Taken with a 136-second exposure and long exposure noise reduction
Over time, every digital camera will develop “hot” or “stuck” pixels that do not work properly. They aren’t usually visible, but when you’re taking long exposures, they become more and more obvious. The easiest way to fix hot pixels is with a camera setting called long exposure noise reduction.
What Are Hot Pixels?
Hot pixels are bright, colorful pixels in your photo that don’t match the rest of the image. You generally cannot see them until you magnify a photo 100% or more on your computer. When you do, they often look like small crosses that are the brightest in the center.
The greenish-blue hot pixel in this image is very obvious, and there are a few other hot pixels throughout the rest of the image as well. This is an extreme crop.
Most post-processing software scans for these pixels and cuts them out, including Lightroom. Sometimes, however, a few sneak through anyway.
Hot pixels are especially visible in long exposures and images taken at high ISO values. The reason is simple: when you capture less light from the scene in front of you in a given moment, the inherent patterns on your camera sensor will be comparatively stronger during that same moment.
What Is Thermal Noise?
Thermal noise is a type of noise in your photo that grows stronger as your sensor grows warmer. It is often associated with long exposures, since camera sensors get hotter and hotter the longer they stay on. Sometimes, thermal noise will have a low-level pattern, while other times it will be essentially random.
For this same reason, you may notice that photography in cold environments has less noise than in hot environments. It’s also why some specialized astrophotographers buy cameras with a fan or liquid cooling rig attached to the back. It simply improves image quality.
What Is Long Exposure Noise Reduction?
Long exposure noise reduction is a camera setting that takes two photos, one after another. The first photo is an ordinary photo of the scene in front of you, while the second photo is known as a “dark frame” because the camera shutter is closed during the exposure.
Dark frame exposure is important because it only contains image noise and hot pixels. Your camera then automatically subtracts the second image from the first, reducing noise and especially hot pixels in your primary shot.
Nasim took this photo with a 262-second shutter speed, long enough that long exposure noise reduction can make a significant difference to the level of hot pixels.
How to Use Long Exposure Noise Reduction
Long exposure noise reduction is a menu option on most cameras today, and it’s very easy to set.
On Nikon, Canon, and Sony, it’s found in the camera’s Shooting Menu (the camera icon menu on all three). Sometimes, long exposure noise reduction will be disabled if you are using an electronic shutter. If it is grayed out in your menu, take a look at the “long exposure noise reduction” chapter in your camera manual to see why.
Once enabled, you don’t need to do any further work to use long exposure
noise reduction correctly in the field. However, note that your camera will not implement long exposure noise reduction