NPF Rule Calculator
Estimate the maximum exposure time to keep stars sharp using a simplified NPF-style rule that accounts for focal length, crop factor and pixel pitch.
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NPF vs 500 rule
The classic 500 rule (max seconds ≈ 500 / effective focal length) was aimed at full-frame film and low-resolution digital. High-megapixel sensors resolve trailing sooner, so stricter limits help. The NPF rule family incorporates pixel size and optional tracking.
This tool uses a simplified practical form for planning. For critical work, shoot a test frame and zoom to 100%.
FAQ
Should I always prefer NPF over 500?
On 40–60 MP bodies, yes — 500 rule exposures often show trails. On older 12–16 MP cameras, 500 rule may still be acceptable.
Does aperture appear in the rule?
Exposure time for star sharpness is about trailing, not brightness. Aperture still controls how much light you collect and how short you can keep the shutter while holding ISO reasonable.
High-resolution night work
Forty- and sixty-megapixel bodies show star trails sooner than older sixteen-megapixel cameras. NPF-style limits help, but seeing conditions and lens sharpness still dominate. Combine short tracked or untracked frames and stack for clean skies without pushing a single exposure too far.