Why does dark-frame subtraction add noise and shift color in my night photos?
Asked 4/16/2021
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2 answers
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I’m shooting night landscapes with stars and tried subtracting dark frames to reduce noise before stacking. My workflow was: shoot a series of frames at the same settings (15 s, ISO 6400), then subtract dark frames from each image, average the foreground frames for noise reduction, and stack the sky in Sequator.
Instead of improving the result, dark-frame subtraction made the image noisier with dark monochrome speckling and also shifted the color/white balance. I shot the dark frames with the lens cap and viewfinder cover on, using the same camera settings and white balance. In Photoshop I used a dark frame layer above the image with Subtract blending mode; I also tried doing the subtraction in Sequator, where it made little or no improvement.
What causes dark-frame subtraction to make an image worse, and how should dark frames be used correctly for astrophotography/night landscapes?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
5
You misunderstand the purpose of dark-frame-subtraction. While it is a technique used to reduce noise, it only reduces noise that is consistently output from the sensor. Any read noise due to the circuitry or uneven output such as hot-pixels. It does not reduce random noise.
When you used dark-frame-subtraction on an image with noise that is primarily random, you are in fact simply adding more noise. Some pixels may look better but overall, there will be more noise. Noise in your image and your dark frame is very high because you are shooting at ISO 6400. It is important to capture using the lowest ISO possible. If you cannot lengthen exposure, open the aperture and if you cannot either, then consider buying a brighter lens.
To reduce random noise using a multi-frame technique, the better approach is to average images. This means capturing the same subject multiple times and then blending the resulting images together, so that random noise gets averaged out. It will not disappear but the more shots you can average, the lower the final noise will be. Of course, anything that moves in the scene causes problems with this but if you are capturing stars within a short period of time, their relative position will be consistent and you can align the image-stack before averaging. If you have foreground elements with foliage, then this technique is not really usable.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
5y ago
0
Generated from our catalog & community — verify before relying on it.
Dark-frame subtraction only removes repeatable sensor artifacts, such as hot pixels and fixed-pattern noise. It does not remove random high-ISO noise. If most of the noise in your image is random, subtracting a dark frame can actually make the image look noisier because the dark frame contains its own noise too.
A second issue is matching: dark frames work best when they closely match the light frames in exposure, ISO, and especially sensor temperature. If the camera warmed up during the session, dark frames shot at a different temperature can subtract incorrectly and leave worse noise or color shifts.
So your result is consistent with either:
- mostly random noise at ISO 6400, where dark subtraction offers little benefit, or
- dark frames that didn’t match the light frames closely enough.
For cleaner results, rely on stacking/averaging multiple light frames to reduce random noise, and use dark frames mainly for fixed sensor artifacts. Also use the lowest ISO you can, if exposure and aperture allow. Dedicated astro software with proper dark-frame calibration can handle this better than a simple Photoshop blend mode workflow.
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