Mired Shift Filter Calculator

Mired Shift

Mired (micro-reciprocal-degree) is the unit photographers use to describe color-correction filter strength. Unlike kelvin, mired values are linear: adding the mired value of a filter to the source’s mired gives the resulting mired exactly.

Formula

mired = 1,000,000 / temperature(K)

mired_shift = mired_target − mired_source

If mired_shift is positive, you need a warming (yellow / orange / amber) filter. If negative, you need a cooling (blue) filter.

Worked Example: Tungsten Film in Daylight

The source is the light you are standing in. The target is the temperature your film or white balance expects.

• Source, daylight: 5500 K → 181.8 mired
• Target, tungsten-balanced film: 3200 K → 312.5 mired
• shift = 312.5 − 181.8 = +130.7 mired

Positive, so you need a warming filter: the Wratten 85B, which is amber and is exactly what is sold for shooting tungsten film outdoors. Without it the frame comes back blue, because daylight carries far more blue than the film was built for.

Run it the other way and you get the mirror image. Daylight film under 3200 K tungsten lamps is 181.8 − 312.5 = −130.7 mired, a cooling filter, and that is the 80A blue.

Getting these two the wrong way round is the classic mistake, and the memory hook is simple: the filter is the colour that is missing. Tungsten light has no blue in it, so you add blue.

Common Filters and Their Mired Shifts

CTO and CTB in Video / Cinema Lighting

Cinematographers describe gels by Color Temperature Orange (CTO) and Color Temperature Blue (CTB):

• 1/8 CTO ≈ +20 mired
• 1/4 CTO ≈ +42 mired
• 1/2 CTO ≈ +81 mired
• Full CTO ≈ +167 mired
• 1/4 CTB ≈ −30 mired
• 1/2 CTB ≈ −68 mired
• Full CTB ≈ −137 mired

These cinema gels are designed in mired increments precisely because mired add linearly while kelvin do not.

Why Not Just Kelvin?

Take a filter that moves a 3000 K source to 4000 K. In mired that is 333 → 250, a shift of −83.

Now put the same piece of glass in front of a 5000 K source. 5000 K is 200 mired, so 200 − 83 = 117 mired, which is 8547 K.

The same filter moved the first source by 1000 K and the second by 3547 K. Its strength never changed; the kelvin scale is just compressed at the top and stretched at the bottom. That is why every filter and gel on the market is specified in mired, and why you cannot add or subtract kelvin the way instinct suggests.

Caveats

Mired only handles the color temperature axis and does not touch green/magenta tints from fluorescent and LED sources. For those, you also need plus-green or minus-green correction (CC filters or Rosco / Lee gel equivalents). A spot-on mired calculation still leaves a residual green tint that must be filtered separately.