Frequently Asked Questions
What is the 500 rule?
A guide for the longest exposure before stars trail: seconds ≈ 500 ÷ (focal length × crop factor). At 20 mm on full frame that is ~25 s. Some use 600 (more lenient) or 300 (stricter for sharp pixels).
What is the NPF rule and why is it better?
A more precise formula accounting for aperture, pixel pitch, and declination: roughly (35 × aperture + 30 × pixel pitch) ÷ (focal × crop). It gives shorter, sharper exposures on modern high-resolution sensors than the 500 rule.
Why does sensor resolution matter?
More megapixels mean smaller pixels that reveal trailing sooner. The 500 rule ignores this; NPF uses pixel pitch (sensor width ÷ horizontal pixels) to compensate.
What if my exposure is too short for a good image?
Open the aperture, raise ISO, or use a star tracker - a tracker lets much longer exposures with pinpoint stars (but trails the foreground).
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