Photo — Night sky

#photo/nightsky · Note 6 of 7 · part of Photography

The 35mm f/1.8 is a genuinely good astro lens — fast, and at ~56mm equivalent tight enough to pick out the Milky Way core without needing a tracker.

The 500 rule, and why it is wrong

The old rule: maximum exposure seconds = 500 / effective focal length.

For this kit: 500 / (35 × 1.6) = 500 / 56 ≈ 8.9 seconds before stars trail.

That rule was written for film. On a 24MP APS-C sensor, examined at 100%, stars smear noticeably sooner. The NPF rule accounts for pixel pitch and aperture and gives roughly 5–6 seconds for this combination.

In practice: shoot 6 seconds, check at 100%, and go to 8 if you are printing small or your standards are kind.

The settings

Manual mode, manual focus
f/1.8, or f/2.2 if the corner stars look like little wings — that is coma, and stopping down one third of a stop cleans it up considerably
6 seconds
ISO 3200 to start; 6400 if the sky is genuinely dark
White balance ~3900K, not auto — auto will chase the light pollution
RAW, always
IS OFF. It is on a tripod. See Photo — Low light.

Focusing on stars

Autofocus will fail. Do this instead:

Live view, digital zoom to 10×
Point at the brightest star or a distant light
Turn the focus ring until the star is the smallest point, not the brightest — go past it and come back
Tape the focus ring so a knock doesn't undo it

What you need besides the camera

A tripod. Non-negotiable — there is no handholding a six-second exposure.
A remote or the 2-second timer. Pressing the shutter shakes the camera.
A dark sky. This matters more than any setting. Bortle 4 or darker; from Bangalore that means driving out, ideally towards Coorg, Kolar, or anywhere away from the city glow.
A red torch, so twenty minutes of dark adaptation survives.

Stacking, when one frame is not enough

Noise is random; the sky is not. Shoot 15–20 identical frames plus a few "dark" frames with the lens cap on, then average them:

Sequator (Windows, free) or Siril (cross-platform, free, open source)
Starry Landscape Stacker on macOS (paid, excellent)

The result is roughly what ISO 400 would have looked like if it had gathered that much light. It is the single biggest quality jump available without a tracker.

Foreground

A sky on its own is a wallpaper. Something in front of it — a ridge, a tree, a building — is a photograph. Expose the sky first, then take a second, longer frame for the foreground and blend the two.

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