Choosing a lens for astrophotography

Photographing the night sky has a hard constraint: the stars move. Expose for too long and points become trails. You cannot solve darkness with time, so you solve it with focal length and aperture.

Why 24mm equivalent or wider

The rough guide is that you can expose for about 500 divided by your equivalent focal length in seconds before trailing shows. At 24mm that is around 20 seconds; at 50mm it is 10. Wider focal lengths buy you exposure time, and they take in more of the sky.

Why f/2.8 or faster

Every stop halves the exposure you need, and that translates directly into less trailing and less noise. f/2.8 is the practical threshold; f/1.8 and faster is better still if the corners hold up.

What the specifications do not tell you

Coma — points of light smearing into wings at the edges of the frame — is the single most important optical property for this work, and it appears in no specification table. Two lenses with identical numbers can behave completely differently. Check the sample photos, and look at reviews specifically for the corner performance wide open.

Manual focus is also normal here: autofocus rarely works on stars, so a lens with a usable focus scale and a hard infinity stop is easier to live with.