You take a beautiful 30-second night photo, zoom in, and see tiny red, green, blue or white dots scattered across the dark areas. Sometimes a corner glows purple. These are not ordinary high-ISO noise. They come from the sensor itself during long exposures, and they get worse when the camera is warm. This lesson explains hot pixels and other long-exposure noise, how to tell them apart from sensor defects, and how to prevent or remove them.
What is long-exposure noise?
Even with no light at all, a sensor slowly builds up a small electrical signal over time, called dark current, mostly caused by heat. In short exposures it is too small to see. In exposures of several seconds or minutes, it builds up enough to appear in the photo. This is why long-exposure noise depends mainly on exposure time and temperature, not just ISO.
| Problem | What you see | Cause | Permanent? |
|---|---|---|---|
| Hot pixels | Bright red, green, blue or white dots, mainly in dark areas of long exposures | Some photosites leak more charge, especially when warm | Varies with time and heat |
| Amp glow | A purple or magenta glow in a corner or edge | Heat from electronics near the sensor edge | Appears in long exposures |
| Stuck pixel | A colored dot in the same place in every photo, even short exposures | A photosite that always reads too high | Usually yes |
| Dead pixel | A black dot in the same place in every photo | A photosite that no longer responds | Yes |
Why heat makes it worse
Dark current increases quickly as the sensor gets warmer. Hot summer nights, long sequences of exposures, live view left on for a long time, and video recording all heat the sensor. Photographers often notice many more hot pixels on a hot evening than on a cold one with the same settings.
- Let the camera rest between long sequences.
- Turn off live view when not needed during long sessions.
- Avoid leaving the camera in the sun before night shooting.
Dark-frame subtraction
Hot pixels and amp glow appear in the same places when exposure time, ISO and temperature are the same. So a photo taken with no light at all, called a dark frame, records only that noise. Subtracting it from the real photo removes most of it.
| In-camera long exposure noise reduction | Manual dark frames | |
|---|---|---|
| How | The camera automatically takes a dark frame after each long exposure and subtracts it | You take separate dark frames with the lens cap on, using the same settings, and subtract them in software |
| Time cost | Doubles the time for each photo | One set of dark frames can serve many photos at the same settings and temperature |
| Best for | Single long exposures | Star trails, astrophotography sequences and time-lapses |
The in-camera option was introduced in reducing noise and protecting image quality. Manual dark frames are used heavily in astrophotography, covered in astrophotography workflow and stacking.
Warning:Dark frames must match the real exposures: same exposure time, same ISO and similar temperature. Take them during or right after your shoot, not hours later.
Pixel mapping for stuck and dead pixels
Stuck and dead pixels appear in every photo, even short ones. Many cameras have a pixel mapping or pixel refresh function (sometimes run automatically, sometimes in a menu or by cleaning the sensor) that finds these pixels and hides them by filling them in from their neighbours. RAW editing software also removes isolated bad pixels automatically in many cases. Check your camera manual for its method.
Remember:A few stuck pixels are normal over a camera's life. A rapidly growing number may indicate a sensor problem worth checking with the manufacturer.
Removing hot pixels after shooting
- Let your RAW software remove isolated hot pixels, which many do automatically.
- Apply moderate color noise reduction; too much will remove real color from stars and lights.
- Use spot healing for remaining obvious dots, covered in healing and cloning.
- For sequences, use dark-frame subtraction or stacking software, which also removes random noise.
Common mistakes
- Thinking hot pixels mean a broken sensor: they are normal in long exposures, especially when warm.
- Taking dark frames at different settings: they will not match.
- Heavy color noise reduction to hide hot pixels: stars and lights lose color.
- Leaving long exposure NR on during star trails: gaps appear between frames; use manual dark frames instead.
How professionals handle long-exposure noise
Night landscape and astrophotographers keep their cameras as cool as possible, use in-camera long exposure noise reduction for single images, and take sets of manual dark frames for sequences. They run pixel mapping when stuck pixels appear and test new cameras with long dark exposures to know their behaviour. Long-exposure planning is covered in long-exposure planning.
Practical examples
A summer night seascape
- Situation
- A 2-minute exposure on a hot night shows many colored dots in the dark sea.
- What to do
- You turn on long exposure noise reduction and accept the extra 2 minutes per shot.
- Why it works
- The dark frame records the same hot pixels and subtracts them.
- Result
- A clean, smooth dark sea.
A star trail sequence
- Situation
- You plan 200 thirty-second exposures for a star trail.
- What to do
- You turn off in-camera noise reduction and take 15 dark frames with the lens cap on at the end.
- Why it works
- In-camera NR would leave gaps between frames; manual dark frames can be subtracted from all of them.
- Result
- Continuous trails with hot pixels removed.
Key points
- Long-exposure noise comes from dark current, which grows with exposure time and heat.
- Hot pixels and amp glow appear in long exposures; stuck and dead pixels appear in every photo.
- Dark-frame subtraction removes hot pixels; in-camera NR doubles time per shot.
- Manual dark frames suit sequences; they must match exposure time, ISO and temperature.
- Pixel mapping hides stuck and dead pixels; RAW software removes isolated ones.
Frequently asked questions
What are hot pixels?
How do I remove hot pixels?
What is the difference between a hot pixel and a stuck pixel?
Why does long exposure noise reduction double the time?
Conclusion
Long exposures bring their own noise from heat and time: hot pixels, amp glow and thermal noise. Keep the camera cool, use dark-frame subtraction, map out stuck pixels and clean up the rest in editing. Next, you will learn how compression affects file size and image quality.
Finished reading? Track your progress through Sensors and RAW.