Long Exposure Noise Reduction: What It Does and When to Use

What long exposure noise reduction does

Long exposure noise reduction is a camera feature designed to clean up the artifacts that appear when you shoot with a slow shutter speed. As the sensor warms up, it records a pattern of fixed dots and uneven brightness, especially at higher ISO settings. This is often called hot pixels or fixed-pattern noise. The camera's in-camera processing can address this using a technique known as dark frame subtraction.

Dark frame subtraction is a straightforward process. After the main exposure, the camera closes the shutter and captures a second frame of the same length and ISO, but with no light reaching the sensor. That second frame is a dark frame that contains only the noise and hot pixels, not the image. The camera then subtracts the dark frame from the main image, removing the pattern while preserving the details.

How dark frame subtraction works

The key to this method is that the dark frame records the exact noise signature of the sensor at that moment. Because the sensor's thermal behavior depends on temperature and exposure time, the dark frame must match the conditions of the original shot. That is why the camera performs the subtraction immediately after the exposure, while the sensor conditions are still similar.

The cost is time. While the camera is capturing the dark frame, you cannot take another photo. The total time for each exposure is effectively doubled. For a single static shot, this is rarely a problem. But when you are shooting a sequence of images, that delay can break the rhythm.

Some cameras let you set the reduction to Auto, On, or Off. In many models, the feature only activates when the shutter speed is slow enough, such as in bulb mode or long-exposure modes. Check your camera's manual for the menu location and trigger conditions.

When to keep it on

For a single static nightscape, such as a starry sky over a mountain, long exposure noise reduction is a great choice. The clean result from dark frame subtraction means you do not need to do as much cleanup in post. You can set your camera on a tripod, compose, and let the camera take the extra dark frame. This works well for any scene where nothing changes between the two exposures, so the subtraction only affects noise.

Leave it on for long exposures of still subjects: architecture at night, silky water in a landscape, or city lights. As long as the framing does not move, the dark frame will match, and the reduction is effective. It is also useful when you want to minimize hot pixels that would otherwise be distracting in smooth areas like sky.

When to turn it off

For star trails, long exposure noise reduction is a problem. A star trail image is typically made from a series of long exposures stacked in post. If the camera takes a dark frame after every shot, there will be a gap in the trail, and the sequence takes twice as long. Turn the feature off and rely on software to remove the noise after you have stacked the images.

Time-lapses have the same issue. A dark frame after each frame interrupts the interval and can cause visible flicker or missing moments. For these applications, it is far better to capture the raw frames quickly and handle noise reduction with dedicated astrophotography software or image editors.

If you are shooting continuous bursts for something like a lightning storm, the delay between frames will cause you to miss events. Keep the feature disabled in any situation where you need to shoot back-to-back shots without delay.

Alternatives to in-camera reduction

Many photographers prefer to do dark frame subtraction in post. If you shoot raw, you can take a dark frame yourself by covering the lens and making an exposure of the same length and ISO, then subtract it in editing software. This gives you more control and lets you reuse a single dark frame for a sequence if conditions are stable.

Another approach is image stacking. By taking multiple shorter exposures and merging them in software, you can average out random noise. This is the standard method for deep-sky astrophotography and produces better results than a single long exposure with dark frame subtraction, but it requires more processing time.

Some cameras also have a separate high ISO noise reduction setting that applies to the raw file, while others only apply it to JPEG. Check your manual to see whether it affects your raw data or only the output format.

Camera settings and considerations

Not all cameras handle long exposure noise reduction the same way. Some allow you to enable it only for bulb or manual exposures, while others have an Auto setting that kicks in when the shutter speed crosses a threshold. Read your camera's menu and understand what the dark frame process does to your workflow.

Keep in mind that battery life will be affected because the camera is doing twice the work per image. Also, the time between shots can be a problem for timed sequences. Plan your shooting schedule accordingly.

We recommend experimenting with your camera's feature while on a tripod. Take the same scene with the reduction on and off, and compare the raw files in post. You will quickly see how much benefit you get and whether the delay is worth it. For night sky work, a full-frame camera with a wide aperture lens is a popular choice, but many APS-C cameras also perform well in this role.

What to pick for your use

Questions

What is dark frame subtraction?

Dark frame subtraction is the process the camera uses for long exposure noise reduction. After the main exposure, it captures a second frame with the shutter closed, matching the same length and ISO. That dark frame records only the sensor's noise and hot pixels. The camera then subtracts it from the main image to remove those artifacts.

Should I use long exposure noise reduction for star trails?

No. Star trails are built from a sequence of long exposures that are stacked later. With the reduction on, the camera adds a dark frame after every shot, creating a gap in the trail and doubling the time needed. Turn it off and handle noise reduction in post-processing.

Does long exposure noise reduction double the exposure time?

Yes, the camera takes two exposures for every image you capture: one for the photo and one as the dark frame. That means the total capture time is effectively doubled, which can interrupt sequences and drain the battery faster. For single shots the delay is usually acceptable.

Can I do dark frame subtraction in post-processing instead?

Yes. You can take your own dark frame by covering the lens and shooting an exposure of the same length and ISO, then subtract it in editing software. This gives you more flexibility and lets you reuse a single dark frame for a series of shots if conditions remain stable.