Astrophotography Exposure Calculator
The longest exposure before stars trail, by the 500 rule, the 600 rule and the NPF rule, with the trail length in pixels.
Results
Longest exposure by the NPF rule
11.583
s
The same by the 500 rule
20.833
s
The same by the 600 rule
25.000
s
Pixel pitch
6.000
µm
Crop factor
1.0000
Trail the 500 rule allows
6.08
px
Trail the NPF rule allows
3.38
px
How fast the sky moves
15.0411
"/s
Sky covered by one pixel
51.566
"/px
What this tool does
The 500 rule was written for film, where a trail of a few hundredths of a millimetre went unnoticed. On a modern sensor it lets stars smear across five or six pixels, which is why the NPF rule exists: it starts from the pixel pitch and the aperture rather than the focal length alone. The page gives all three and, more usefully, the trail length each one actually allows — so you can decide rather than trust a rule of thumb.
Formula
500 rule: t = 500 ÷ (focal × crop) · NPF: t = (35 N + 30 p) ÷ focal · trail = 0.072919 × focal × t ÷ p
Variables
| Symbol | Meaning | Unit |
|---|---|---|
fl | Focal length as marked | mm |
sw | Sensor width | mm |
sh | Sensor height | mm |
px | Pixels across the sensor | — |
ap | f-number | — |
dec | Declination of the stars | ° |
NP | Longest exposure by the NPF rule | s |
R5 | The same by the 500 rule | s |
R6 | The same by the 600 rule | s |
PP | Pixel pitch | µm |
CF | Crop factor | — |
T5 | Trail the 500 rule allows | px |
TN | Trail the NPF rule allows | px |
AR | How fast the sky moves | "/s |
SC | Sky covered by one pixel | "/px |
Worked example
- Focal length as marked24 mm
- Sensor width36 mm
- Sensor height24 mm
- Pixels across the sensor6000
- f-number2.8
- Declination of the stars0 °
- Longest exposure by the NPF rule11.583 s
- The same by the 500 rule20.833 s
- The same by the 600 rule25.000 s
- Pixel pitch6.000 µm
- Crop factor1.0000
- Trail the 500 rule allows6.08 px
- Trail the NPF rule allows3.38 px
- How fast the sky moves15.0411 "/s
- Sky covered by one pixel51.566 "/px
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.