The astrophotography exposure calculator finds the maximum shutter speed before stars begin to trail (appear as lines rather than points). It uses two methods: the simple Rule of 500 and the more accurate NPF rule based on your camera's pixel pitch.
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How to Use the Astrophotography Exposure Calculator
This free astrophotography calculator finds the maximum shutter speed for sharp stars. Enter your lens focal length, aperture, and sensor crop factor. The tool computes both the classic Rule of 500 and the more precise NPF rule.
Rule of 500 vs NPF Rule
The Rule of 500 is simple and widely used: divide 500 by your effective focal length (focal length × crop factor). A 24mm lens on an APS-C camera has an effective focal length of 36mm, giving 500 ÷ 36 = 13.9 seconds. Round down for safety.
The NPF rule accounts for your camera's pixel density. Higher-resolution sensors (smaller pixels) show trailing sooner because the trail covers more pixels. The NPF formula considers aperture, pixel pitch, and focal length for a more accurate result. Use NPF when using modern 24MP+ cameras.
Practical Tips for Milky Way Photography
Use your widest lens (14–24mm) at maximum aperture (f/2.8 or faster). Start at ISO 3200 and your calculated max shutter speed. Take 5–10 test shots, zooming in to 100% to check for trailing. If stars show slight trails, reduce shutter speed by 20%. For final images, stack 20–30 exposures to reduce noise — this beats using a single long exposure at very high ISO.
Deep Sky vs Wide-Field Targets
For tracked deep-sky photography (using an equatorial mount), the trailing constraint is removed — you can expose for 2–5 minutes per frame. At 2 minutes, ISO 800–1600 on a 600mm telescope reveals galaxies and nebulae that single-exposure wide-field photography cannot match. The calculator's "Deep Sky" preset provides settings for tracked imaging.
FAQ
What is the Rule of 500?
The Rule of 500 states that the maximum exposure time before stars trail is 500 divided by the effective focal length (focal length × crop factor). For a 24mm lens on a full-frame camera, that's 500 ÷ 24 = about 21 seconds. Some photographers use 400 for a more conservative (sharper) result.
What is the NPF rule?
The NPF rule is a more accurate formula that accounts for the aperture (N = f-number), pixel pitch (P), and focal length (F). It calculates the shutter speed where stars appear as point sources rather than short trails, based on the actual pixel size of your sensor. It's more complex but gives sharper results than the Rule of 500.
What ISO should I use for Milky Way photography?
For Milky Way photography, ISO 1600–6400 is typical. Start at ISO 3200 with a 20–25 second exposure at f/2.8 on a full-frame camera to gauge your sky brightness. In darker skies (Bortle 3–4), you can push ISO 6400. Noisier images can often be cleaned up with stacking multiple exposures.
What crop factor does my camera have?
Full-frame cameras (Sony A7, Nikon Z6, Canon R5) have crop factor 1.0. APS-C cameras (Sony A6000, Nikon Z50, Canon 90D) have crop factor 1.5–1.6. Micro Four Thirds cameras (Olympus, Panasonic Lumix) have crop factor 2.0.
Is this tool free?
Yes, completely free with no signup required.