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Since the beginning of the digital age in photography, stacking images has become much more common than it used to be earlier. But what are the benefits of stacking images? Well, as simple as it may sound, you are able to achieve results you wouldn't have been able to obtain with a single photo. You can get a cleaner, sharper image and you can compress tones to gain better dynamic range in your photos by stacking. Let's look at them in detail.
1. Creating HDR Photos
Stacking 3 or more images shot at different exposure settings (underexposed, normal, and overexposed) can be used in order to expand the dynamic range beyond the capabilities of the camera. Usually, 3 shots 1 EV apart do the trick. However, there are no limitations (at least by software) to the number of photos you can use. Generally, more than 5 photos is not worth the hassle since they don’t help much. With the improvement HDR software solutions offers, processing RAW bracketed files is an option nowadays. That is great improvement on the amount of quality and detail over JPEG HDR processing. That’s true especially when it comes to noise.
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The 3 sample images are blended together through HDR software to create a composite with greater amount of dynamic range
2. Focus Stacking for Macro Photography
We can call it “macro photographer’s way of life”. Having enough depth of field to squeeze the whole subject into focus when focusing up close is really hard. The only way to go usually is to shoot more images at different focus and then stack them together in order to create a composite image which will have bigger field of focus than a single shot would have. This process usually requires a tripod and some fiddling to get it right, however image alignment in Photoshop is so advanced nowadays that you can do it handheld with some practice. The only issue here is to have the same light during these shots. If the light changes it might pose some issues with the stacking. Whether it will confuse the algorithm or it will show up when stacked, the fact is that you’ll have issues and the result won’t be perfect. Make sure your light is constant between the shots and that you don’t shift your perspective too much. Image alignment and focus stacking algorithms in Photoshop will do the rest for you.
Example of a focus stacked image. Photos shot hand-held.
3. Image Stacking for Noise Reduction
If you are photographing some products and you want the most noise-free image possible, image stacking is the best way to go. This means essentially shooting the identical image over and over again, and then stacking all of it with median to reduce the noise. Median essentially averages out each pixel position value between shot, thus effectively reducing random noise. You can achieve further noise reduction if you shoot a black frame and then use that to reduce hot pixel noise (usually present with longer exposures). Hot pixel noise occurs when some pixels get exposed differently than others, and since that is more or less constant, it is easy to eliminate. Exposing a bit to the right (over exposing but without clipping) will ease the process because you’ll have better signal-to-noise ratio. Combine exposing to the right, black frame and image stacking and your product shots won’t have a hint of noise.
Example of the amount of noise being reduced with just 10 photos merged together.
4. Star Trails and Cloud Brushing
All those who are inclined towards more abstract and artistic styles will be happy to know that image stacking can make cool effects, too. Like star trails, for example. When shooting stars you use longer exposures than the 500 rule states in order to create star trails in your image, and you essentially shoot the same image over and over again. The stars moving on the horizon create trails, so when you stack those photos together with a software like StarStax you can create whole trails of stars from the beginning to the end of the horizon, all depending on the amount of frames you have shot. You’ll ask: what about the time between frames, wouldn’t that create intermissions in the trails? The answer is yes. But do not despair, StarStax and similar software solutions correct that so the lines are whole and look good in the picture.
Multiple star trail images being stacked into one. Photo by Robert Hensley
The same thing can be done with moving clouds in a cool landscape. You could easily do it with a long exposure, but clouds often move too slow for the longest exposure to make nice and long cloud motions, thus you’ll need more than one exposure – the longest possible ones for the smoothest effect.
So, I suppose you’ll use stacking more often from now on to get better results. Photographers use this technique for even more than what’s mentioned in the article. Do you know any other creative ideas where stacking plays a crucial part? Let me know in the comments.