Star Trails Max Shutter Guide

What it does

Calculates the longest shutter speed you can use before stars visibly trail into streaks rather than staying as points, using three different rules of thumb of increasing strictness: the simple 500 Rule (focal length only), the stricter 400 Rule (same formula, tighter constant for modern sensors), and the tightest NPF Rule, which also accounts for aperture and how densely packed your sensor's pixels are.

What you'll need

• Focal length in mm and sensor format, for the 500 and 400 Rules.
• Aperture and sensor resolution, additionally, for the more precise NPF Rule.

Steps

1. Enter your focal length and sensor format.
2. For the tighter NPF result, also enter your aperture and pick the closest sensor resolution.
3. Use whichever result fits how strict you want to be about pinpoint stars.

Reading the result

All three results are the longest single-exposure shutter speed before trailing becomes visible at normal viewing size, from most permissive (500 Rule) to strictest (NPF Rule). The NPF Rule is generally the most accurate, especially on high-resolution sensors where trailing becomes visible sooner than the simpler 500 or 400 Rules suggest — if they disagree, trust the NPF result for genuinely pinpoint stars.

Tips

• If you want intentional long star trails (streaks across the whole frame) rather than pinpoint stars, ignore both numbers and expose for much longer instead — or better, stack many shorter exposures (each at or under the max shutter shown here) in software afterwards, which avoids the sensor noise and battery drain of one very long exposure and gives you more control over the final trail length.
• Wider focal lengths and full-frame sensors generally allow longer shutter speeds before trailing than telephoto lenses or cropped sensors — the sky appears to move faster through a narrower, more magnified field of view.

What are the 500, 400 and NPF Rules?

All three are rules of thumb for the longest single-exposure shutter speed you can use before stars — which are actually moving across the frame the whole time, thanks to Earth's rotation — visibly smear into short streaks instead of staying as sharp points. The 500 Rule is the oldest and simplest: divide 500 by your focal length, adjusted for crop factor. The 400 Rule uses the same formula with a stricter constant, reflecting that modern high-resolution sensors reveal trailing sooner than the 500 Rule assumed. The NPF Rule is the tightest of the three, additionally accounting for your aperture and how densely packed your sensor's pixels are.

Why photographers use it

Night sky and astro photographers generally want pinpoint stars, not streaks, in a typical single-frame Milky Way or star-field shot. Going even a few seconds past the safe shutter speed for your specific setup turns crisp points of light into visibly elongated smears — especially once the image is viewed at full size or printed large, where a shortfall that looked fine on a phone screen suddenly doesn't.

Which rule should I trust — 500, 400 or NPF?

The 500 Rule is quick to work out in your head in the field, but was calibrated for older, lower-resolution sensors — treat it as a rough, often slightly optimistic starting point on a modern camera. The 400 Rule is a safer default with barely more effort, using the same simple formula. The NPF Rule is the most accurate of the three, especially on modern high-resolution sensors where trailing becomes visible sooner than either the 500 or 400 Rule suggests. When they disagree and you want genuinely pinpoint stars, trust the NPF result — the extra margin the simpler rules give up can still be a reasonable trade if you're deliberately fine with very slightly softer stars in exchange for more light per frame.

Common mistakes

• Using the 500 or 400 Rule on a modern high-megapixel body without adjustment. neither rule knows your sensor's actual resolution. A 45-60MP body reveals trailing at noticeably shorter shutter speeds than either assumes — the NPF Rule accounts for this, the simpler rules don't.
• Forgetting to apply crop factor. the 500 and 400 Rules both need the full-frame-equivalent focal length. Plugging in a lens's physical focal length on a crop-sensor body without adjusting for the crop factor gives an overly generous, wrong answer.
• Maxing out the allowed shutter speed and still getting an underexposed frame. the max-shutter number is a trailing limit, not an exposure target. If the frame is still too dark at that shutter speed, the fix is a wider aperture or higher ISO — not pushing the shutter speed past the limit.
• Ignoring where in the frame the stars actually are. trailing is most visible near the celestial equator and least visible near the poles (close to Polaris in the northern hemisphere, or the south celestial pole). A composition weighted toward one area can tolerate a slightly different shutter speed than the blanket rule suggests.
• Treating the number as a hard pass/fail line. trailing is a gradual effect, not a cliff edge. A shutter speed just over the limit may still look acceptable at normal viewing size or downsized for social media, while looking soft at a 100% crop.

Example settings

FAQ

Why do stars trail at all if my shutter speed feels "fast"?

Earth's rotation moves the sky continuously, and even a 10-20 second exposure is long enough at typical astro focal lengths for that motion to become visible at the pixel level — it's a much smaller margin than daytime handheld-shake shutter speeds.

Which rule is most accurate?

The NPF Rule generally, since it accounts for more of what actually determines visible trailing — aperture and pixel density — rather than focal length alone. The 400 Rule is a reasonable middle ground if you want something almost as quick to eyeball as the 500 Rule but a bit safer.

What if I want intentional long star trails instead of pinpoint stars?

Ignore both numbers and either expose for much longer in one shot, or, better, stack many shorter exposures — each at or under the max shutter shown here — in editing software afterward. Stacking avoids the extra noise and battery drain of one very long exposure and gives you more control over the final trail length.

Does a wider lens really let me use a longer shutter speed?

Yes — through a wider, less magnified field of view, the sky's apparent movement is less noticeable per pixel, so wider focal lengths and full-frame sensors generally tolerate longer shutter speeds before trailing shows.

Do I need my sensor's exact pixel pitch, or is the preset list close enough?

The preset list, grouped by common sensor format and resolution combinations, is close enough for the NPF Rule's purposes — this is already an approximation tool, not a lab-grade calculation, so exact pixel pitch to the micron isn't necessary.