ZoomCam

Night-sky exposure lab

Measure star movement before it becomes a trail.

Compare the NPF and 500 rules, estimate motion at the sensor in pixels, and plan a complete night-sky sequence with time and storage totals.

Camera and sequence

Use physical lens and sensor values for the NPF estimate.

Physical millimeters.
Use 1 for full frame.
Enter the f-number.
Micrometers per pixel.
0 deg is the celestial equator.
Approximate pixels of motion.
Seconds per frame.
Seconds.
Megabytes.
Degrees of Earth rotation represented.
NPF Rule--Camera-specific estimate
500 Rule----
Conservative start--Shorter of NPF and 500
Planned star movement--Estimated sensor pixels

Exposure comparison

Longer bars represent longer shutter times for the entered camera.

NPF limit
--
500 Rule
--
Your exposure
--

Sequence and star-trail plan

Includes exposure time, gaps and the estimated file size entered.

Time for desired trail arc--
Total sequence duration--
Sequence storage--
Estimated movement in each frame--

Interpret the result

Both rules are starting points, not guarantees.

Sharper starsStart below the NPF time

High-resolution sensors reveal motion earlier, especially when viewed at full size.

More lightBalance shutter, aperture and ISO

A shorter shutter may require a wider aperture, higher ISO or multiple stacked frames.

Intentional trailsPlan the complete arc

Use many consistent frames with short gaps when building a smooth star-trail sequence.

Use pixel-level movement instead of one universal rule

The 500 Rule divides 500 by full-frame equivalent focal length. It is quick, but it ignores aperture and pixel density. The simplified NPF Rule uses focal length, aperture and pixel pitch, making it a more camera-specific starting point.

Why declination changes the result

Stars near the celestial equator appear to move faster across the frame than stars near the celestial poles. The calculator adjusts the estimated pixel movement using target declination, while limiting extreme values near the poles.

Test under real conditions

  • Take one frame and inspect stars at full size.
  • Shorten exposure when wind, vibration or tracking error adds blur.
  • Use consistent gaps and manual settings for a star-trail sequence.
  • Leave storage and battery margin for discarded frames.

More advanced planning tools

Moon and lightPlan sky direction, lunar phase and twilight windowsFraming visualizerEstimate the moon or distant subject size in frameStorage planningCalculate recording capacity from bitrate and duration

Take the plan outside with your phone.

ZoomCam provides mobile controls for distant and night-sky photo or video.

Download for iPhoneGet it for Android