The Leica electronic depth of field indicator. A short video from Red Dot Forum posted here in response to questions about achieving deep, sharp focus.

 https://www.youtube.com/watch?v=74fD5TAjbCk&t=26s

Or, how to use depth of quick, electronic, depth of field calibrations to control the drop off of focus from an object you want to be sharp against a background that you want to have less sharp....