How can I reduce hot pixels in very long exposures?
Asked 8/17/2015
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I shot a very dark foreground with a single 4-minute exposure and ended up with lots of red, green, and white hot pixels—too many to clean up easily afterward. Is there a good way to avoid or reduce hot pixels when photographing dark scenes with long exposures? I've heard that taking multiple shorter exposures at lower ISO and stacking them can help. How does that work, and are there other practical options besides switching to film?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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You can't really prevent hot pixels on long exposures, you can only deal with them. For a single four minute exposure the easiest way is to use what is known as Dark Frame Subtraction. Different manufacturers have different names for in camera versions of it.
Canon, the brand I shoot, calls it Long Exposure Noise Reduction. After an image is taken the camera exposes another frame while the shutter remains closed. The data from the sensor obtained from the "dark" exposure is then subtracted from the shot taken with the shutter open. Be aware that if your exposure is four minutes, then it will take an additional four minutes to create the dark frame. During this time you will not be able to take your next photo.
There are also photo processing software applications that can subtract a single dark frame created by you from a batch of multiple images. Most full suites, such as Adobe Photoshop or RawTherapee support dark frame subtraction. The easiest way to create a dark frame is to make an exposure with the same settings but with the lens cap left on the camera. Just be sure the cap does not leak light! If there is any light source behind the camera you may also want to mask the viewfinder to prevent light from leaking around the raised mirror and making it into the mirror box.
Even in your case with an image you took a while back, you might be able to improve the image that you now consider unusable. Set the camera to the exact same settings that you used when you took the image. Place the camera in an environment (especially temperature) as close to the environment where the original image was taken. Be sure to allow enough time for the camera to adjust to the ambient temperature. Then take a dark frame with the lens cap in place and use that frame in an application such as Photoshop to subtract the hot pixels in the dark frame from the previous file. FOr how to do it in Photoshop, see this link.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
10y ago
0
Generated from our catalog & community — verify before relying on it.
Hot pixels are a common side effect of long exposures, especially as the sensor heats up. You usually don’t completely prevent them—you reduce or remove them.
The simplest fix for a single long exposure is dark-frame subtraction. Many cameras offer this as long-exposure noise reduction: after the main exposure, the camera makes a second exposure of equal length with the shutter closed, then subtracts hot-pixel noise from the image. The downside is that a 4-minute shot takes another 4 minutes to process, and you can’t shoot during that time.
Stacking shorter exposures is also effective. Instead of one 4-minute frame, shoot many shorter frames at a lower ISO, then combine them in software. Averaging/stacking reduces random noise and makes hot pixels less prominent while preserving real scene detail. In practice, more frames generally give cleaner results.
So your main options are:
- enable in-camera long-exposure noise reduction
- shoot multiple shorter exposures and stack them later
- use software that applies dark-frame subtraction in post
For very dark foregrounds, stacking multiple shorter frames is often the most flexible approach.
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