I can understand why it's hard for some. I've landed on that wiki page a while ago and couldn't figure it out. Then found a similar thing on an itch.io page that was easier for me to figure out.

In these later examples (starting with the easy puzzle of the OP, and your 3d examples), I find that I do the process in two stages.

Unfocus my sight until the third image shows up in the middle at the correct size (as a blurry mess). Then try to focus the center image.