Home/Astronomy & Astrophysics/Astrophotography 500 Rule

Formula & Calculator

Astrophotography 500 Rule

Rule-of-thumb maximum exposure time (in seconds) before stars begin to visibly trail in a wide-field night-sky photo.

Written by the Vorzep Team

AstronomyAstrophotographyPractical

Astrophotography 500 Rule Calculator

tmax = 500 / ( f · CF )
Solve for tmax, f, or CF
tmax⟷f, CF
s
mm
—
Solve for:
Result —
Copied!
Max Exposure vs. Focal Length tmax(f) = 500 / (f · CF)
tmax(f) for fixed CF Computed point
f > 0, CF > 0 • tmax in seconds

Variables

SymbolQuantityUnit
t_maxMaximum exposure times
focal_lengthLens focal lengthmm
crop_factorCamera sensor crop factor

What it means

The 500 rule is a rule of thumb for untracked astrophotography to estimate the longest exposure time (in seconds) before the motion of the stars becomes noticeable as trailing. It divides 500 by the effective focal length (focal length × crop factor for digital cameras). It assumes a full‑frame sensor and provides a starting point. It is used by astrophotographers to capture sharp star images without a tracking mount. The rule is approximate; a more conservative 200‑300 rule may be preferred for high‑resolution sensors. Understanding this helps beginners avoid star trails and plan exposures.

Worked example

Astrophotography 500 Rule – Two Detailed Examples

Real‑World
Scenario: An astrophotographer is using a full‑frame camera (crop factor 1.0) with a 24 mm lens. They want to capture a wide‑field Milky Way image without star trailing. Using the 500 rule, t_max = 500 / (focal_length × crop_factor) = 500 / (24 × 1) = 20.83 seconds. They set their exposure time to 20 seconds, which keeps stars as points and yields a stunning image.
ParameterValue
focal_length (mm)24
crop_factor1.0
1t_max = 500 / (24 × 1) = 20.83 s
Result ~20.8 s ✓ Max exposure without trailing
Scenario: A photographer using a crop‑sensor camera (crop factor 1.5) with a 50 mm lens wants to capture a star field. They compute t_max = 500 / (50 × 1.5) = 500 / 75 = 6.67 seconds. They use a 6‑second exposure, which prevents star trails and allows them to capture a crisp image of a bright constellation.
ParameterValue
focal_length50
crop_factor1.5
1t_max = 500 / (50 × 1.5) = 6.67 s
Result ~6.7 s ✓ Safe exposure
Insight: The 500 rule is a simple guideline to avoid star trails. It assumes a full‑frame sensor and works best for wide‑angle lenses. For modern high‑resolution sensors, a stricter rule (e.g., 400 or 300) is sometimes recommended.

Common mistakes

  • 500 rule: t_max = 500 / (focal_length × crop_factor) – to avoid star trails.
  • Focal length: In mm – for full‑frame (crop_factor=1).
  • Crop factor: For APS‑C (~1.5), Micro 4/3 (~2).
  • Result: Maximum exposure time in seconds.
  • Rule of thumb: Not precise – use NPF rule for better accuracy.

Applications

The 500 rule for astrophotography, t_max = 500 / (focal_length * crop_factor), estimates the maximum exposure time before star trails become noticeable due to Earth's rotation. This is a simple guideline for untracked astrophotography. Amateur astrophotographers use it to set camera exposures for Milky Way and wide‑field shots. A shorter focal length allows longer exposures without trailing. While not precise (the NPF rule is more accurate), it is a quick starting point. By using the 500 rule, photographers can capture sharp stars without a tracking mount.

  • Untracked astrophotography – setting exposure times for Milky Way, constellations
  • Planning for star trail photography
  • Comparison of lenses for sky photography
  • Education on Earth's rotation effects
  • Quick field guide for beginner astrophotographers