How to choose the correct white balance for star trail photography (Nikon D3200)

Question

These last days I've been practising star trails photography. It's not complicated, but there's a problem that keeps me giving headaches: the white balance.

I know, for this type of situations, I have to choose color temperature close to blue or "cold". The problem start when I'm trying to configurate the white balance in order to get a precise color temperature in my Nikon D3200. This is how the color temperature grid looks like this:

It has no Kelvin temperature indicator. Instead, it has two text boxes, one labeled A-B and the other one G-M.

My question is: what are the closest values to the perfect white balance for star trail photography (using these both values)?

P.D.: What do both of them mean?

Answer 1

A-B stands for Amber-Blue. This is your color temperature/Kelvin scale, and is primarily what you will be concerned with for your star trails.

G-M stands for Green-Magenta, and is used to correct for color cast stemming from artificial light sources which do not very evenly emit all wavelengths of visible light. For example old fluorescent lights are notorious for creating yellow-green skin tones, which need to be corrected for with a shift on this axis.

Back to your question, there is no "correct" white balance setting for star trails. The color temperature you choose will greatly depend on the light sources illuminating the sky around you, as well as personal preference, and can even change throughout the evening as the sun/moon rises/sets or clouds roll through (assuming you're taking multiple exposures through the night).

If you are intent on setting a specific Kelvin temp however, you could try to correlate various settings along the A-B axis with reference Kelvin temperature settings through trial and error.

Instead though, why not shoot in RAW and keep the entire color temperature spectrum in play? This allows you the flexibility to try out photos with different temperature profiles, and would even allow you to set a specific Kelvin value in post if you were still so inclined.

Answer 2

What you are looking at is the White-Balance Fine-Tuning screen. This lets you adjust a Preset WB at least, although some cameras allow Custom WB and even Auto WB to be adjusted.

The point is that you must start with some white-balance, say Daylight which is a specific Kelvin color temperature and then you can shift that along either the Amber-Blue (A-B) axis or the Green-Magenta (G-M) axis. The steps you see are arbitrary and depend on the camera. Not only that but some digital cameras use linear steps and some logarithmic ones.

The A-B axis affects color temperature but not the G-M axis. If you shift the offset towards the Blue, the camera makes images cooler, corresponding to correction needed for a warmer white-balance, so be careful which way around you adjust.

There is probably no perfect WB for this but star trails are usually blown out, so they will probably be white regardless of white-balance in the end. Stars themselves have different color temperature that depend on the composition of the particular star, so there is no one WB for all of them.

Answer 3

I am not an expert, at best I might be intermediate. I have the D3200. If I'm shooting the sky at night I'm using manual settings, but set the white balance to auto. The camera does a great job. After you take the shot the camera processes the white balance. You can tell this by the small green light that remains on for a short period of time. It seems to stay on the same amount of time of the exposure. When the lights goes off your white balance is done and you're ready for your next shot. I've recently started shooting RAW. You can get amazing results.