Thank you for making this. It's the "small, but useful" kind. Another good one to add to my arsenal.
Initially, I was seeing the wrong image because in the project I tested it on already had a "focus" image.
Jumping to focus_test is straightforward to see it in action. I intent to use it but for different purpose (likely, with tweaks).
I just wish I didn't have to edit variable variable names and image names for it (in my case, the "focus" image and the "focusing" variable).