About this calculator
Because the earth rotates, any exposure that’s too long will render stars as short trails instead of sharp points. The classic ‘500 rule’ (500 ÷ full-frame-equivalent focal length) is a quick, simple estimate. The ‘NPF rule’ is more accurate, especially on today’s high-resolution sensors, because it also factors in your aperture and your camera’s actual pixel pitch — smaller pixels reveal trailing sooner than the 500 rule accounts for.
How the calculation works
The 500 rule gives exposure time in seconds as 500 ÷ (focal length × crop factor). The NPF rule uses (35 × f-number + 30 × pixel pitch in μm) ÷ focal length, so it accounts for your aperture and the real size of your sensor's pixels.
Pixel pitch is sensor width divided by horizontal pixel count, and the pixel count comes from megapixels and aspect ratio. The closer your sensor size and resolution presets are to your camera, the closer the result is to reality.
Worked examples
- Full frame, 24 mm f/2.8, about 45 MP (pixel pitch about 4.4 μm): the 500 rule gives about 21 s and the NPF rule about 9.6 s. The higher the resolution, the bigger the gap.
- The same lens on a roughly 24.2 MP full-frame body (about 6.0 μm) stretches the NPF result to about 12 s. Fewer megapixels mean larger pixels, so a slightly longer exposure is still safe.
- Full frame, 14 mm f/2.8, about 61 MP (about 3.8 μm): 500 rule about 36 s, NPF about 15 s.
- APS-C (crop 1.5), 16 mm f/2.8, about 33 MP (about 3.3 μm): 500 rule about 21 s, NPF about 12 s.
When to use it
Before heading out for the Milky Way or star photos, work out the maximum exposure for each lens and camera combination so you are not calculating in the dark on location. Seeing both rules side by side also shows which one is more conservative.
Limits and common mistakes
- Both rules estimate whether stars look like points at normal viewing size. If you print large or view at 100%, shoot shorter.
- The estimates assume a sky near the celestial equator. Stars near the celestial pole, such as around Polaris, move more slowly and can take longer exposures, while stars near the celestial equator trail faster.
- On a crop body, enter the real focal length; the crop factor is applied through the sensor size you pick. Entering an equivalent focal length counts it twice.
- A shorter exposure gathers less light. How to balance it with a wider aperture and higher ISO is a separate decision.
- If you do not know your exact megapixel count, choose the closest preset. A small difference barely changes the result.
Guides that pair well with this calculator
Both rules are estimates for a ‘no visible trailing at normal viewing size’ standard — extreme pixel-peeping, very large prints, or unusually long lenses may need a shorter exposure than either rule suggests.
Frequently Asked Questions
Why do the 500 rule and NPF rule give different answers?+
The 500 rule only considers focal length and crop factor. The NPF rule also accounts for your aperture and your camera’s pixel pitch, which is why two cameras with the same focal length but different resolutions get different NPF results.
Which rule should I trust more?+
The NPF rule is generally considered more accurate, especially on modern high-megapixel cameras, since smaller pixels make star trailing visible at shorter exposure times than the simple 500 rule assumes.
I don’t know my camera’s exact megapixel count — what do I do?+
Pick the resolution preset closest to your camera’s actual megapixel count; the listed example models can help you match it.