Watch a star smear across a camera’s pixels as the exposure grows, from Georgetown, Guyana, and see where the 500 rule and the stricter NPF rule stop it.
Shows Antares from Georgetown (6.80° N, 58.16° W) at 21:00 GYT on Thu 15 Jul 2027, through a 24 mm f/2.8 lens on an APS-C camera with 3.9 µm pixels; the pixels are simulated, the star’s motion is real.
A silent explainer drawn from the site’s own exposure code and sky kit. It follows Antares through a 24 mm f/2.8 lens on an APS-C camera (crop 1.5, 3.9 µm pixels, a 53.1° by 36.9° field) from Georgetown, Guyana, at Thu 15 Jul, 21:00 GYT, and shows the star’s image on the sensor’s pixels as the exposure lengthens to 30 s. The pixels are simulated; the star’s motion is real.
Antares is 57° up in the S. Each pixel covers 33.5″ of sky, and the sky turns 13.5″ a second there, so the image moves 0.40 pixels a second. The NPF rule limits the exposure to 9.0 s (3.6 pixels of trail); the 500 rule to 13.9 s (5.6 pixels).
On the same camera: 14 mm, NPF 15.4 s, 500 rule 23.8 s; 20 mm, NPF 10.8 s, 500 rule 16.7 s; 24 mm, NPF 9 s, 500 rule 13.9 s; 35 mm, NPF 6.1 s, 500 rule 9.5 s; 50 mm, NPF 4.3 s, 500 rule 6.7 s.
0:00 Leave the shutter open and the stars smear into lines. How long can you expose before they do?
0:03 A 24 mm f/2.8 lens on an APS-C camera, aimed at Antares, 57° up in the S from Georgetown. Each pixel covers 33.5″ of sky.
0:09 The sky turns 13.5″ a second at Antares’s declination, so its image slides 0.40 pixels a second across the sensor.
0:16 The NPF rule stops at 9.0 s, when the star has moved 3.6 pixels: the limit it sets for a point at 100% view.
0:21 The 500 rule allows 13.9 s, 1.6 times as long: 5.6 pixels, a short dash at 100%. It was made for film and coarser sensors.
0:26 Keep going to 30 s and Antares is a 12.0-pixel line.
0:30 Wider lenses buy time, longer ones lose it. On every lens the NPF limit is the shorter one.
0:36 Start at the NPF limit for your camera, check a frame at 100%, and lengthen only if the stars stay round.