cmcguinness/focusstack — reverse-engineered prompt

Build me a simple Python tool for focus stacking photos. I want to put a set of images taken at different focus depths into an input folder, run the program, and get one merged image where the sharpest parts from each photo are combined into a single final picture.

Keep it easy to use from the command line. If there are sample images, use them so I can test it right away, and save the result as merged.png by default. The main logic should stay cleanly separated from the small runner script so I can reuse it later.

Please make the code straightforward and readable, with sensible error messages if the input folder is missing or there aren’t any images. Use common Python image processing tools, and look up current docs online if you need to.