In-camera noise reduction for long exposure

[batmura]

What is everyone's opinion on using the in-camera noise reduction feature when doing long exposure photography? To this day, I have used it, but the waiting can sometimes be really frustrating. While reading an article the other day, the author suggested turning it off and adjusting it in post. Is this a good method and as effective or does having nr reduction on in camera have irreversible effects?

Thanks!

 

[SCraig]

I want my camera to do as little as possible to my images. I shoot in RAW and adjust things to the way that I want to see them during post processing, not the way a Nikon engineer wanted them to be. That includes noise reduction. In fact it ESPECIALLY includes noise reduction because that is a very destructive process.

My suggestion would be to try it both ways and decide for yourself which works best for you.

 

[Overread]

The long exposure noise reduction in camera works differently to many other noise reduction methods. It's specifically aimed at dealing with noise generated by overheating of the sensor and is intended to be used on exposures well over 30 seconds and well into minutes.

As the sensor is used it heats up - as it heats up noise levels increase. When this happens for something like startrail photography the results can be very damaging to the final image. As a result what Long Exposure Noise reduction does is to take a second photo, for the same length of exposure as the first, right after the first photo is taken. This second photo is taken with the shutter baldes shut - as a result it is a purely black scene with no light - any "colour" or such that appears on the shot will only be thus created by the heat induced noise.

The camera then processes the two shots one over the other - deducting the noise from the black shot from the original photo - and thus having the greatest chance to remove heat induced noise and preserve actual details present on the photo.

 

[TCampbell]

Some types of noise are random, others are predictable for YOUR specific sensor. e.g. at any given temperature and taking a photo of a given length, there is certain noise which will re-appear on the same pixels. If you grabbed another camera body of the same model as yours, the noisy pixels would be in a different spot. The repeatable noise is like a fingerprint... unique to your sensor.

In-camera noise reduction can remove that type of noise in a way that post-processed images cannot. Your computer software wont know the difference between the two types of noise. If you shoot RAW images, you can shoot "dark" frames and process the noise out on the computer -- the dark frame essentially tells the computer where your "repeatable" noise is (and usually you shoot many of them so it can assess which pixels are consistently noise and which pixels are randomly noisy.) I don't know of any terrestrial photographers who shoot dark-frames (although it wouldn't surprise me to learn some people do it) but astrophotographers (who mostly shoot long exposures) shoot these routinely.

You take them at the same time (e.g. after taking your normal frame, shoot a few dark frames) because you want them captured when the outdoor ambient temperature and the inside-camera temperature are the same as they were for your normal exposure.

You can also reduce noise by taking multiple exposures and "stacking" them. The idea is that elements in the image that are really there will be there in every frame. Pixels which represent noise will appear randomly. By "stacking" you can allow the computer to notice the difference. This will eliminate random noise -- but it wont eliminate the consistent noise. That's why you shoot the dark-frames. HOWEVER... there is another technique called "dithering" in which the camera is shifted _very_ slightly between frames. You'll re-align the offset images later, but the point of this is that any consistent noise will remain in the same place even though your subject moved over just slightly. This means the repeatable noise wont in the same location "relative" to the position of your subject -- making it easy for a computer to detect and eliminate.

Again... these are all techniques practiced regularly by astro-images. Keep in mind their exposure times are often VERY long (e.g. they might conceivably last an hour each.)

If you're shooting long exposure JPEGs and just want to get it in one frame with minimum hassle, then I'd probably turn the long exposure NR on and wait it out.