The calculator
Maximum shutter speed for stars without trails
Change either input and results update instantly. Results are rules of thumb — see the formulas section for what they assume.
Lens & sensor
In millimeters — the number printed on your lens. Wide-angle lenses (14–24 mm) are most common for Milky Way work.
Sets the crop factor — see the table below for all five constants.
500-rule limit:
How it works — the formulas in full
The 500 and 300 rules are rules of thumb, not precise physics. They give you a practical field limit for shutter time by dividing a constant into the full-frame-equivalent focal length of your lens-and-sensor combination.
Variable definitions
What each symbol means
f — focal length in millimeters, as printed on the lens.
CF — crop factor (dimensionless). Accounts for sensor size relative to a 35 mm full-frame sensor. Full frame = 1.0; see the crop-factor table below for all five values.
feff — full-frame-equivalent focal length = f × CF. This is the effective angle-of-view multiplier that controls how fast stars cross the frame.
t500 — maximum shutter time in seconds under the 500 rule.
t300 — maximum shutter time in seconds under the stricter 300 rule.
Equations
f_eff = f × CF
All lengths in mm. A 20 mm lens on a full-frame body (CF = 1.0) gives f_eff = 20 mm. The same lens on an APS-C 1.5× body (CF = 1.5) gives f_eff = 30 mm — a narrower angle of view — so stars cross the frame faster relative to frame width.
t_500 = 500 / f_eff = 500 / (f × CF)
Result in seconds. A practical limit for casual Milky Way photography and lower-resolution bodies (under ~24 MP). Stars are acceptably sharp at normal screen sizes and smaller prints, but may show faint trailing at 100% pixel view.
t_300 = 300 / f_eff = 300 / (f × CF)
A tighter limit recommended for cameras of 24 megapixels or more, for images that will be viewed at 100% zoom, or for large-format prints where pixel-level sharpness matters. Stars will remain point-like at full-resolution zoom.
A note on the NPF rule. The 500 and 300 rules assume a fixed constant divided by effective focal length. A more precise modern alternative — the NPF rule, named for aperture (N), pixel pitch (P), and focal length (F) — also accounts for your lens aperture, the pixel density of your specific sensor, and the declination of the stars in your frame. NPF produces a tighter, camera-specific limit that can be 30–50% shorter than the 500 rule on a dense-pixel modern body. If you are shooting for large prints or professional publication, the NPF rule (available in PhotoPills and dedicated apps) is worth calculating. For most field use and casual Milky Way photography, the 500 and 300 rules are fast and reliable starting points.
Crop factors by sensor format
These are the five crop-factor constants used by this calculator. The active row highlights when you select a format above.
| Sensor format | Crop factor (CF) | Typical cameras | 500-rule at 20 mm |
|---|---|---|---|
| Full frame (35 mm) | 1.0× | Sony A7, Nikon Z6/Z7, Canon R5/R6 | 25.0 s |
| APS-C 1.5× | 1.5× | Nikon Z50, Sony A6xxx, Fuji X series | 16.7 s |
| Canon APS-C 1.6× | 1.6× | Canon Rebel, R50, R10, 90D | 15.6 s |
| Micro Four Thirds | 2.0× | Olympus OM-D, Panasonic G series | 12.5 s |
| 1-inch sensor | 2.7× | Sony RX100 series, Nikon 1 | 9.3 s |
500-rule values above use a 20 mm lens as a representative wide-angle example: 500 ÷ (20 × CF). These are rules of thumb. All five crop-factor constants are fixed labeled values — not derived or estimated here.
Worked example — step by step
A 20 mm lens on a full-frame camera (crop factor 1.0) — these are the calculator's default inputs. Every figure below can be reproduced by hand.
Inputs
20 mm · Full frame (crop 1.0×)
f = 20 mm | CF = 1.0 (full frame) | feff = 20 × 1.0 = 20 mm
Step 1 — Full-frame-equivalent focal length
f_eff = f × CF
f_eff = 20 mm × 1.0 = 20 mm
Because this is a full-frame camera, the crop factor is 1.0 and the effective
focal length equals the physical focal length.
Step 2 — 500-rule maximum shutter time
t500 = 500 / f_eff
t500 = 500 / 20 = 25.0 seconds
At shutter times beyond 25 s, stars will begin to trail visibly at normal
viewing sizes on this lens-and-body combination.
Step 3 — 300-rule maximum shutter time
t300 = 300 / f_eff
t300 = 300 / 20 = 15.0 seconds
The stricter limit — use this when shooting at high resolution (24 MP+) or
when you will view the image at 100% zoom or print it large.
Step 4 — Interpret and use the results
Set your camera shutter to 25 s (500 rule) as a starting point. If you are shooting on a 24 MP or higher body and plan to pixel-peep or make large prints, try 15 s (300 rule) instead. Take a test shot, zoom to 100% on the camera LCD, and check whether stars appear as sharp points — move shorter if you see any elongation. Wide aperture (f/1.8 to f/2.8) and high ISO let you achieve proper exposure within either limit.
Step 5 — Crop-sensor example for comparison
Same 20 mm lens on an APS-C 1.5× body:
f_eff = 20 × 1.5 = 30 mm
t500 = 500 / 30 = 16.7 s |
t300 = 300 / 30 = 10.0 s
The narrower angle of view on the crop sensor means stars sweep across the
frame more quickly, so the allowed shutter time drops.
Common mistakes with the 500 rule
These are the errors that lead to disappointing star photos or wasted shots in the field.
Forgetting to multiply by the crop factor
On a crop sensor, using the raw focal length in the denominator — rather than the full-frame-equivalent focal length — makes your limit too generous. A 20 mm lens on an APS-C 1.5× body behaves like a 30 mm on full frame. Dividing into 500 without the crop factor gives 25 s; the correct limit is 500 ÷ 30 ≈ 16.7 s. The extra 8-plus seconds will produce visible trailing on any crop body.
Trusting the 500 rule on a 45 MP body when pixel-peeping
The 500 rule was developed when 12–16 MP sensors were common. On a modern 45 MP or 60 MP sensor, each pixel is much smaller, and a star trail that would span sub-pixel distances on an older body now spans several pixels clearly. At high resolution, even the 300 rule may show faint trailing at 100% zoom. Use the NPF rule, or bracket your shutter times and check at full zoom in the field.
Assuming wider aperture lets you extend the shutter beyond the rule
Opening the aperture increases the light per second but does not change how fast stars cross the sensor — the trailing limit is purely geometric, driven by Earth's rotation and your field of view. A wider aperture means you can get a properly exposed frame at a shorter shutter time, which actually helps you stay under the 500-rule limit rather than letting you push past it.
Applying the same limit to the entire sky
The 500 and 300 rules assume stars near the celestial equator, where apparent motion is fastest. Stars near the poles move more slowly — Polaris barely moves at all. So the rules are worst-case for equatorial stars and conservative elsewhere. If your frame is pointed toward the galactic core (typically low and south in the Northern Hemisphere), you are near the equatorial worst case and the rules are approximately right. Pointing near Polaris, you may have more time than the rule suggests.
Using a zoom lens set to a different focal length without recalculating
If you use a zoom lens and change focal length in the field, the 500-rule limit changes. A 14–24 mm zoom at 14 mm gives a much more generous limit than the same zoom at 24 mm. Recalculate whenever you reframe with a zoom, or default to the limit for the longest focal length you intend to use in a session.