Image Stacking is the term for another post-processing workflow used heavily in macro photography and astronomical photography. The reasons for image stacking in these two types of photography are totally different.
First, astronomical photography and, in particular, photographing deep sky objects are challenging because of the rotation of the earth which causes the apparent movement of the stars across the night sky. For deep sky objects, photographers need a motor drive attached to the tripod that mimics the rotation of the earth. Most photographers set up their tripods with camera and telescope with the attached motor drive and the setup is likely portable. This setup theoretically requires perfect polar alignment and perfect movement on both the right ascension and declination axis. This perfect setup is almost impossible so photographers take a series of images of two to three minutes in length. A relatively short exposure like two to three minutes won't show any star trails. To compensate for a two hour exposure that is needed, the photographer will take a series of 60 - two minute exposures that are then aligned and stacked in software specifically created for such photography. The light from those images are added together. Dark frames and flat frames are subtracted that decreases the effect of sensor noise from the camera and to add contrast to the final image. I hope to do more experimentation with this process in the future. Check out some of the images created by Jason Dain from Nova Scotia. His work can be found at...
Secondly, image stacking in macro photography is for a very different reason. If you have ever experimented with macro photography using a dedicated lens or with macro tubes between the camera body and the lens, you will quickly find that the depth of field is razor thin. That is, the area that has perfect focus is very small. For example, if you wanted to take an extreme closeup of a small insect, you would have to take many images focused on different parts of the insect. One photograph would only show a small part of the insect in focus and the rest out of focus. The series of images would then be aligned and stacked in software such that the final result would be one image showing the whole of the insect in sharp focus. To see the results of such work, check out Don Komarechka at https://www.donkom.ca/ . His work with snowflake photography is quite amazing.
This same stacking process can be used to a lesser degree in landscape photography. Rather than use many images in the stacking process, only a few can be effective. Focusing on a foreground object and then focusing on a distant object and combining the two in post-processing can make for an effective image. The following image is where I combined two images taken at different focal lengths and then stacking them.
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