TriCast · Camera Calculators
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What Is the NPF Rule for Star Photography?

More accurate than the 500 Rule for high-resolution cameras. The NPF Rule factors in aperture and pixel pitch so your stars stay sharp. TriCast calculates it for your exact setup.

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Three variables instead of one

Why the NPF Rule Gives Better Results on Modern Cameras

The 500 Rule only uses focal length. The NPF Rule adds aperture and pixel pitch to the calculation. Pixel pitch is the key: smaller pixels on high-megapixel sensors are more sensitive to trailing than larger pixels on lower-resolution sensors at the same shutter speed. A 45MP Sony and a 12MP Sony use the same focal length but need very different maximum shutter speeds to keep stars as points.

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N: Aperture (f-number)
Wider apertures (lower f-numbers) allow shorter exposures for the same brightness, reducing trailing risk. The NPF formula directly accounts for this tradeoff rather than treating all apertures as equivalent the way the 500 Rule does.
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P: Pixel Pitch (microns)
The physical size of each pixel on your sensor. Smaller pixels require shorter shutter speeds to keep stars as single-pixel points. This is why the 500 Rule fails on high-resolution sensors: it was calibrated for pixel sizes that are now outdated. TriCast looks up pixel pitch by camera body automatically.
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F: Focal Length (mm)
Same as the 500 Rule: longer focal lengths trail faster and require shorter shutter speeds. On a 500mm telephoto, stars trail in under a second. On a 14mm ultra-wide, you have significantly more time — but the NPF Rule will still give a shorter number than the 500 Rule at high resolution.
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When to Use It
Use the NPF Rule for any camera above 20 megapixels, for shots where you'll examine star sharpness at 100% crop, or when printing large. For a quick field estimate on a lower-resolution body, the 500 Rule is fast enough. TriCast shows both results side by side.
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TriCast Knows Your Camera's Pixel Pitch

To use the NPF Rule manually, you need your camera's pixel pitch in microns — a number most photographers don't have memorized. TriCast includes a camera database so it can look up pixel pitch by body, then calculate both the 500 Rule and NPF Rule results for your focal length and aperture instantly. No lookup tables, no mental math at 2am.

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App Exclusive
TriCast is in the iOS app, alongside StarCast, FogCast, CloudCast, GoldCast, and DroneCast. StarCast sends push alerts when night sky conditions score high for your location — arrive already knowing your shutter speed.
Common Questions
What is the NPF Rule?
A formula that calculates maximum shutter speed before star trailing using aperture, pixel pitch, and focal length. More accurate than the 500 Rule for cameras above 20MP. TriCast calculates it for any setup in the LightCast iOS app.
Why does the NPF Rule give shorter shutter speeds than the 500 Rule?
Because it accounts for pixel pitch. High-resolution sensors have smaller pixels that capture trailing more visibly than larger pixels on lower-resolution sensors. The 500 Rule doesn't know this and gives the same result regardless of megapixel count.
Should I use NPF or 500 Rule?
NPF for cameras above 20MP or any shot you'll examine closely. 500 Rule for a quick estimate on lower-resolution bodies. TriCast shows both so you can choose based on your camera and how critical the sharpness needs to be.
What is pixel pitch and where do I find it?
The physical size of each sensor pixel in microns. Calculated from sensor width divided by pixel count. TriCast looks it up for your camera body automatically — you just select your camera from the list.
What is LightCast TriCast?
Camera calculators covering the NPF Rule, 500 Rule, hyperfocal distance, depth of field, and more. Exclusive to the LightCast iOS app. $2.99/month after a 7-day free trial.
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