What is the 500 Rule?

The 500 Rule states that the maximum exposure time (in seconds) before stars begin to trail is 500 divided by your effective focal length (focal length × crop factor). For example, a 24 mm lens on a full-frame camera gives 500 ÷ 24 ≈ 20 seconds.

What is the NPF Rule and why is it more accurate?

The NPF Rule (developed by Frédéric Michaud) incorporates aperture (N), focal length (f), and pixel pitch (p) to produce a more precise limit: t = (35 × N + 30 × p) / f. It accounts for the physical size of your sensor pixels, making it much more reliable for high-megapixel cameras where the 500 Rule over-exposes.

What is crop factor and where do I find mine?

Crop factor (also called focal length multiplier) describes how your sensor size compares to a 35 mm full-frame sensor. Full-frame = 1×, APS-C Canon ≈ 1.6×, APS-C Nikon/Sony ≈ 1.5×, Micro Four Thirds = 2×. Check your camera's spec sheet or the manufacturer's website.

Pixel pitch is the physical distance between the centers of adjacent pixels on your sensor, measured in micrometers (µm). You can find it in detailed camera specs or calculate it as: sensor width (mm) ÷ horizontal pixel count × 1000. Typical values range from 3 µm (high-res mirrorless) to 8 µm (older DSLRs).

What does the Bortle scale represent?

The Bortle scale rates sky darkness from 1 (pristine dark site, Milky Way casts shadows) to 9 (inner-city sky, only the Moon and a few stars visible). A lower Bortle number means you can use longer exposures before sky glow overwhelms your image.

Should I stack multiple exposures instead of one long one?

Yes — for most astrophotography, stacking many shorter exposures (using software like DeepSkyStacker or Sequator) produces less noise and avoids satellite trails better than a single long exposure. Use this calculator to find the per-frame maximum, then shoot as many frames as time allows.

Astrophotography Exposure Calculator

Name: Astrophotography Exposure Calculator
Availability: InStock
Author: Nham Vu

Find the maximum exposure time before stars trail, using the 500 Rule, NPF Rule, or your custom sky conditions.

Camera & Lens Settings

Calculation Method

Sensor Crop Factor

Focal Length (mm)

Recommended Exposure

Enter your settings and press Calculate.

Shooting Tips

1 Use manual focus and focus on a bright star at maximum magnification (Live View 10×) before starting your sequence.
2 Shoot RAW for maximum dynamic range and easier white balance correction in post.
3 Stack at least 20 frames to reduce noise significantly. Tools like DeepSkyStacker, Sequator, and Siril are free.
4 Shoot flat frames (with a light pad or white T-shirt over the lens) to correct vignetting and dust spots.
5 The NPF Rule is more accurate for sensors with pixel pitch below 5 µm — use it for modern 24+ MP cameras.

Summary