Shutter speed
A longer shutter gathers light for longer, but stars keep moving across the sensor. The useful limit depends on focal length, sensor sampling and sky position, not one universal 500-rule number.
Astro exposure: balance star motion, light and noise
Change focal length, aperture, ISO and shutter time. See when a starting exposure begins to trade pinpoint stars for trails — then verify it in the field.
Night-sky exposure previewLonger shutter times brighten the frame but increase apparent star motion.
Decision ·
Motion starting point
Exposure value
Star motion
Why
Physical fact: Earth rotation drives apparent stellar motion.
Earth rotates about 15° per hour. A longer focal length magnifies that apparent motion, so the same shutter time produces longer trails.
Teaching approximation: The motion threshold here is a coarse 300-rule starting estimate, not a guarantee.
In the field
Use this as a starting point, not a universal best setting. Take a test frame, magnify a bright star, check the histogram, then adjust shutter and ISO.
What to look for
Pinpoint stars, clipped highlights, shadow noise, foreground detail and whether your final output size actually reveals motion.
Try it
- Set 14 mm, f/2.8, ISO 3200 and compare 10 s vs 25 s.
- Switch to 50 mm without changing shutter time.
- Keep shutter fixed and open/close the aperture to see EV change.
Common mistakes
Treating one “500 rule” number as exact; ignoring sensor resolution, declination, print size, tracking and local sky brightness.
Related tools
Why the controls change the photo
Aperture
Opening the aperture gathers more photons, but many lenses show stronger corner coma and softness wide open. Stopping down can improve star shape while demanding more time or ISO.
ISO
ISO does not create photons. It changes electronic gain and signal placement. Raise it enough to keep faint structure clear of read noise while protecting bright stars and foreground lights from clipping.
Field workflow
Set composition and focus first, choose the longest shutter that keeps acceptable star shape, open the lens as far as image quality allows, then use ISO to place the histogram safely. Judge a magnified test frame, not a recipe.