Tuesday, September 13, 2011

Using Long Exposure Noise Reduction

Long exp. NR (Long Exposure Noise Reduction) is designed to combat visual noise in long exposures. Many of us do not often take exposures long enough to cause Long Exp. NR to kick in. However, I would like to encourage you to shoot some time exposures (longer than 8 seconds) and take advantage of Nikon's excellent Long exp. NR. Nikon has prepared your camera for nearly any type of shooting. Long exposures is one of them!

As a digital photographer, you know that noise is that ugly, grainy look in images when they are underexposed and then brightened or when a really high ISO sensitivity setting is used. If you ever shoot film, you know how faster films have lots more grain. Noise is like that, except uglier. It’s the digital equivalent of static in music. Who wants static in their images?

Nikon knows that their sensors might exhibit more noise than is acceptable in exposures longer than 8 seconds. The sensor can start to warm up a little when longer exposures are used. This causes a condition called amp noise, in which warmer sections of the sensor start to create more noise than cooler sections. There are two settings for Long exp. NR (figure 2):

  • On – When you select On and an exposure is longer than 8 seconds, the camera will take two exposures with approximately same time for each. Nikon says that the D7000 can sometimes take the second exposure in as little as half the time as the first one. The first exposure is the normal picture-taking exposure. The second one is a black-frame subtraction exposure, which is a second image that is exposed for the about the same duration as the first image, but the shutter is closed. The noise in the black-frame exposure is examined and then subtracted from the original image. It’s really quite effective and beats having to blur the image to get rid of noise. I’ve taken exposures of around 30 seconds and had perfectly usable results. The only drawback is that the total exposure time can be as much as doubled because two exposures are made. The black-frame exposure is not written to the memory card, so you’ll have only one image, with much less noise, in the end. While the black-frame exposure is being processed, a message of Job nr will blink in any active displays. When Job nr is flashing, you cannot use the camera. If you turn it off while Job nr is flashing, the camera will keep the first image, but it won’t do any noise reduction on it. If Long exp. NR is set to On, the frame advance rate may slow down a little in Continuous release mode and the capacity of the in-camera memory buffer will drop, usually by one image. (We’ll discuss Continuous release mode in the chapter titled: Autofocus, AF-Area, and Release Modes).

  • Off – If you select Off, Long Exposure Noise Reduction will be disabled in exposures longer than 8 seconds.

Figure 1 – 30-second exposure with Long exp. NR enabled - No noise!

In Figure 1 is an image I took with a 30-second exposure and Long exp. NR set to On. I left the shutter open for 30 seconds in several exposures, hoping to catch a lightning strike in a storm near my home. The camera used black-frame subtraction with the Long exp. NR setting to remove noise from my lighting strike image, without seriously blurring the image in the process. I can’t see any noise, can you?

Disclaimer: It can be a lot of fun to capture lightning images, but be sure you have a safe place to shoot from so you won’t attract the lightning. I was shooting from my upstairs bedroom window. The camera was on a tripod with a wide-angle lens looking out the open window. I got four or five nice lightning shots for my efforts— such as the one shown.

Figure 2 – Using Long exp. NR

Figure 2 shows the two Shooting Menu screens needed to choose Long Exp. NR.

My Recommendation: I like the benefits of Long exp. NR. I shoot a lot of waterfall and stream shots where I want exposures longer than 8 seconds to really blur the water. Also, I like to take midnight shots of the sky and shots of city scenes at night. Even though it may slow down the frame rate slightly and give me one less image in the in-camera memory buffer, I still use it most of the time.

If I were a sports or action shooter using Continuous release mode, I may leave Long exp. NR set to Off. It’s unlikely I would be using exposures longer than 8 seconds, and I would want the maximum frames per second and the ability to cram as many images into the camera buffer as possible. I wouldn’t want my camera to slow down while writing black-frame subtraction exposures to the memory card.

Your style of shooting will govern whether this function is useful to you. Ask yourself one simple question, “Do I often shoot exposures longer than 8 seconds?” If so, you may want to keep Long exp. NR set to On. Compare how your images look with and without it. I think you’ll like Long exp. NR.

Keep on capturing time...
Darrell Young
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  1. Figure 1 – 30-second lightning exposure:
    Which ISO did you use. I've used high ISO's and still had considerable noise in long exposures. Black-frame/Dark-frame Subtraction concerns eliminating the noise from the sensors "pixels" heating up from long exposures / excessive use. I don't believe "grainy" noise is effected.

  2. Why would it matter for sports photography if you had the Long exposure NR on when it doesn't even kick in until doing shots at least 8 seconds long?

    1. Even with it set to on, you will not see any disadvantage, except for maybe half as many frames available in maximum image buffer size during long exposure processing. Some have reported one less frame all the time Long EXP NR is on.

  3. Have you done extra long exposures with a D7000 ? I did a 10 min exposure and it locked the camera up for about 7 min processing the image.... How do I speed that up ? Shutting the noise reduction off helped the process times but not much. Most of my long exposures are below I so 400 and don't need it anyway.
    A friend with a full frame Nikon, I can't recall which, did the same and his was ready in no time flat. Is the hardware what made that difference ?

  4. Paul,

    Make sure that you are not shutting off High ISO NR, instead of Long Exp. NR. Long exposure noise reduction (Long Exp. NR) is what adds the extra processing time. You will see Job NR flashing in the control panel during the Long Exp. NR processing. With it off, the camera will finish processing as soon as the shutter has closed. To reduce the processing time, shut off Long Exp. NR. However, the images will be more noisy as the noise is not caused so much by how high or low you set the ISO. Rather, long exposure noise is caused by heating up of the sensor during exposures of 8 seconds or longer.

  5. Thank you. Wish you a great 2016