What shutter speed should I use for the Milky Way at 24mm on full frame?
At 24mm on a typical 24-megapixel full-frame camera, the simple 500 rule suggests roughly 21 seconds (500 ÷ 24); the more accurate NPF rule, accounting for that sensor's real pixel pitch, suggests a shorter roughly 12 seconds before star trailing becomes visible at full resolution. Which one to actually use depends on what you're doing with the file afterward.
Why the Two Numbers Diverge This Much
24 megapixels on a full-frame sensor gives a pixel pitch of exactly 6.0 microns, and the NPF formula's sensitivity to that figure is exactly why its answer runs shorter than the resolution-blind 500 rule — a higher-resolution full-frame body would push the NPF number shorter still, while a lower-resolution one would bring it closer to the 500 rule's estimate.
A Practical Starting Setup
For a 24mm Milky Way shot on this sensor, a reasonable starting point is f/2.8 (or your lens's widest available aperture), the NPF-derived roughly 12-second shutter speed, and ISO adjusted until the histogram shows a well-exposed sky — often somewhere in the 3200-6400 range depending on how dark your location actually is. Take a test shot, zoom in on a bright star at full resolution to confirm it still reads as a clean point rather than a short streak, and adjust from there.
Get the exact figure for your own camera's resolution and any declination in the Astro 500/NPF Rule Calculator, pre-filled with 24mm f/2.8 on a 24MP full-frame body.
Frequently Asked Questions
Should I use the 500 rule's 21 seconds or the NPF rule's 12 seconds?
The NPF figure is more accurate if you plan to view the image at full resolution, crop into it, or print large; the 500 rule's more generous 21 seconds is a reasonable choice if the final output is small (social media, a modest print) and gathering more total light matters more than pixel-level star sharpness.
Does aiming at a different part of the sky change this shutter speed?
Yes, meaningfully — this figure assumes shooting near the celestial equator, where the Milky Way's core sits and where stars trail fastest. Aiming higher in declination toward the celestial pole allows a longer exposure before trailing becomes visible, at the identical focal length and aperture.