What is pixel pitch and how does it affect star trailing?

Pixel pitch is the physical size of each pixel on the camera sensor, measured in micrometres (µm). Smaller pixels capture finer detail but also show star motion more readily. A 45-megapixel full-frame sensor has smaller pixels than a 12-megapixel sensor of the same physical size, so it requires a shorter maximum shutter speed to avoid visible trailing. The NPF rule accounts for pixel pitch directly.

NPF Rule Calculator — Astrophotography Exposure Calculator

Q: What is the difference between the NPF rule and the 500 rule?

The 500 rule (500 ÷ focal length) was developed for film cameras and early digital sensors with large pixels. It does not account for aperture or pixel size. On modern cameras with 24+ megapixels, the 500 rule often produces exposures that show visible star trailing when viewed at full resolution. The NPF rule adds aperture and pixel pitch to the calculation, giving a shorter, more accurate maximum exposure for high-resolution sensors.

Q: What focal length is best for Milky Way photography?

Wide-angle lenses between 14mm and 24mm are most commonly used for Milky Way photography. Wider focal lengths allow longer exposures before trailing and capture more of the galactic arc in a single frame. A 14mm f/1.8 or 20mm f/1.8 on a full-frame camera is a common choice among professional astrophotographers.

Q: How do I avoid star trails in astrophotography?

To avoid star trails, keep your shutter speed at or below the value calculated by the NPF rule for your camera and lens combination. Use a wide aperture (f/1.4 to f/2.8), a high ISO (1600 to 6400), and a wide-angle lens to compensate for the shorter exposure time. Mount your camera on a sturdy tripod and use a remote shutter release or self-timer to avoid camera shake.

You set up your tripod, nail the composition, get home, open the RAW file at 100% zoom — and there they are. Elongated stars. Not terrible, but not round. You were close. The 500 rule said 20 seconds, you shot 20 seconds, and it still trailed.

This is the problem the NPF rule solves. Use the calculator on the right to get your number, then keep reading to understand why it works.

The NPF rule formula

The NPF rule was developed by photographer Frédéric Michaud as a more accurate replacement for the 500 rule. It accounts for three variables that the 500 rule ignores: aperture, pixel size, and the declination of your target.

t = (35 × N + 30 × p) ÷ f

tMax shutter (seconds)

NAperture (f-number)

pPixel pitch (µm)

fEffective focal length (mm)

Pixel pitch (p) is the physical size of each pixel on your sensor in micrometres. It's calculated from your sensor's physical dimensions and megapixel count. A Sony A7R V (61MP full-frame) has a pixel pitch of about 3.76µm, while a Sony A7S III (12MP full-frame) has a pixel pitch of about 8.37µm — the higher-resolution sensor needs a shorter shutter speed to keep stars round.

The formula above is simplified for the Milky Way's galactic core, which sits at approximately -29° declination. The full NPF formula includes a cosine correction for declination. At Dec -29°, cos(-29°) ≈ 0.875, which is already factored into the constant 35 in the simplified version. For targets near the celestial equator (Dec 0°), results will be slightly more conservative than necessary — but for Milky Way photography this version is exactly right.

NPF vs. the 500 rule: real numbers

Here's how the two rules compare on common camera and lens combinations. All examples assume the galactic core as the target.

Camera	Lens	500 Rule	NPF Rule	Difference
Sony A7 III (24MP FF)	20mm f/1.8	25s	22s	-3s
Sony A7R V (61MP FF)	20mm f/1.8	25s	12s	-13s
Nikon Z6 II (24MP FF)	14mm f/2.8	35s	28s	-7s
Nikon Z8 (45MP FF)	14mm f/2.8	35s	19s	-16s
Canon R6 Mark II (24MP FF)	24mm f/1.4	20s	18s	-2s
Canon R5 (45MP FF)	24mm f/1.4	20s	13s	-7s
Sony A6700 (26MP APS-C)	16mm f/2.8	20s	15s	-5s

The difference is small on older 24MP bodies at wide angles and grows dramatically on high-resolution sensors or longer focal lengths. If you're shooting on a 45MP or higher camera and using the 500 rule, you're almost certainly getting soft stars at 100% zoom — the NPF rule will fix that.

How to use the result

The NPF calculator gives you the theoretical maximum based on sensor geometry. In practice, treat it as the starting point for a bracket, not a hard ceiling.

Start at the NPF value and shoot one frame
Zoom to 100% on your LCD and check a corner star — corners are always the first place trailing appears
Add 2 seconds per subsequent frame until you see elongation, then back off one step
Conditions vary: atmospheric turbulence and lens aberrations at wide apertures can make stars appear to trail at shorter times than predicted
Shoot RAW so you can evaluate trailing accurately at full resolution in post

ISO and aperture strategy

Because the NPF rule often gives you a shorter maximum shutter than the 500 rule, especially on high-res bodies, you'll want to compensate with aperture and ISO.

Use the widest aperture your lens allows

A wider aperture does two things: it gathers more light per second, and it also increases the NPF result (wider aperture = longer allowable shutter). An f/1.8 lens gives you roughly double the light of an f/2.8 lens and also allows a longer exposure. For Milky Way work, f/1.4 to f/2.8 is the practical range. Going wider than f/2.8 starts to introduce significant coma on most lenses — test your specific glass to find its best aperture for sharp stars.

Raise ISO to compensate for shorter shutter

On a high-resolution camera where NPF gives you 12 seconds instead of the 500 rule's 25, you'll need to push ISO higher to maintain exposure. Modern full-frame sensors handle ISO 3200 to 6400 well. Shoot RAW and evaluate noise in your own editing environment rather than on the camera's LCD. Many photographers find that a slightly underexposed shot at ISO 3200 is more recoverable than a properly exposed shot at ISO 6400.

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In the Milky Way Tonight app: the exposure calculator is built in and automatically uses your sensor's pixel pitch for the NPF calculation. Open the Photography Guide, tap Exposure Calculator, select your camera and lens, and get your number before you leave home — no mental arithmetic in the field at midnight.

Frequently asked questions

What is the NPF rule in astrophotography? ▾

The NPF rule is a formula for calculating the maximum shutter speed before stars begin to trail in a photograph. It was developed as a more accurate replacement for the older 500 rule. NPF accounts for three variables: aperture (N), pixel pitch (P), and focal length (F). The formula is: shutter speed = (35 × aperture + 30 × pixel pitch in µm) ÷ effective focal length. It is the standard recommendation for modern digital cameras with 24 megapixels or more.

What is the difference between the NPF rule and the 500 rule? ▾

The 500 rule divides 500 by focal length to get a maximum shutter speed. It was developed for film and early digital sensors with large pixels (6µm and above). It does not account for aperture or pixel size, so on modern high-resolution sensors it often produces exposures where trailing is visible at full zoom. The NPF rule adds aperture and pixel pitch to the calculation. On a 24MP camera the difference is a few seconds. On a 45MP or 61MP camera, the NPF rule can be 10 to 15 seconds shorter than the 500 rule at the same focal length.

What is pixel pitch and why does it matter? ▾

Pixel pitch is the physical size of each individual pixel on a camera sensor, measured in micrometres (µm). Two sensors of the same physical size with different megapixel counts will have different pixel pitches. A 12MP full-frame sensor has pixels around 8.4µm wide. A 61MP full-frame sensor has pixels around 3.76µm wide. Smaller pixels capture finer detail but also record the motion of stars more readily, requiring a shorter maximum shutter speed to keep stars round. The NPF rule accounts for pixel pitch to give you the correct result for your specific camera.

What focal length is best for Milky Way photography? ▾

Wide-angle lenses between 14mm and 24mm on full-frame are the most common choice for Milky Way photography. Wider focal lengths allow longer exposures before trailing and capture more of the galactic arc in a single frame. A 14mm to 20mm f/1.8 or faster is the most common professional setup. On APS-C cameras, 10mm to 16mm is equivalent. Going wider than 14mm on full-frame can introduce strong distortion and vignetting depending on the lens. The SIGMA 14mm f/1.4 DG DN Art and SIGMA 20mm f/1.4 DG DN Art are widely used in the astrophotography community for their correction of coma at wide apertures.

How do I stop stars from trailing in my photos? ▾

Use the NPF rule (calculator above) to find the maximum shutter speed for your camera and lens. Stay at or below that value. Use a wide aperture (f/1.4 to f/2.8), a high ISO (1600 to 6400), and a wide-angle lens to compensate for the shorter exposure time. Mount your camera on a sturdy tripod and use a remote shutter release or 2-second self-timer to avoid camera shake introducing its own blur. Zoom to 100% on your LCD and check a corner star after your first shot.

Does the NPF rule work for APS-C and Micro Four Thirds cameras? ▾

Yes. The NPF rule works for any sensor format. The calculator above includes APS-C (Nikon, Pentax, Sony, Fuji 1.5× crop), APS-C Canon (1.6× crop), and Micro Four Thirds (2× crop). It uses each format's actual sensor width and typical megapixel count to calculate pixel pitch. For crop-sensor cameras, the effective focal length used in the formula is the actual focal length multiplied by the crop factor — the calculator handles this automatically.

NPF Rule Calculator:No More Star Trails