The color temperature within
photography is the apparent color of the
light and measure the red/blue balance. This does not mean that the actual temperature of the source of the light is that hot. The
sun is the source of light for
sunrise,
dusk,
daylight,
overcast and
hazy sky and yet has a range from 3,100 K to 10,000 K.
Often color temperature is measured as "MIRED Value" which stands for
MIcroREciprocal Degree. This can be calculated as
1,000,000 / Color Temperature
source Color Temp (K) MIRED Value
Candlelight 1,930 518
75 Watt 2,800 357
100 Watt 2,900 345
200 Watt 3,000 333
Sunrise/set 3,100 323
Tungsten 3,200 312
Photolamp 3,400 294
Dawn/dusk 3,600 278
predawn/dusk 4,000 250
"Daylight" 4,500 222
fluorescent
Carbon arc 5,200 192
Daylight 5,500 182
Camera flash 6,000 167
Overcast sky 7,000 143
Hazy sky 10,000 100
sunless blue 11,000 91
sky
clear sky in 20,000 50
mountains
Most film is balanced for daylight at 5500 K. Using a light source that is warmer (lower color temperature) or cooler (higher color temperature) requires a filter to readjust the light. For example, if you are taking a picture without a flash of a person with daylight film under a 200 watt bulb the photograph will appear very warm (the color balance would be shifted to the red). Likewise, a photograph on a bright hazy day will appear much bluer.
Filter Conversion (K) MIRED
80A 3200 to 5500 -131 | Cooling
80B 3400 to 5500 -112 | Filters
80C 3800 to 5500 - 81 |
80D 4200 to 5500 - 56 | (blue)
82C 2800 to 3200 - 45 |
82B 2900 to 3200 - 32 |
82A 3000 to 3200 - 21 |
82 3100 to 3200 - 10 |
---------------------------------------
81 3300 to 3200 + 9 |
81A 3400 to 3200 + 18 | Warming
81B 3500 to 3200 + 27 | Filters
81C 3600 to 3200 + 35 |
81D 3700 to 3200 + 42 | (amber)
81EF 3850 to 3200 + 53 |
85C 5500 to 3800 + 81 |
85 5500 to 3400 +112 |
85B 5500 to 3200 +131 |
It is easy to see that these filters mostly deal with daylight balanced film (5500K) and tungsten-balanced film (3200K). Furthermore, slide film (typically the only place you find the tungsten balance) is much more accurate with its color renditions and thus the increments are 100K rather than the 200K or 400K units with print film.
The MIRED shift value above is defined as:
1000 * (1000/T2 - 1000/T1)
Where
T1 is the color temperature present and
T2 is the desired color temperature (most often that of the film). This value is often called the
Light Balance Index of the filter.
The using the light balance for a filter is not specific to that color and can be used for other color shifts. Furthermore, it is additive so that stacking two filters will add the values together.
Take for example you are photographing a person with daylight film at dusk and you want the skin tone rendered correctly. Looking at the chart you see that the color temperature is about 4000K, but alas there is no 4000K to 5500K shift. So, you whip out your calculator:
1000 * (1000/5500 - 1000/4000)
A color shift of -68. Glance at your filters and... oh no! You've got a -81 color shift, and a -56 and a whole bunch more - but nothing that is -68. Relax, You can stack a 82 (LB index of -10) on top of the 80D (LB index of -56) for a shift of -66. Or you could put a 82A (LB index of -21) on top of a 82C (LB index of -45) also for a shift of -66.
A common complaint with flash photography is that the light is too cool -
and it is. The camera flash is 6000K while the film is at 5500K often giving a blueish cast to things. This requires some warming. Once again the table has nothing that outright says "6000K to 5500K"... so...
1000 * (1000/5500 - 1000/6000)
A +15 color shift. In most cases, people recommend an 81A for work with
a flash.
http://cybaea.com/photo/color-correction.html
The Nikon Field Guide