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Please check this 100% crop of a night sky.

enter image description here

I shot this on a Nikon D750 with a Nikkor 50mm f/1.4 D @ f/8, ISO 100, RAW format.

I had already noticed that kind of noise / hot pixels (all these small RGB dots) on much longer shots, anywhere above 10-15 minutes. But this shot was only 269 seconds (less than 5 minutes).

Is this a sign that my sensor is slowly dying? And is there anything I could do to prevent this to happen aside from turning on the long exposure noise reduction?

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    \$\begingroup\$ Hot pixels are normal for long exposures. 269 seconds is a long exposure. Your image looks fine to me. \$\endgroup\$
    – Mike Sowsun
    Commented Mar 28, 2020 at 14:56
  • \$\begingroup\$ Ok, maybe I hadn't noticed it... I'll dig in my older shots to see if that had happened before. \$\endgroup\$
    – MrUpsidown
    Commented Mar 28, 2020 at 16:12
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    \$\begingroup\$ Philosophically speaking, all sensors are dying. Each time a gamma ray strikes your camera's sensor, it damages a small part of it, usually restricted to a single photosite or "pixel well". But hot pixels are very normal during long exposures. Dark frame subtraction is a way to deal with them. \$\endgroup\$
    – Michael C
    Commented Apr 5, 2020 at 9:29
  • \$\begingroup\$ Thanks. I apparently also have dust or oil (or both) on top of my sensor so, when I shoot long exposures with a relatively small aperture, I end up spending a lot of time in post removing all the stuff that was not supposed to be on my photos and find this rather less enjoyable than taking the pictures :/ \$\endgroup\$
    – MrUpsidown
    Commented Apr 5, 2020 at 16:06

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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.

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  • \$\begingroup\$ I know about LENR and mentioned it in my question. LENR works but when shooting a 20 minutes exposure which is already long enough, you need to wait another 20 minutes for the processing. That's a lot of wasted time and battery life. \$\endgroup\$
    – MrUpsidown
    Commented Mar 29, 2020 at 11:12
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    \$\begingroup\$ Sorry, missed that line at the end... If you use Photoshop you may be able to get away with shooting one dark frame and subtracting it from exposures of similar length. \$\endgroup\$
    – BobT
    Commented Mar 29, 2020 at 15:05
  • \$\begingroup\$ Yes, that's actually what I did now. I just shot a 6 min dark frame and surprisingly, most of the hot pixels are in the same place on the black shot... but not all of them (I compared it with 2 other long exposures shots). I suppose it depends on "how hot" the sensor is? Anyway my main question really was: Is this a sign that my sensor is going to die, and I suppose the answer is no, or at least not too soon. Is this usually getting worse over time? \$\endgroup\$
    – MrUpsidown
    Commented Mar 29, 2020 at 15:29
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    \$\begingroup\$ @MrUpsidown Hot pixels are normal. Your camera cam out of the factory with hot pixels. The factory almost certainly did what is known as pixel mapping to remove the results of those hot pixels. When taking long exposures, pixels that are not "hot" for shorter exposure times will heat up more because of the longer exposure time. Gamma rays striking your camera's sensor can also create more hot pixels. You can have your sensor remapped by a factory service center. With some cameras, you can do it yourself, though the method may not always be obvious. Try a google search for D750 pixel mapping. \$\endgroup\$
    – Michael C
    Commented Apr 5, 2020 at 9:26

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