Colored light sources at night

There's a certain amount of standard advice for shooting night photography. One of them is to shoot either Fujichrome 64T or Kodak Ektachrome 160T for film. Both films are well-behaved and well-characterized tungsten balanced slide film. With slide film, you don't need to worry about automatic machine corrections like you do with print films, so that's easy to understand. Both films have about as reasonable figures for long-exposure reciprocity as you'll find in a manufacturer datasheet.

But, tungsten color balance? Why do most night photographers shoot using tungsten color balanced film, or the tungsten setting on their digital SLRs?

I hate hearing standard advice without a justification. I like to research things to understand why the advice is given and to know when I can break these rules. To understand why this is the standard advice, and furthermore, why this is advice I ignore frequently, you need to understand the color of night.

The eye compensates for a lot that film can't compensate for. For example, if you look at a white sheet of paper on a clear day, it will appear to be white. If you take that sheet of paper indoors and look at it under a normal incandescent light, it will still appear to be white. However, if you were to photograph that sheet of paper with daylight balanced film (or the daylight color balance on your digital camera), it will be white outdoors and distinctly yellow indoors.

Mesopic vision

At night, things get even more interesting. The correct measured color temperature for night is actually daylight. However, if you are out at night, you wouldn't believe it, because night always looks awfully blue tinged, very much like tungsten balanced film.

In order to understand what's going on, you need to understand the way the eye works. Most people know that the eye has rods and cones and that the rod cells are active when there's not much light and the cone cells are active the rest of the time.

As light dims, the higher total sensitivity of your blue-sensitive cone cells and the way that the rod cells are processed by your brain, your perception of color shifts things towards blue, so even though you are dealing with normal daylight colors, things start to look blueish.

Movies take full advantage of this. If you underexpose your images by a stop or two and shift your colors towards blue, it will look like night, even in broad daylight.

Low Pressure Sodium vapor lights

These are the most efficient form of illumination out there. They rely solely on the illumination from a sodium vapor arc. Light is concentrated on a single monochromatic line of wavelength 589nm. It doesn't matter if you are shooting tungsten or daylight, you aren't going to get anything other than yellow, no matter how many filters you add.

Astronomers love these lights because you can get a filter that just knocks out the 589nm line and nothing else and pretty much nothing observable in space has that 589nm line.

High Pressure Sodium vapor lights

These lights add mercury to the mix, so they aren't totally monochromatic. With several stop's worth of filters, you can make them look white on daylight film. They look blue tinged on tungsten film.

You can tell the difference between the two because the low pressure lights are a more saturated and pure yellow. Out where I live, there are places with both types right next to each other, which helps one tell the difference.

Florescent Lights and Mercury Vapor lights

These are much closer to white than either type of sodium vapor lights, but they tend to appear green on film unless they have been carefully color balanced, or you use sufficient amounts of filtration.

Tungsten and other incandescent lights

These lights are just lightbulbs and will appear yellow on daylight film and white on tungsten film.

Colored lights

At night is when the colored lights come out. Colored neon signs light up, red warning beacons light up on antenna towers, and all sorts of colored architectural bulbs are lit.

Moonlight

The moon is pretty close to gray, so moonlight is actually rather white. I've taken pictures at night where you can't tell that it's actually night because I used daylight film.

This is why the standard advice is to shoot moonlight with a tungsten color balance. Because you are usually in Mesopic vision or Scotopic vision (meaning, full night vision where you don't perceive colors) when there's a full moon out, having a blue tint to the world matches what the eye sees, even though the eye isn't going to agree with what a color temperature meter would measure.

So why shoot daylight film at night?

Troy Paiva of Lost America writes "The other problem is with shooting in urban conditions where man-made light sources will shift colors turning your images a murky green or brown. Sodium Vapor is the worst and it's, unfortunately, the most common"

I do not want my images of nighttime urban scenery to look that majestic. I prefer my sodium vapor lights to become like the brilliant gold of pyrite, unless I'm trying to say something.