The Best Way to Reduce Noise at Night
June 21, 2018
Noise is the great enemy of night photographers. Sure, the noise-reduction sliders in the Detail panel in Lightroom work pretty well, but the result is always a compromise. Noise-reduction software attempts to suppress the high-frequency variation in brightness and color that is unwanted noise while preserving the high-frequency variation in brightness and color that is desirable texture and detail. Inevitably, the more you reduce noise, the more you lose fine detail.
Instead of attempting to smooth out noise, delete it using Photoshop’s Stack Mode>Median. The basic idea is to shoot a series of identical frames and stack them as layers in a Photoshop file. Photoshop can then drill down through the layer stack, comparing the value of each pixel to the value of all the other pixels directly above and below it. Photoshop then chooses the median value—the number in the middle of the range—and displays it.
Here’s a simplified example. Let’s say you shoot four identical images. As Photoshop drills down through the layer stack at a particular point in the scene, it finds four pixels. Three have RGB values averaging out to 100. The fourth has RGB values averaging out to 200. The pixels with an average RGB value of 100 are pixels showing the correct brightness and color. The pixel with an average RGB value of 200 is noise—a random and undesirable variation from the true value you want to capture. The median value of that set of four numbers is 100, so that is the value Photoshop displays. Instead of smoothing out the noise, you’ve removed it.
In the real world, of course, the effect isn’t quite that dramatic. In my testing, I’ve found that shooting four frames at ISO 6400, then reducing noise with this method, produced an image that looks like it was shot at ISO 1600. Shooting 10 identical frames shot at ISO 6400 then stacking them to reduce noise produced an image that looked like it was shot at ISO 800 or even lower. Results from your camera may be different, of course.
It’s easiest to apply this technique to the land, rather than the sky, since the land doesn’t move between frames. Fortunately, the land portion of a night image is usually the part that shows the most noise. Noise is always worse in the deep shadows, and the land will always be darker than the sky. In addition, noise-reduction software does a better job with the sky than it does with the land. Usually you can find a setting that will smooth the sky while also preserving faint stars. Applying heavy noise reduction to the land, on the other hand, can easily smooth out textures to the point that the land looks like injection-molded plastic.
You can also shoot multiple frames using the correct exposure for the sky, but the stars will move enough between frames that they won’t align when you stack the images in a layered Photoshop file. Photoshop’s Auto-Align Layers utility is designed to identify features in the land portion of the frame and make them align. It won’t work to align stars. Deleting the land from all the frames won’t help; in my experience, Photoshop will simply report that it can’t align the images at all. Registering the stars requires specialized astronomical software. As I write this, the best programs I’ve found for this purpose are RegiStar from Auriga Software (PC only) or Starry Landscape Stacker (Mac only). There may be other programs available by the time you read this. Once the stars are aligned, you can use the Stack Mode>Median-Mean utility built into RegiStar or the noise-reduction utility built in to Starry Landscape Stacker to reduce noise. You’ll then need to stack the good-sky and good-land images in Photoshop and blend them. The process of aligning the stars turns the land into a blurry mess, so you can count on lots of cloning along the skyline to create a clean boundary between the two images.
Here, in detail, is the procedure for using Stack Mode>Median to reduce noise in the land. Start by shooting at least four, and preferably more, identical frames using the correct land exposure. The more frames you shoot, the more you’ll be able to reduce the noise. Once you’ve collected the necessary frames, download them to Lightroom, select all of them, and open one in the Develop module. Be sure Auto-Sync is enabled so that a change applied to one will be applied to all, then edit the good-land images to taste. Don’t apply any luminance noise reduction at this stage. Experiment with turning off color noise reduction as well, then applying it again once you’ve created the finished image. You may find this helps preserve additional detail. With all the images still selected, right-click on one of them and choose Edit In>Open as Layers in Photoshop. In Photoshop, select all the layers by clicking on the top layer and Shift-clicking on the bottom layer. Right-click on one of the layers and choose Convert to Smart Object. Then choose Layer>Smart Objects>Stack Mode>Median. To reduce file size, right-click on the Smart Object layer and choose Rasterize Layer. That’s it—you’ve just learned the best way to reduce noise in night landscapes.
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