Oh purple mountain majesty, above the fruited...
Hey! The photographs I took up in the Sierras... the mountains were
not that purple - what gives?
Welcome to Taking Pictures On Mountains 101 - cross-listed with
color temperature 202 and camera shake 203.
There are a number of issues with photographing at high altitudes that are not realized at first. These range from the color balance to the thinner atmosphere.
Color temperature is the red/blue balance in a photograph. This is most often seen by computer types on their monitor allowing a person to set the white point at 5500K, 6500K or about 9000K. As anyone who has fiddled with this selection will quickly recognize is that the brighter whites are the higher values and the lower values are redder.
Color film is calibrated to a particular temperature at which it renders what you see as white as white. With color print film this is almost always 5500K, while color slide can be rated at 5500K or 3200K (the later is for indoor work at about the level of incandescent bulbs). The standard for daylight film is rated at the color of daylight at noon June 21 in Washington DC (outside the Bureau of Standards).
Now, you're up in the mountains and the air is so much cleaner... and less of it too. This will change the amount of red/blue in the scene - it actually raises the blue to between 6000K to 7000K. When shooting film this may mean there is a bit too much blue (or not enough amber) in the photograph. With color prints, this can be corrected by the person running the lab - though it is impossible to correct with slides - the colors that are on the film (and thus the slide) are fixed and set in silver.
To correct for this before the light hits the film, one uses a warming filter. Color temperature is often measured in mired - the micro-reciprocal of the actual color temperature. To compute this, take the temperature and divide it into 1,000,000. 5500K becomes 182 mireds, 6000K becomes 167 mireds, 6500K becomes 154 mireds, and 7000K becomes 143 mireds. The difference between two mired values gives you the mired shift. The following table is for color temperature adjustment for various altitudes (sea level is 5500K or 182 mireds)
- 4,000 feet to 6,000 feet; 6000K; 167 mireds; -15 mired shift
- 6,000 feet to 8,500 feet; 6500K; 154 mireds; -28 mired shift
- 8,500 feet to 10,000 feet; 7000K; 143 mireds; -39 mired shift
To compensate for the
mired shift, a warming filter is used. The following is a list of standard warming filters and the amount of a the shift. (A mired shift of +/-10 is at the edge of detection for the human eye)
- 81 = +10 mireds
- 81A = +18 mireds
- 81B = +37 mireds
- 81C = +35 mireds
- 81D = +42 mireds
- 81EF = +53 mireds
- 85C = +81 mireds
- 85 = +112 mireds
Photographs taken with
black and white film can safely ignore this.
Along with the thinner air changing the color of the surroundings, the thinner air results in less energy available to a person's muscles and they tire more easily. This is especially noticeable when holding up the barrel of a lens and camera. The general rule of thumb for how fast you can hand hold a lens at the altitude one is acclimated to is in the ball-park of: 1/focal length. For a normal lens (50mm), this is frequently around 1/30 to 1/60 of a second. When dealing with telephoto lenses at 200mm, it is a good idea to error on the side of 'too fast'.
Going higher than the altitude it is advised to shoot hand held 2x to 4x faster than one is normally accustom to. The normal lens should now be shot from 1/60 to 1/250 of a second. For anything telephoto, a tripod is highly recommended.