colour so small that it would not be noticed. However, the same change of 500K from 3,000K to 3,500K would bring about a significant colour shift. This problem is solved by the micro-reciprocal degrees scale (mired), which takes such variations in to account.

Mired values are calculated by multiplying the inverse of the colour temperature by one million, and are used as the unit of colour temperature measurement for colour temperature compensation filters. For example, a colour temperature of 5,000K is represented by a value of 1 / 5,000 x 106 = 200. The mired correction, which is typically used for selecting appropriate colour temperature compensation filters, is then calculated by subtracting the mired value of the light source from the mired value of the white balance setting. If a photographer is working in a room illuminated by tungsten lighting with a colour temperature of 2,900K (mired value 345) and the camera is set to a white balance value of 5,000K (mired value 200), the difference is 200 - 345 = -145 mired. The appropriate correction can therefore be achieved using a cool (blue) filter with a mired value of 145. Had the correction value been positive, the filter would have been warm (amber).

The alternative approach, with a modern DSLR, is of course to set the white balance correctly before beginning work.


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