Blue hour and night photography

Set up after dark and you get a black sky full of blown white dots. The time to shoot a city is the fifteen to twenty-five minutes after sunset, while the sky still has colour and the lights are already on.

1. Which stretch blue hour is

Using the definition from golden and blue hour, blue hour is when the sun sits roughly 4 to 6 degrees below the horizon. The sun has gone but the upper atmosphere is still lit, and scattering turns the whole sky into one huge, blue light source. Light comes from everywhere, so nothing casts a hard shadow.

How long it lasts depends on latitude and date. The steeper the sun drops, the faster it crosses those two degrees. Near the equator that takes around ten minutes all year; a high-latitude summer stretches it out for ages. Today's times have to be computed for your place and date.

2. The window where things match

You are not waiting for a pretty sky. You are waiting for the moment the sky and the street lighting are about equally bright.

• Too early: the lamps are lost against a bright sky, and you have an ordinary evening snap.
• Right on time, a clear layer of blue remains, windows and streetlights are on, and both sit in the middle of the histogram together (see how to read a histogram).
• Too late, the sky is dead black with white dots on it. You're photographing lamps now.

City lights come on gradually after sunset, so early in blue hour they're often half lit. What works: be set up forty minutes early, framing and focus done, then fire the same frame every two or three minutes until the sky is gone. Pick at home; on location your eyes keep adjusting and can't judge the balance.

3. Setting white balance

Auto white balance sees a frame full of blue, calls it a cast, and neutralises exactly what you came for. Instead:

1. Shoot RAW, so white balance can be reset losslessly later; see RAW or JPEG.
2. Set a fixed Kelvin value and leave it. 3800K to 4500K is a sensible start: lower under warm sodium lamps, higher under white LED. With a fixed value the whole sequence matches and can be graded together.
3. Don't chase accuracy. Cool sky and warm lamps together are what this time of day looks like. Neutralise both and the blue hour is gone; see white balance and colour temperature.

4. Long exposure or high ISO

Two easy things to forget. Focus while there's still light, then switch to manual and don't touch the ring again, because autofocus hunts in the dark. And turn stabilisation off on a tripod (see hand-holding and stabilisation).

5. Where starbursts come from

Starbursts come from diffraction at the edges of the aperture blades, with no filter or editing involved. Three rules:

• The smaller the aperture, the stronger the effect. They start around f/8 and are cleanest between f/11 and f/16.
• Blade count sets the number of rays. An even number of blades gives that many rays; an odd number gives twice as many, so a seven-blade aperture makes fourteen.
• You need small, bright point sources: streetlights, distant windows, a sliver of sun. An evenly lit panel won't burst.

The price is diffraction softening the frame, plus a longer exposure. Don't stop down to f/22 chasing rays. By then the whole picture is soft.

How this works in Lenscript. The LCT platform computes sunrise, golden hour, sunset and blue hour for a given place and date in local time, to within about two minutes, so you don't have to look up when to be there. The platform is still being built, and weather and air quality only get connected once it opens. For now it tells you when to leave, not whether there will be cloud.

6. FAQ