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Photoshop’s image stacks
A lesser-known feature in Photoshop is image stacking. One of the best uses for image stacks is to remove the digital noise that appears in photographs when you use very high ISO settings or leave the shutter open for long periods of time. Digital noise appears as unwanted spots of color, especially in darker areas of constant color and tone. These spots of color are random, so if you shoot the same scene multiple times with the same settings, the noise will appear in different spots in each image.
You can see an example of digital noise in this photo which is the result of using a high ISO, in this case, ISO 6400. Most image-editing programs have noise reduction settings, but these settings usually reduce noise by blurring the image very slightly, which affects the sharpness of the image. With Photoshop’s image stacks function, you can combine multiple versions of the same image taken with the same exposure settings, and Photoshop will average all of the pixels and remove any artifact that does not appear in every frame, effectively reducing the appearance of digital noise in the final image. This technique works best when you have quite a few images to work with, like 8 or 10. Just make sure that your camera is mounted on a tripod and that you photograph the same subject with the same settings every time to ensure that the only thing that varies in the images are the random spots of digital noise.
Making an image stack
Once you’ve captured your source images, it’s relatively easy to create and process a stack of images in Adobe Photoshop:
- 1 Open [File > Scripts > Load Files into Stack]. In the Load Layers dialog, select the files you want to use and check the two boxes at the bottom of the window: Attempt to Automatically Align Source Images and Create Smart Object after Loading Layers (Figure 36.3). Smart Objects allow you to do nondestructive editing on the layer, which is key when applying a Stack Mode in the next step.
- 2 Choose [Layer > Smart Objects > Stack Mode], and then select one of the modes from the submenu. There are a few choices and they all do slightly different things with the image stack. The best Stack Mode for removing noise from an image is the Mean Stack Mode.
When you select the Mean Stack Mode, the software looks at all of the nontransparent pixels and averages the values for each. The software does nothing to the pixels that are the same in each image, but wherever noise is present, the pixels are averaged with those that don’t have noise in other images and the noise effectively disappears. You can see the difference this makes in the figures below. Figure 36.4 shows just one exposure, and Figure 36.5 was created from a stack of 10 images. I used an extremely high ISO for these images to show how well the stacks method actually works when you use it to reduce digital noise. It works just as well for images shot at lower ISOs.
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