What ISO should I use for Milky Way photography?
Set your aperture (as wide as your lens allows) and shutter speed (your trailing-limited maximum, from the NPF or 500 rule) first, then raise ISO until the histogram shows a well-exposed sky — commonly somewhere in the 1600 to 6400 range, though the exact number depends on how genuinely dark your location is and how much noise your specific camera can tolerate.
Why ISO Is the Last Variable to Set, Not the First
Aperture and shutter speed both have hard, physics-driven ceilings in astro work — aperture at your lens's widest usable setting, shutter speed at the trailing threshold — while ISO is the one variable with genuine room to adjust after the other two are locked in. Treating ISO as the free variable that absorbs whatever brightness the fixed aperture and shutter speed didn't provide is the correct order of operations, rather than picking an ISO first and hoping the exposure works out.
Why a Modern ISO-Invariant Sensor Changes This Slightly
On a genuinely ISO-invariant camera, shooting at a lower ISO and deliberately underexposing, then brightening the file in post-processing, can produce very similar final noise to raising ISO in-camera at the time of capture — a real, useful option for astro work specifically, since it means slightly underexposing to protect highlight detail (like a bright foreground light source) doesn't cost as much noise penalty as it would on a non-invariant sensor. Test your own camera's ISO invariance before relying on this technique for critical shots.
Work out your trailing-limited shutter speed first in the Astro 500/NPF Rule Calculator, then choose ISO from there.
Frequently Asked Questions
Is ISO 6400 too noisy for Milky Way photography?
It depends heavily on your specific camera's sensor and how much noise reduction and post-processing you're willing to apply — many current full-frame bodies produce genuinely usable files at ISO 6400 for astro work, though noise becomes progressively more visible in shadow detail as ISO climbs.
Should I underexpose slightly to protect a bright foreground element like the moon or city lights?
It's a reasonable technique on an ISO-invariant camera, since you can recover shadow detail in post with a relatively small noise penalty — but it's worth testing your own camera's behavior first rather than assuming every sensor handles this equally well.