How to use it
- Type the Focal length (mm) of your lens and the Aperture (f-number) you will shoot at.
- Type your camera's Crop factor, or 1 for full frame, and its Pixel pitch (micrometres).
- Read the Longest exposure, NPF rule and the Longest exposure, 500 rule. The shorter figure is the safer one.
- Use the table to compare focal lengths with the same camera and aperture.
The two rules
The 500 rule is a rule of thumb for keeping stars point-like. The Astrophotography article on Wikipedia gives it as the maximum exposure in seconds: 500 divided by the focal length in millimetres, multiplied by the crop factor. It does not depend on the aperture or the ISO. The article's example is a 35 mm lens on an APS-C sensor: 500 ÷ (35 × 1.5) is about 9.5 seconds.
The NPF rule is the more accurate rule the same article gives. It adds the aperture and the pixel pitch: (35 × f-number + 30 × pixel pitch in micrometres) ÷ focal length in millimetres, for a star at a declination of 60 degrees. The article's example is an 11 mm lens at f/2.8 on a Nikon D3300, with a pixel pitch of 3.91 micrometres: about 19.6 seconds.
500 rule: longest exposure (s) = 500 ÷ (focal length (mm) × crop factor)
NPF rule: longest exposure (s) = (35 × f-number + 30 × pixel pitch (µm)) ÷ focal length (mm)
Pixel pitch = sensor width in micrometres ÷ horizontal pixelsWhat the rules do not cover
- Stars in the field of the lens: the declination of the star field changes how fast stars move, and the NPF rule assumes 60 degrees.
- The sharpness of the lens and the focus: a very sharp lens shows trails that a softer one would hide.
- The longer exposures for star trails: the article notes that intentional trails take several minutes or hours; this calculator is for points of light.
Worked example: 24 mm at f/2.8 on APS-C with a 3.91 µm pixel pitch
The 500 rule gives 500 ÷ (24 × 1.5) = 13.9 seconds. The NPF rule gives (35 × 2.8 + 30 × 3.91) ÷ 24 = 8.97, or 9.0 seconds.
The NPF time is the shorter, so it is the safer limit: the aperture and the pixel pitch make the stars trail sooner than the 500 rule alone suggests.
| Rule | Longest exposure |
|---|---|
| 500 rule | 13.9 seconds |
| NPF rule | 9.0 seconds |
Frequently asked questions
What is the 500 rule?
The 500 rule is a rule of thumb: divide 500 by the focal length and by the crop factor to get the longest exposure in seconds before stars trail. The Astrophotography article on Wikipedia gives it with an example for a 35 mm lens on an APS-C sensor.
What is the NPF rule?
The NPF rule is the more accurate rule the same article describes. It adds the f-number and the pixel pitch, (35 × N + 30 × pixel pitch) ÷ focal length, and assumes a star at a declination of 60 degrees.
What is a pixel pitch?
The pixel pitch is the size of one pixel on the sensor. Divide the sensor's width in micrometres by the number of pixels across it; a camera with a 23.5 mm wide sensor and 6,000 pixels across has a pitch of about 3.9 micrometres.
Why are the two rules different?
The 500 rule uses only the focal length and the crop factor. The NPF rule also counts the aperture and the pixel size, so a smaller pixel makes the stars show trailing sooner. The article notes that the more accurate calculation takes pixel pitch and declination into account.
Is the crop factor the same as the sensor size?
Not exactly. The crop factor is the full-frame diagonal divided by the sensor's diagonal, so a smaller sensor has a larger crop factor. The Crop factor calculator on this site shows the value for each sensor size.
Sources
Figures and text last checked on Tuesday, November 10, 2026.