How to use the 500 rule calculator
- Enter the focal length of your lens in millimeters.
- Enter the crop factor of your camera. Use 1 for full frame, or find it with the Field of View Calculator.
- Choose the rule number: 500, 400 or 300. Larger numbers allow a longer exposure but more trailing.
- Open More options to enter the aperture and the pixel pitch to see the NPF figure as well.
Formula
equivalent focal length = focal length x crop factor
longest exposure = rule number / equivalent focal length
NPF exposure = (35 x aperture + 30 x pixel pitch in micrometers) / focal length
The NPF rule uses the actual focal length, the aperture and the size of the pixels, so it adapts to the camera. Both rules are rough guides.
A 20 mm lens at f/2.8
On full frame, the 500 rule gives 500 / 20 = 25 seconds. With a pixel pitch of 5.9 micrometers the NPF rule gives (35 x 2.8 + 30 x 5.9) / 20 = 13.75 seconds, which is much shorter because it expects sharper stars.
A 24 mm lens on a camera with a crop factor of 1.5 has an equivalent focal length of 36 mm, so the 500 rule gives 13.9 seconds and the 300 rule 8.3 seconds. A 50 mm lens with a crop factor of 1.6 and the 400 rule gives 400 / 80 = 5 seconds.
Why the rules disagree
The 500 rule is a simple rule of thumb, and on high resolution cameras it lets stars trail by several pixels. The NPF rule takes the aperture and the pixel size into account, so high resolution cameras get shorter times. Both are guides. Take a test frame, zoom to 100 percent and look at the stars before you settle on a time.
Other things that change trailing
Stars near the celestial equator move faster across the frame than stars near the pole, so pointing toward the pole allows longer exposures. A wider lens allows longer exposures than a telephoto lens. Tracking mounts remove trailing completely and are outside this page.
Related photography tools
Check the view with the Field of View Calculator, plan filters with the ND Filter Exposure Calculator, plan a sequence with the Timelapse Calculator, and check sky times with the Sunrise and Sunset Calculator.
Assumptions and limits
- The crop factor and focal length are accurate.
- The camera points at the sky away from the celestial pole, which is the stricter case.
- Pixel pitch is the distance between pixel centers in micrometers.
- The result is a guide and not a promise of sharp stars.
Frequently asked questions
What is the 500 rule in astrophotography?
Divide 500 by the full frame equivalent focal length to get the longest exposure in seconds before stars show trails. A 20 mm lens on full frame gives 25 seconds.
How do I use the 500 rule with a crop sensor?
Multiply the focal length by the crop factor first. A 24 mm lens with a crop factor of 1.5 acts like 36 mm, so the 500 rule gives 13.9 seconds.
What is the NPF rule?
It is a rule that adds the effects of the aperture and the pixel size. Its exposure time is 35 times the aperture plus 30 times the pixel pitch in micrometers, divided by the focal length.
Why are my stars still trailing?
The rules are rough guides, and high resolution sensors show trails the old rule allows. Shorten the exposure, use the NPF rule or point away from the celestial equator.