Star Trail Calculator
Calculate exposure settings and rotation time for star trail photography. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Max Exposure = 500 / (Focal Length x Crop Factor) | Shoot Time = Trail Degrees / 15.04 degrees per hour
The 500 Rule determines maximum single-frame exposure before stars trail. Earth rotates at approximately 15.04 degrees per hour (360 degrees in 23h 56m 4s sidereal day). Total shooting time equals desired trail length in degrees divided by rotation rate. Total frames equals shooting time divided by individual exposure time.
Worked Examples
Example 1: Quarter-Circle Star Trails with 24mm Lens
Problem:Calculate the shooting time and frame count for 90-degree star trails using a 24mm f/2.8 lens on a full-frame camera at ISO 1600.
Solution:500 Rule max exposure: 500 / 24 = 20.8 seconds per frame\nEarth rotation rate: 15.04 degrees per hour\nTime for 90 degrees: 90 / 15.04 = 5.98 hours\nTotal frames needed: (5.98 x 3600) / 20.8 = 1,035 frames\nStorage: 1,035 x 25MB = 25.9GB\nField of view: 73.7 degrees
Result:Shoot for ~6 hours | 1,035 frames | 25.9GB storage needed
Example 2: Short Star Trails on Crop Sensor
Problem:Calculate settings for 30-degree trails with a 18mm lens on a 1.5x crop sensor camera.
Solution:Effective focal length: 18 x 1.5 = 27mm\n500 Rule: 500 / 27 = 18.5 seconds per frame\nTime for 30 degrees: 30 / 15.04 = 2.0 hours\nTotal frames: (2.0 x 3600) / 18.5 = 389 frames\nStorage: 389 x 25MB = 9.7GB
Result:Shoot for ~2 hours | 389 frames | 9.7GB storage
Frequently Asked Questions
How do I find the North Star (Polaris) for centering star trails?
To find Polaris in the Northern Hemisphere, first locate the Big Dipper (Ursa Major) constellation. Draw an imaginary line through the two pointer stars at the outer edge of the Big Dipper bowl (Dubhe and Merak) and extend it approximately five times the distance between them. This line points directly to Polaris, which sits at the end of the Little Dipper handle. Polaris is not the brightest star in the sky but maintains a nearly fixed position at the north celestial pole. For Southern Hemisphere photographers, there is no bright pole star, so use the Southern Cross to estimate the south celestial pole position. Smartphone apps like Stellarium and PhotoPills can precisely locate the poles.
How long do I need to shoot for impressive star trails?
The shooting duration directly determines the length of the star trails. Earth rotates at 15 degrees per hour, so a one-hour shoot produces trails spanning approximately 15 degrees of arc. For a quarter circle (90 degrees), you need about 6 hours. For a full circle (360 degrees), you would need nearly 24 hours, which is impractical. Most photographers find that 1-3 hours produces pleasing results with clearly visible curved trails. Shorter sessions of 30-60 minutes create shorter streaks that can still be effective, especially with wider lenses. The visual impact also depends on focal length: wider lenses show more of the sky with shorter apparent trails, while telephoto lenses magnify the trail length but cover a smaller field of view.
What camera settings should I use for star trail stacking?
For star trail stacking, set your camera to manual mode with the widest aperture available (f/1.4 to f/2.8 is ideal). Use ISO 1600-3200 for bright trails against a moderately dark sky, or ISO 800 for subtler trails with less noise. Set the shutter speed using the 500 Rule to keep stars as points in each individual frame. Enable long exposure noise reduction only if NOT shooting continuous stacked frames, as it doubles the time between shots and creates gaps in trails. Use manual focus set to infinity (verify with live view magnification on a bright star). Shoot RAW for maximum post-processing flexibility and use an intervalometer to automate the continuous capture process.
What software is best for stacking star trail images?
Several excellent software options exist for stacking star trail images. StarStaX is a free, dedicated star trail stacking application available for Windows, Mac, and Linux, offering lighten blending, gap filling, and comet mode effects. Adobe Photoshop can stack trails using the Lighten blending mode across image layers, though it is memory-intensive for hundreds of frames. Sequator is a free Windows tool that handles both star stacking and trail stacking with advanced alignment. StarTrails.exe is another free Windows tool specifically designed for star trail creation. For advanced users, ImageMagick command-line tools can batch-process thousands of frames efficiently. Most photographers find StarStaX sufficient for the majority of star trail projects.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy