Today’s Question: How necessary is Long Exposure Noise Reduction for night sky pictures?
Tim’s Quick Answer: I consider long exposure noise reduction to be very helpful in terms of reducing overall noise, and so do highly recommend using it for night photography or any other scenario where exposure time will be longer than about one second.
More Detail: A wide variety of cameras support long exposure noise reduction, though the specifics of when this noise reduction get implemented vary. Some cameras will even determine whether noise is likely to be an issue with a long exposure based on current conditions, and only apply the noise reduction when it is needed.
The key benefit of long exposure noise reduction compared to applying noise reduction in post-processing with software, is that in-camera long exposure noise reduction is measuring the actual signal from the image sensor to calculate (and subtract) the noise.
The process of in-camera long exposure noise reduction involves essentially capturing two photos instead of one. First, the actual exposure is created, and then the camera captures another exposure of equal duration, but without actually recording light from the scene (such as by keeping the shutter closed). The noise from this “dark exposure” can then be subtracted from the original capture.
Of course, there is a drawback to long exposure noise reduction. Because each exposure is doubled, you won’t be able to respond as quickly to changing conditions to capture a photo that is optimally timed. A 30-second exposure, for example, will require a full minute. I can tell you from personal experience that the time added for the long exposure noise reduction capture can prove very frustrating! But in many cases, of course, the timing of an additional capture is not especially critical.
I consider the benefit of long exposure noise reduction to outweigh the additional time that is required. In-camera noise reduction can greatly improve the overall quality of the image and helps ensure that much less noise reduction will be needed (if at all) in post-processing.