π Astrophotography Exposure Calculator
Maximum shutter speed without star trails, via the 500 rule and the sharper NPF rule.
NPF rule maximum exposure (sharper result)
How to Use This Calculator
Stars move across the sky as the Earth rotates, and at some shutter speed that motion turns points of light into short streaks. This calculator tells you where that limit is for your exact lens, sensor, and aperture, using both the classic 500 rule and the more precise NPF rule. Enter your four numbers and use the NPF result as your field limit.
Enter the Focal Length of your lens in mm and select your Crop Factor. The calculator multiplies these to get the effective focal length, which is what both rules actually use.
Enter the Aperture f-number you plan to shoot at. Wider apertures (smaller f-numbers) let the NPF rule give you a slightly longer exposure, because the rule factors in lens speed.
Enter your Pixel Pitch in microns from your camera specs. If you do not know it, 5.9 is typical for 24 MP full frame, 4.3 for 45 MP full frame, and 3.8 for 61 MP full frame.
Compare the two results. The 500 rule time is the optimistic ceiling; the NPF time is the sharp-stars limit. Set your shutter at or below the NPF time, open the aperture, and dial ISO to taste.
The Formulas: 500 Rule vs NPF Rule
The 500 rule dates from the film era, when 35mm frames were scanned or enlarged less aggressively than today's 60 MP files are pixel-peeped. It divides the constant 500 by the effective focal length: a 24mm lens on full frame gives 20.8 seconds, a 14mm gives 35.7 seconds. It is fast mental math in the dark, and it is still useful as a rough ceiling.
The NPF rule is stricter because it models reality more carefully. Its numerator adds two terms: 35 times the aperture number and 30 times the pixel pitch in microns, all divided by the actual focal length. Denser sensors (smaller pixel pitch) record the star's drift sooner, so the rule shortens the exposure for high-resolution bodies. On a 61 MP camera at 24mm f/2.8, the NPF rule allows about 8.8 seconds while the 500 rule claims 21, and the stars in the longer shot will show visible trails at 100 percent zoom.
Worked Example
Setup: 24mm f/2.8 lens on a full-frame camera (crop factor 1.0) with 5.9 micron pixel pitch. 500 rule: 500 / (24 x 1.0) = 20.8 seconds. NPF rule: (35 x 2.8 + 30 x 5.9) / 24 = (98 + 177) / 24 = 11.5 seconds. The NPF limit is barely half the 500 rule, and it is the one that keeps stars pinpoint on this sensor.
Change one variable to see the sensitivity. Swap the body for a 61 MP camera (3.8 microns): NPF = (98 + 114) / 24 = 8.8 seconds. Move the same lens to an APS-C body (1.5x crop) with the original sensor density: effective focal length 36mm, 500 rule gives 13.9 seconds, and the NPF rule does not use crop factor directly, so it stays 11.5 seconds. This is exactly why the calculator shows both rules side by side.
Practical Tips
Treat the NPF time as a maximum, not a target. If your foreground is dark and noisy at the ISO you need, stacking multiple NPF-limited frames in software like Sequator or Starry Landscape Stacker beats one long trailed exposure every time. Ten 10-second frames stacked give you the light of 100 seconds with round stars.
Declination matters too: stars near the celestial pole move slowly across the frame while stars on the celestial equator streak fastest. If you are shooting the Milky Way core, the rules hold as stated. Pointing near Polaris buys you extra seconds beyond the calculator. Also remember these rules assume the camera is static on a tripod; a star tracker mount removes the limit entirely and is the real upgrade when you outgrow the rules.
Frequently Asked Questions
What is the 500 rule in astrophotography?
The 500 rule says your maximum shutter speed in seconds is 500 divided by your effective focal length (focal length times crop factor). A 24mm lens on a full-frame camera gives about 21 seconds. It is a quick field estimate, but it tends to overestimate on high-resolution sensors, which is why the NPF rule exists.
What is the NPF rule and is it more accurate?
The NPF rule accounts for your aperture and pixel pitch (sensor resolution) in addition to focal length: max seconds = (35 x aperture + 30 x pixel pitch) / focal length. Because denser sensors record star movement sooner, the NPF rule gives shorter, more realistic times for modern high-megapixel cameras. Most serious Milky Way shooters use it.
Do I need to use the crop factor with the 500 rule?
Yes. The rule uses effective focal length, so multiply the lens focal length by your crop factor: 1.5 for APS-C, 1.6 for Canon APS-C, 2.0 for Micro Four Thirds. A 24mm lens on APS-C behaves like 36mm, so the 500 rule gives about 14 seconds instead of 21.
What is pixel pitch and where do I find it?
Pixel pitch is the physical size of one photosite on your sensor, measured in microns. You can find it in your camera's spec sheet or calculate it: sensor width in mm x 1000 / image width in pixels. A 24 MP full-frame sensor has about 5.9 microns; a 61 MP full-frame sensor has about 3.8 microns. Smaller pitch means the NPF rule gives a shorter exposure.
Should I shoot wide open for the Milky Way?
Usually yes. A fast aperture like f/1.8 to f/2.8 gathers far more light in the short exposure the rules allow. The exposure calculator lets you enter your aperture so the NPF rule reflects your actual lens. If your lens is soft wide open, try f/2.8 or f/3.5 and compensate with higher ISO rather than a longer shutter that trails the stars.
What ISO should I pair with these shutter speeds?
Start around ISO 3200 to 6400 on modern full-frame cameras and check the histogram: the peak should sit in the left third, clearly off the left edge. High ISO does not create noise by itself; it amplifies the signal you captured. Underexposing at low ISO and brightening later creates far more noise than shooting at the right ISO in the first place.