Frequently Asked Questions
Why does the NPF rule give a shorter time than the 500 rule?
The NPF rule factors in aperture and pixel pitch, which the 500 rule ignores. Modern sensors pack more pixels into the same sensor area, so a star moves across a pixel faster than older, lower-resolution cameras.
Does the 600 rule apply to different camera types?
The 600 rule is sometimes used for full-frame cameras with older, lower-resolution sensors. For contemporary 24+ MP bodies, the NPF rule is more reliable.
How does crop factor affect the result?
A crop sensor multiplies the effective focal length, shortening the safe shutter time proportionally. A 24 mm lens on an APS-C body (crop 1.53) behaves like 37 mm, cutting the safe time by about 35%.
Can I use these rules with a motorized star tracker?
No - a tracker cancels Earth's rotation, removing the trailing limit. Without a tracker you are limited to these short exposures.
What if my megapixel count is not a standard option?
Select the sensor preset closest to your camera or use a custom entry. The pixel pitch estimate shifts slightly but remains a reliable guide.
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Important Disclaimer: Estimates for informational purposes only.
This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.