Astrophotography Exposure Calculator
Calculate maximum exposure time before star trailing from focal length and sensor using the 500 rule.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
500 Rule: t = 500 / (Focal Length x Crop Factor)
The 500 Rule divides 500 by the effective focal length to give maximum exposure in seconds. The 300 Rule uses 300 for a more conservative limit. The NPF Rule uses (35 + 30 x pixel_size) / (effective_focal_length x cos(declination)) for pixel-accurate results. All three rules aim to keep star trailing below a visible threshold.
Worked Examples
Example 1: Wide-Angle Milky Way on APS-C
Problem:Calculate maximum exposure for a 24mm lens on APS-C camera (1.5x crop, 4.3 micron pixels) pointed at the celestial equator.
Solution:Effective focal length: 24 x 1.5 = 36mm\n500 Rule: 500 / 36 = 13.9 seconds\n300 Rule: 300 / 36 = 8.3 seconds\nNPF Rule: (35 + 30 x 4.3) / (36 x cos(0)) = 164 / 36 = 4.6 seconds\nImage scale: (4.3 / 36) x 206.265 = 24.6 arcsec/pixel\nFor 30 min integration at NPF: 30 x 60 / 4.6 = 391 frames
Result:500 Rule: 13.9s | 300 Rule: 8.3s | NPF Rule: 4.6s
Example 2: Telephoto Deep Sky on Full Frame
Problem:Calculate max exposure for 200mm on full-frame (crop 1.0, 5.9 micron pixels) targeting M31 at +41 degrees declination.
Solution:Effective focal length: 200 x 1.0 = 200mm\n500 Rule: 500 / 200 = 2.5 seconds\n300 Rule: 300 / 200 = 1.5 seconds\nNPF Rule: (35 + 30 x 5.9) / (200 x cos(41)) = 212 / 150.9 = 1.4 seconds\nDeclination benefit: cos(41) = 0.755, allowing ~32% longer than equator\nImage scale: (5.9 / 200) x 206.265 = 6.08 arcsec/pixel
Result:500 Rule: 2.5s | 300 Rule: 1.5s | NPF Rule: 1.4s | Tracker recommended
Frequently Asked Questions
What is the 500 Rule in astrophotography?
The 500 Rule is a simple guideline for calculating the maximum exposure time before star trailing becomes visible in an untracked astrophotography image. The formula divides 500 by the effective focal length (focal length multiplied by the sensor crop factor) to yield the maximum exposure in seconds. For a 50mm lens on a full-frame camera: 500/50 = 10 seconds maximum. For a 200mm lens on an APS-C sensor with 1.5x crop: 500/(200 x 1.5) = 1.67 seconds. The rule was developed in the film era when grain masked small amounts of trailing, and it tends to be overly generous for modern high-resolution digital sensors where even slight trailing becomes visible when images are viewed at full resolution. Many astrophotographers now prefer the more conservative 300 Rule or the pixel-based NPF Rule.
Why do stars trail in long exposure photographs?
Star trailing occurs because Earth rotates on its axis at approximately 15 degrees per hour (one full 360-degree rotation every 23 hours and 56 minutes), causing celestial objects to appear to move across the sky from the perspective of a camera fixed to the ground. During a long exposure, this apparent motion records as streaks rather than points of light. The rate of trailing depends on the star declination (celestial latitude): stars near the celestial equator trail the fastest at 15.04 arcseconds per second, while stars near the celestial poles trail progressively slower and Polaris barely moves at all. Longer focal lengths magnify this motion proportionally, which is why wide-angle lenses allow much longer exposures than telephoto lenses before trailing becomes visible.
What is the crop factor and how does it affect astrophotography exposure times?
The crop factor describes how much smaller a camera sensor is compared to a full-frame (35mm) sensor, and it directly affects the effective field of view and maximum exposure time. APS-C sensors have a crop factor of approximately 1.5x (Nikon/Sony) or 1.6x (Canon), Micro Four Thirds sensors have 2x, and full-frame sensors have 1x. When calculating maximum exposure using the 500 Rule or similar formulas, you must multiply the lens focal length by the crop factor to get the effective focal length. A 100mm lens on an APS-C body acts like a 150mm lens on full-frame in terms of both field of view and star trailing sensitivity. This means smaller sensor cameras require shorter maximum exposures at the same focal length, reducing the signal gathered per frame and potentially requiring more frames for the same total integration time.
How does declination affect maximum exposure time?
Declination is the celestial equivalent of latitude, measuring how far north or south an object is from the celestial equator, and it significantly affects how fast stars appear to trail across the sensor. Stars at the celestial equator (0 degrees declination) move at the maximum rate of 15.04 arcseconds per second of time. As declination increases toward the celestial poles, the apparent motion decreases by the cosine of the declination angle. At 45 degrees declination, trailing speed is reduced to about 70 percent of equatorial rate. At 60 degrees, it drops to 50 percent, and near the pole at 85 degrees, it is only about 9 percent. This means you can use longer exposures when photographing objects near the poles, such as the North America Nebula near Polaris, compared to equatorial targets like the Orion Nebula.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy