What is the 500 Rule?

The 500 Rule states that the maximum exposure time before stars visibly trail is 500 divided by the full-frame equivalent focal length (your focal length multiplied by the crop factor). For example, a 24mm lens on a 1.5× crop sensor gives an EFL of 36mm, so the max exposure is 500 ÷ 36 ≈ 13.9 seconds.

Why do I need to account for crop factor?

A crop sensor effectively magnifies the view compared to full frame. The higher apparent magnification means stars move across more pixels per second, so trailing becomes visible sooner. Multiplying the focal length by the crop factor gives the full-frame equivalent, which is what the 500 Rule is calibrated for.

What is the NPF Rule and when should I use it?

The NPF Rule (Nightscape Photography Formula) is a more detailed estimate: (35 × aperture + 30 × pixel pitch in µm) ÷ physical focal length. Crop factor affects the calculated pixel pitch; using equivalent focal length again would count the crop twice. Use NPF when sensor resolution and aperture should be included in the estimate.

What is pixel pitch and how is it calculated?

Pixel pitch is the physical size of each sensor pixel in micrometers. It is derived from the sensor area divided by the total pixel count. This calculator estimates pixel pitch from your sensor format (which determines the sensor area based on a 3:2 aspect ratio) and your megapixel count.

Why does a narrower aperture allow a longer exposure under the NPF Rule?

A narrower aperture has a higher f-number and a larger diffraction disk. In the NPF formula, that raises the 35 × aperture term and therefore the estimated exposure limit. A wider aperture has a lower f-number and produces the opposite result.

Does the 500 Rule work for all sensor sizes?

It was originally calibrated for full-frame 35mm film at typical print sizes. Modern high-resolution sensors can resolve finer detail, making star trailing visible at shorter exposures than the 500 Rule predicts. On sensors above 24 MP, the NPF Rule gives more reliable results. On sensors below 20 MP, the 500 Rule is generally sufficient.

500 Rule Astrophotography Calculator

Enter your focal length and crop factor to get the maximum exposure time before stars visibly trail, using the 500 Rule or the more accurate NPF Rule.

Camera & Lens

Sensor Format

Focal Length (mm)

Calculation Method

500 Rule NPF Rule Both

Set your camera and lens, then click Calculate.

Full-Frame Equivalent Focal Length

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Focal length × crop factor — used by the 500 Rule. NPF uses the lens's physical focal length.

500 Rule Quick estimate

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Max shutter speed before star trailing is visible

NPF Rule More accurate for high-res sensors

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Max shutter speed before star trailing is visible

500 Rule — Common Focal Lengths

Focal Length	1.0× (FF)	1.5× (APS-C)	2.0× (MFT)

Times in seconds. Highlighted row is your current focal length.

Summary

The 500 Rule gives astrophotographers a quick formula for the longest shutter speed that keeps stars as points: divide 500 by the full-frame equivalent focal length. This calculator applies that formula with crop factor correction and also offers the NPF Rule — a more precise formula that accounts for aperture and sensor pixel pitch — so you can compare both methods and choose the right exposure for your camera and lens combination.

How it works

Select your camera sensor format to set the crop factor (or enter a custom value).
Enter your lens focal length in millimeters.
For the NPF Rule, also enter your aperture (f-stop) and sensor megapixel count.
Choose between the 500 Rule (quick) and the NPF Rule (more accurate).
The calculator displays the effective focal length and the maximum safe exposure time.
Compare both results side by side to decide how conservative to be.

Use cases

Find the maximum shutter speed for sharp Milky Way photos without star trails.
Compare the 500 Rule and NPF Rule for your specific camera body.
Quickly check the exposure limit when switching between lenses in the field.
Plan astrophotography shoots and configure your intervalometer.
Understand how crop factor changes the effective exposure limit.
Verify that a recommended exposure from a guide matches your actual gear.

Frequently Asked Questions

Last updated: 2026-06-14 · Reviewed by Nham Vu