The second test image appears to highlight the fresnel ridges, allowing you to locate the sweet spot even more accurately using the high contrast glare. Update: As per the suggestion of /u/CocoCarnage I've included his test image as well, just press spacebar to toggle between the two. Update: 3rd version, this one now has text for the crosshairs that can be toggled with the 't' key as suggested by /u/Milopapa
