What is the best aperture for Milky Way photography?
The widest usable aperture your lens offers, typically somewhere in the f/1.4 to f/2.8 range for dedicated astro lenses, is generally the right choice — unlike ordinary daylight photography, aperture here isn't primarily a depth-of-field decision, since your shutter speed is capped by star trailing rather than by choice, and gathering more light per second directly reduces how much you need to lean on ISO.
Why Wider Almost Always Wins Here
A full stop of extra light-gathering at a fixed, trailing-limited shutter speed means either a stop lower ISO for the same brightness, or a genuinely brighter, cleaner exposure at the same ISO — going from f/2.8 to f/1.4 is a full two-stop gain, computed directly from the aperture ratio, which is a real, substantial improvement in a genre where every bit of light matters and shutter speed isn't a free variable to trade against.
Where Wide Open Isn't Automatically Correct
Many lenses show visible softness, vignetting or coma (star-distorting smearing toward the frame edges) at their absolute widest aperture, and stopping down half a stop to a full stop from maximum often meaningfully improves edge sharpness with only a modest light-gathering cost — a full-frame f/1.4 lens shot at f/1.8 still gathers substantially more light than an f/2.8 lens, while correcting much of the coma its widest setting would show. Testing your specific lens's coma and sharpness at a few aperture settings is worth doing before committing to always shooting wide open.
See how much shutter-speed or ISO headroom a wider aperture actually buys you in the Astro 500/NPF Rule Calculator, pre-filled at f/1.4.
Frequently Asked Questions
Does aperture choice affect the safe shutter speed under the NPF rule?
Yes, slightly — the NPF formula includes the f-number directly, with a narrower aperture very slightly extending the safe shutter speed, though focal length, pixel pitch and declination are all considerably larger factors in the calculation than aperture is.
Is coma really a big enough problem to justify stopping down for astro?
On many lenses, yes, especially toward the corners — visible coma turns stars near the frame edge into small comet-shaped smears rather than clean points, and a modest stop-down often improves this dramatically for only a small light-gathering cost, worth testing on your specific lens.