Are hot pixels in a 5-minute night-sky exposure a sign of sensor failure?
Asked 3/28/2020
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I took a 269-second night-sky exposure on a Nikon D750 with a Nikkor 50mm f/1.4D at f/8, ISO 100, shooting RAW. At 100% I can see many small colored dots/hot pixels. I had noticed similar artifacts on much longer exposures (10–15 minutes and beyond), but this was under 5 minutes. Is this normal for long exposures, or could it indicate that the sensor is failing? Is there anything I can do to reduce it besides enabling Long Exposure Noise Reduction?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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Hot pixels are normal for long exposures. Additionally, the sensor and electronics get warm and this increases the noise level. The noise level is random, but the hot pixels are not and can be fixed in-camera
Your camera has a feature called Long Exposure Noise Reduction. If you turn it on, the camera will take two exposures. The first will be a normal exposure for the time you set (in this case 269 seconds), the second will be for the same length of time except with the shutter closed. The camera then subtracts the 'dark frame' noise from the image. This pretty much eliminates the hot pixels from the final image. Note that the LENR in the D750 is only applied to exposures longer than 1 second. Your manual should give you more information.
Originally by user11772. Source · Licensed CC BY-SA 4.0
user11772
6y ago
0
Generated from our catalog & community — verify before relying on it.
This is normal for long exposures and is not, by itself, a sign that your sensor is dying.
With multi-minute exposures, the sensor and surrounding electronics warm up, which increases noise. Some pixels show up repeatedly as bright red/green/blue “hot pixels,” especially in night-sky shots and when viewed at 100%.
Your Nikon D750’s Long Exposure Noise Reduction is the standard fix. It makes a second exposure of the same length with the shutter closed, then subtracts that dark-frame noise from the image. This is specifically meant to reduce hot pixels and similar fixed-pattern noise in long exposures.
The tradeoff is that a 269-second exposure effectively takes twice as long to complete. On the D750, this applies to exposures longer than 1 second.
So: what you’re seeing is expected behavior in long exposures, and using LENR/dark-frame subtraction is the normal way to deal with it.
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