Astrophotography Calculator
Canon EOS R5
Sensor: 36x24mm, 45MP, 4.38um pitch
Crop: Full Frame
Stacking Recommendations
Based on 7s sub-exposures for Milky Way imaging
| Frames↕ | Total Time (min)↕ | SNR Gain↕ | Equiv. Single Exp.↕ |
|---|---|---|---|
| 10 | 1.2 | 3.2x | 1m 10s |
| 20 | 2.3 | 4.5x | 2m 20s |
| 50 | 5.8 | 7.1x | 5m 50s |
Rule Comparison
The NPF rule accounts for pixel pitch and aperture for more precise results. Use the most conservative (shortest) value for sharp stars.
This astrophotography calculator finds the longest exposure you can take before the stars trail, using both the precise NPF rule (which accounts for your sensor's pixel pitch and aperture) and the classic 500 rule. It then recommends a sub-exposure length adjusted for light pollution on the Bortle scale and your target type, and shows how stacking multiple frames improves signal-to-noise ratio and total integration time.
Formula
t_NPF = (35 × N + 30 × p) ÷ f; t_500 = 500 ÷ (f × crop); SNR gain = √(frames)
- N
- Aperture f-number (e.g. 2.8)
- p
- Sensor pixel pitch in micrometers
- f
- Lens focal length in millimeters
- crop
- Camera crop factor relative to full-frame (1.0 for full-frame)
How it works
- Select your camera from the sensor database (which supplies pixel pitch and crop factor), then enter the lens focal length, aperture (f-number), ISO, and the Bortle sky-brightness level.
- Pick a target type — Milky Way, deep sky, or star trails. The tool computes the NPF and 500-rule limits, takes the more conservative of the NPF and crop-adjusted 500 limit, then scales it by a Bortle multiplier for the recommended sub-exposure.
- Review the stacking table: for several frame counts it shows total integration minutes, the √N signal-to-noise improvement, and the equivalent single-exposure time you would otherwise need.
Worked example
A full-frame camera (4.38 µm pixel pitch) with a 24 mm f/2.8 lens shooting the Milky Way under a Bortle 4 sky.
- NPF rule: t = (35 × 2.8 + 30 × 4.38) ÷ 24 = (98 + 131.4) ÷ 24 ≈ 9.6 s.
- 500 rule: t = 500 ÷ 24 ≈ 20.8 s (crop factor 1.0, so the crop-adjusted value is the same).
- Base = min(NPF 9.6, 500-crop 20.8) = 9.6 s; Bortle 4 multiplier = 1 − (4−1)×0.09375 = 0.71875.
- Recommended sub = round(9.6 × 0.71875) ≈ 7 s.
NPF limit ≈ 9.6 s, 500 rule ≈ 20.8 s, recommended sub-exposure ≈ 7 s; stacking 50 frames yields a 7.1× SNR gain over a single frame.
Frequently asked questions
- What is the difference between the NPF rule and the 500 rule?
- The 500 rule (500 ÷ focal length) is a quick approximation. The NPF rule additionally accounts for aperture and sensor pixel pitch, so it gives a tighter, more accurate exposure limit for sharp pinpoint stars on modern high-resolution sensors.
- How does stacking improve my images?
- Stacking N aligned frames improves the signal-to-noise ratio by roughly the square root of N. Stacking 100 short subs gives about a 10× SNR improvement and the same total integration as one very long exposure, but without star trailing or a single ruined frame.
- Why does a darker sky allow longer exposures?
- Under bright skies, light pollution (sky glow) accumulates and washes out faint detail quickly, so shorter subs are preferred. The calculator applies a Bortle multiplier that allows longer recommended subs at darker sites (Bortle 1–3) and shortens them in bright urban skies (Bortle 8–9).
- Does a crop-sensor camera change the exposure limit?
- Yes. A crop sensor effectively magnifies the field, so stars cross pixels faster. The crop-adjusted 500 rule divides by both focal length and crop factor, shortening the allowed exposure compared with a full-frame body using the same lens.