Long Exposure Noise Reduction

All photographers have demons to slay. Wildlife photographers have to exercise more patience than I ever will have. Baby photographers have to deal with…well…babies. Wedding photographers have to deal with the bride’s mother. I wouldn’t want to minimize the burdens of my brethren, but I have to say that I think night photographers face more than their share of hardship.

(I will admit, though, that facing the bride’s mother is an ominous thought. But hey, at least wedding photographers can see the dang camera when they shoot!)

Among a night photographer’s biggest demons is noise. It’s a problem no matter what camera you have, as even the best sensor is going to record some noise in the shadows with long exposures, particularly in warm temperatures.

So we do what we can to minimize, if not eliminate noise. This is such an important and involved issue that it requires a lot more attention than I can give in a single article. Here, I’m going to talk about the easiest technique you can use to reduce the noise—enabling Long Exposure Noise Reduction on your camera. (Well, only shooing when it’s very cold outside is pretty easy as well, but it’s not an option I would choose.)

First, let’s make sure our wave patterns are in sync. Most cameras offer two types of user-enabled noise reduction. One is called “High ISO Noise Reduction” and the other “Long Exposure Noise Reduction” (LENR). The actual terminology might differ between brands and even different cameras from the same brand, but it’s going to be something similar to this.

Long Expsoure Noise Reduction example photo

15 seconds, ISO 3200, Nikon D800. Long Exposure Noise Reduction disabled. 200% crop. Curves adjustment layer applied to illustrate the effect better.

Long Exposure Noise Reduction example photo

15 seconds, ISO 3200, Nikon D800. Long Exposure Noise Reduction enabled. 200% crop. Curves adjustment layer applied to illustrate the effect better.

High ISO Noise Reduction is a totally different beast from LENR. I have no idea exactly how the camera software algorithms work to reduce high ISO noise, nor do I want to know. All I need to know is that the camera attempts to reduce it and it doesn’t do as good a job as I can do with other software such as Nik Dfine or Photoshop. All cameras are different, and I’m sure some can do a pretty good job reducing the noise in high ISO images, but my experience shows better results from dealing with it in the computer instead of the camera. I keep High ISO Noise Reduction set to “Norm.” Never “High.”

Update: The above statement is misleading. High ISO Noise Reduction does not affect RAW files, at least not in the Nikon D700 and D800 that I currently use. It writes a tag to the file. Nikon’s Capture NX reads this tag and applies the noise reduction algorithm to the RAW file when it is converted. Adobe Camera RAW cannot read the tag, and I assume no other software besides Nikon’s can either. So, if you shoot RAW, HIGH ISO NR will have no affect on the photo. It only affects JPEGs and the LCD preview image on the camera.

LENR works much differently. It is based on the fact that two identical exposures that are shot successively will display the same noise characteristics. As long as the shutter speed, ISO, and temperature are the same, and as long as there has not been a long interval between the shots (noise increases as the camera gets older), each photo will have almost precisely the same noise. And here’s the important part: The noise records regardless of whether there is any light coming through the lens. In other words, you could put the lens cap on for one of the exposures and both photos will still display the same noise.

This is important because it gives the camera a way not only to measure the noise, but also to remove it. With LENR enabled, you shoot the photo just as you normally would, but after the exposure is finished, the camera takes a second exposure, using the same settings, but without allowing any light to reach the sensor. This second exposure records only the noise, the same noise that recorded on the first shot. So now the camera has two photos to work with, one with the picture you shot, which we call the “light frame” and one that shows only the noise. The second frame is referred to as a “dark frame” because it did not record any light.

Long Exposure Noise Reduction example photo

30 seconds, ISO 1600, Nikon D800. Long Exposure Noise Reduction disabled. 200% crop. Curves adjustment layer applied to illustrate the effect better.

Long Exposure Noise Reduction example photo

30 seconds, ISO 1600, Nikon D800. Long Exposure Noise Reduction enabled. 200% crop. Curves adjustment layer applied to illustrate the effect better.

The software looks at the dark frame, measures all the noise, and determines precisely where it is located on the frame. Then it goes back to the light frame and plucks away all the noise that it measured. The process is called “dark frame subtraction.” One way to visualize what’s going on is to imagine the noise as a real object, say a red marble. If you take a long exposure of a box full of blue marbles, you’re going record some red marbles (noise) as well. The dark frame records ONLY those red marbles, so when the camera software looks at it can say, “Okay, I can see a lot of red marbles scattered all over the frame. Now I’m going to go to the light frame and if I see any red marbles in the same position, I’ll get rid of them.”

Theoretically. There are different kinds of noise and the camera can’t get rid of all them. It does a good job of removing “hot” or “stuck” pixels—those tiny blue, red, or green squares or rectangles—and it will remove “amp glow”—the purple or pinkish color that shows on the corners of the frame when the sensor heats up. But it won’t remove “random noise,” which makes up the bulk of what we perceive as noise in an image. Random noise can be reduced using post-processing software, although with an unavoidable loss of sharpness.

Although LENR won’t remove all the noise, it stills removes enough to make it very beneficial in some shooting situations. I’ve spent many tedious hours with the Clone Stamp and Healing Brush tools in Photoshop removing hot pixels and I’m telling you it ain’t any fun. So, when practical, I shoot with LENR enabled.

When practical.

Unfortunately, LENR is not always a good thing. Since it doubles the exposure time, you can’t use it for some types of shots. For instance, you can’t shoot can’t shoot star trails. With star trails, you need the shortest interval between exposures as possible to prevent gaps between the trails. If you’re shooting 60-second exposures with LENR, you’re going to end up with 60-second gaps between the trails. Not good.

LENR also doesn’t work when shooting time lapses with an external intervalometer, at least as far as I can tell. With my timers, LENR screws up the signal and causes the camera to shoot short exposures in between the desired ones. It’s really weird. I suppose you could manually fire the shutter, but who wants to stand behind the camera for hours while staring at a watch and pressing a shutter button at regular intervals.

Another reason why you might not want to use LENR is when the light or subject matter is changing. Let’s say you’re shooting night-sky scenes under moonlight with clouds blowing in the wind. You want to be ready to fire the shutter when a nice cloud blows into the scene. If half of your time is taken up by LENR, you’re going to miss some clouds.

Similarly, if you’re shooting a meteor shower, you want your camera exposing for as much of the night as possible. You can bet that if you shoot meteors with LENR, the biggest and brightest fireballs of the night will occur while the camera is performing its dark frame subtractions.

Yet another situation where LENR is not a good idea is when you shoot multiple exposures to increase the light parts of a scene. For example, you might shoot a dozen exposures of a cityscape in order to increase the car light streaks. You’d stack the exposures as layers in Photoshop and change the blend mode to Lighten. When you do this, only the lightest portion of each frame shows up, and since noise does not occur in very light areas of an image, none of the noise that might be in each of the frames carries through. Of course, you do have to deal with whatever noise might occur in the bottom layer, so for that one you could use LENR and then turn it off for the subsequent frames.

Long Exposure Noise Reduction example photo

60 seconds, ISO 800, Nikon D800. Long Exposure Noise Reduction disabled. 200% crop. Curves adjustment layer applied to illustrate the effect better.

Long Exposure Noise Reduction example photo

60 seconds, ISO 800, Nikon D800. Long Exposure Noise Reduction enabled. 200% crop. Curves adjustment layer applied to illustrate the effect better.

Did you catch that part about noise not showing up in the very lightest parts of an image? If you didn’t, listen up, because this is important. If you shoot a night scene that includes street lights, car lights, light from a flashlight, stars, moon, comets, or any other bright light source, you won’t see any noise in the lights. The dark areas surrounding the lights might look horrible, but the lights will be clean. I don’t know at what point this ceases to be true, but I can tell you that I’ve shot 20-minute exposures on a hot summer night and see no noise at all in the lights. The shadows suck, but the lights look great. So, anytime you are using the Lighten blend mode to increase the lights in a photo, it doesn’t matter how much noise is in the shadows. Something very useful to keep in mind.

So LENR is only practical for situations where it’s okay to spend half of your shooting time humming the theme song to Jeopardy. But this still leaves a lot of scenarios where it is worthwhile and I recommend using it whenever you can.

Is there anything you can do to lessen the noise in those situations where LENR is not practical? Absolutely, but that’s going to have to be the topic for another article. But just so you don’t think I’m being flippant about withholding valuable information until the time suits me, I’ll tell you that for most night photographers, using LENR during capture and/or using software (i.e. Nik Dfine, Photoshop) when post processing is going to be the whole bag of tricks. The techniques I’ll be discussing in an upcoming article are for obsessive folks like me who, in all honesty, really need to step away from the noise sometimes and just let it be.

When to use Long Exposure Noise Reduction.

  1. Photos where the light or other elements in the scene are not changing quickly.
  2. Photos that do not require brief intervals between subsequent shots.

When not to use Long Exposure Noise Reduction.

  1. When shooting star trails or any other situation where you need the shortest interval possible between shots.
  2. When shooting time lapses, unless your timer system happens to work with LENR enabled and the interval between shots is longer than the shutter speed.
  3. When you don’t have a lot of time and need to get as many shots as quickly as you can.
  4. When the only portion of the photo you’ll be using is a bright light source, as when shooting multiple exposures of car light streaks for layer stacking using the Lighten blend mode.

Some random tidbits about Long Exposure Noise Reduction.

  1. The shutter speed at which the camera performs LENR varies. With my Nikon D800, LENR is active with any exposure longer than 1 second, but this may be different for your camera.
  2. While LENR is being performed, your camera will display an indication in the viewfinder. Nikon cameras flash “Job nr.” Don’t know what the other brands do.
  3. LENR affects both RAW and JPEG files.
  4. The noise characteristics of a camera are not fixed. The number of hot pixels on the sensor increases as the camera is used, making it even more important to enable LENR for older cameras.

One final, and important, note about noise is that post processing can have a dramatic effect on how pronounced it appears in an image. If you do any testing and judge the amount of noise only by how it looks in the RAW file, you might be in for a big shock when you look at the processed image. Stretching the image using Curves, Levels, or some other method to lighten the shadows will explode the noise.

You can take two lessons from this. First, you should always try to get your night exposures as close as possible in capture so that you don’t have to stretch them in post. Of course, this isn’t always practical, especially with night sky photos that usually need some stretching, but the idea is always to get as much exposure as possible IN THE CAPTURE.

The second lesson is to be very careful with the post processing. Aggressive adjustments affect the noise just as they do the rest of the image, so regardless of how insignificant it might look in the RAW file, you’re going to have to address it at some point during post.

As a rule, I perform noise reduction as the first step during post processing and then pay careful attention to any adjustments made afterward. You can certainly apply noise reduction after making other adjustments, but my experience shows that the noise-reduction software does a better job on the otherwise unaltered file. All adjustments you make will alter the image to some extent, and once altered, the software seems to have a more difficult time figuring out what is noise and removing it appropriately.

The most important thing you can do when noise is the subject? Keep quiet!

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3 Responses to “Long Exposure Noise Reduction”

  1. Ross Says:

    Kevin, this so timely for me personally. I have heard opinions lately of LENR from never to use, to always use, both being said without much verbal documentation. Your is the best I have heard or read, and I plan to follow it to the letter. Thank you.

  2. Kevin Adams Says:

    Thanks Ross! Glad the article is helpful for you. Yeah, there’s a lot of confusing and contradicting info out there about LENR.

  3. marsusf Says:

    Very interesting article. That white pixels issue is my main (and serious, as I do a lot of night photography) gripe with the D800. I love the images this camera produces otherwise.

    “The techniques I’ll be discussing in an upcoming article are for obsessive folks like me who, in all honesty, really need to step away from the noise sometimes and just let it be.”

    I’m one of those, and I can’t wait to read the article following this one!

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