NPF, standard starting point
—
More forgiving, for normal-sized viewing. Try a shorter exposure for large prints.
Gear Lab · Night-sky photography
Untracked Star Exposure Planner
Find a practical starting shutter speed for Milky Way and star-field photos on a fixed tripod. Compare the NPF guideline, a strict pixel-drift limit, and the older 500 rule.
No account needed
Calculates on your device
No camera data uploaded
Your camera and sky
Build the shot.
Your results
How long can you expose?
These are planning estimates for a stationary tripod, not a tracked equatorial mount.
NPF, strict
—
More conservative for inspecting stars closely.
Your pixel-drift limit
—
When predicted drift reaches the tolerance you selected. This is a separate, stricter geometric check.
500 rule, comparison only
—
Older shortcut, often too long for modern high-resolution cameras.
Image scale
—
Sky drift speed
—
Drift at your shutter
—
Method and limits
Understand the number before using it.
The NPF estimate uses lens aperture (N), focal length (F), pixel pitch (P), and the minimum absolute declination in the frame. The standard setting permits more apparent star movement than the strict setting. Image scale and drift are separate geometric calculations based on Earth's sidereal rotation. ISO changes brightness, not how far a star moves.
NPF is a starting point
We use the approximate Michaud expression: t = k × (16.856N + 0.0997F + 13.713P) / (F × cos δ). k = 2 for standard and k = 1 for strict. Check a test photo at the size you plan to display it.
Pixel drift is geometric
Image scale ≈ 206.265 × P / F arcseconds per pixel. Drift ≈ 15.041 × cos δ arcseconds per second. At wide angles, the corners can contain faster-moving stars than the center.
The 500 rule is older
500 ÷ (focal length × crop factor) ignores aperture and pixel size. We show it to explain why old advice may allow visible trailing, not as the recommended exposure.
Field check: Use declination 0° when you do not know the minimum declination of stars in the frame. Focus carefully, take a short test frame, zoom in on corner stars, and shorten the shutter if you see elongation. Lens coma, wind, tripod movement, tracking, and camera processing can all change what looks sharp. At declinations very near the pole, this simple model is less useful, so the input is limited to 85°.
Reference reading: PhotoPills Milky Way photography guide and PhotoPills' NPF explanation. StarStation's numbers are independently calculated, not PhotoPills app results.
Build the rest of the imaging setup
Keep planning.
Frame your target, estimate battery runtime, and compare camera and telescope combinations before you head into the field.