What causes red and blue specks in a 36-minute long exposure?
Asked 1/15/2012
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I made a 36-minute BULB exposure on a Canon EOS 1000D at f/29, ISO 800 while testing for star photography. The camera was indoors behind a window in a dark room. In the image, I see red and blue dots/specks. What causes this noise in such a long exposure, and how can it be reduced?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
15
Well, perhaps you should have gone out after all :)
The noise is thermal noise, which will become noticeable as your sensor heats up during a long exposure. In astrophotography, it's quite a common problem.
Some ways to reduce such noise:
- cool sensor down, e.g. by shooting in the cold weather. Note that cold also negatively affects battery life.
- set the camera to take a dark frame with same settings (to get just the noise) and subtract it from the image; this can be either done manually (taking a second frame with lens cap on) or automatically by camera (the feature is called Long Exposure Noise Reduction or something similar).
- using a stronger noise reduction setting in camera; this may also make the photo softer overall.
- shoot several shorter exposures in a sequence and add them together in post processing (this is called "stacking"); you could use DeepSky Stacker for that. Make sure you do not combine this with dark frame technique to avoid dotted trails.
Originally by user4390. Source · Licensed CC BY-SA 4.0
user4390
14y ago
0
Generated from our catalog & community — verify before relying on it.
Those red and blue specks are mainly long-exposure sensor noise, especially thermal noise (dark current) and hot pixels. During a very long exposure, the sensor warms up and some pixels generate extra signal even when little or no light is hitting them. That often shows up as colored dots.
There is also normal random image noise from very low light levels (photon shot noise), which becomes more visible in dark scenes.
Ways to reduce it:
- keep the sensor cooler; colder conditions help
- enable Long Exposure Noise Reduction, which uses a dark frame to subtract much of this noise
- manually shoot a dark frame with the same settings and subtract it in processing
- use stronger noise reduction if needed, though it can soften detail
- shoot multiple shorter exposures and stack them instead of one very long exposure
For astrophotography, this kind of noise is common, so dark-frame subtraction and stacking are standard solutions.
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