(Editor’s note: Photofocus is proud to add Rocky Nook as a vibrant contributor for our readers. Rocky Nook is a small — seven-person independent publishing company. Their commitment is producing high-quality books written by pros for photographers who want to learn more about their craft. Over the next several weeks, Photofocus features a series on multi-shot photography excerpted from “The Enthusiast’s Guide to Multi-Shot Techniques” by Alan Hess.)
Image stacking in Photoshop is commonly used in the field of astrophotography, which involves taking photos of the night sky, including stars that are very dim and are not always visible to the naked eye. The main issue when photographing the night sky is that very little light reaches the camera. When you point your camera at the sky and take a single image, you’re only able to capture the brightest stars, even if you use a long shutter speed and high ISO as shown in the photo below.
The goal of stacking multiple images is to increase the signal-to-noise ratio, which gives you more of the information you want with less noise. This will not increase the color or make the overall image brighter, but it will give you more information so you can make those adjustments with image-editing software.
The other issue you can run into when photographing the sky at night is that if you use a longer shutter speed, the stars will look less like stars and more like streaks of light in the sky. This is great if you want to create star trails like those in the photograph that opens this article, but that isn’t the type of image we are looking for here.
Eliminating noise and other random stuff
The solution to this problem is to capture multiple images and use the computational power of the editing software to get the average brightness of the scene. This helps to eliminate the random stuff and makes the details stand out.
Point the camera at an interesting part of the sky and take multiple images, each with a shutter speed of about 10 seconds, a wide open aperture, and an ISO of 800. Make sure the focus is set to infinity and the autofocus is turned off. The number of frames you use for the image stack makes a difference. In short, the more images you use the better. You also need to take a set of images with the same settings with the lens cap covering the lens. These images are called dark frames and the image-editing software uses them to reduce noise through a process called dark-frame subtraction. The stacking software usually requires the same number of dark frames as normal frames, so if you take 50 frames of the sky, you need to take 50 frames with the lens cap on. Once you’ve taken these images, you can use image-editing software to stack and process them (see lesson 39).
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