It is a special tool that helps determine how good the focus of the camera is when taking close ups!
After you file and debur your piston rings you still aren’t finished with today’s metric size and narrow width pis- ton rings. Lapping the rings on both sides gives them a much better fit in the pistons. This lapping removes all high spots and any ring coating buildup. All you need is 400 to 600 grit wet and dry sand paper and a flat plate of granite or steel, or a piece of thick glass.
I do not mind if you now call me a " know it all", I have been called it so many times it's like water running off a ducks back.