The OMTech 60 W laser cutter has an 18 mm lens with a 2 inch = 50.8 mm focus distance:
The tip of the cone sits 18 mm over the focal point.
The red “pen” is the auto-focus probe switch. It has essentially zero overtravel after tripping, with enough pretravel to hover barely 6 mm above the focal point.
Lightburn can produce a focus test pattern:
The left pattern comes from a piece of paper with the focus point at the surface, corresponding to the “0.0” line.
The right pattern is on another piece of paper atop a 3 mm cardboard sheet, so the “0.0” line sits 3 mm above the focal point. The pen tip brushed a protruding corner after the “1.0” line, showing the need for a really really flat sheet of whatever you’re engraving / cutting and definitely limiting the height of any clamps / fixtures you might think of applying to the sheet.
I ran a manual test firing 50 ms pulses into thin cardboard atop a 9+ mm (3/8 inch + protective papers) sheet of plastic (distance to lens increases to the right, the numbers are the nominal surface elevation relative to the autofocus level):
The “0” pinhole may have happened near the actual focal point, but the true distance isn’t particularly accurate.
A closer look suggests the focal point sat 1 mm above the nominal zero:
In round numbers, the lens produces decent focus within ±1 mm of the focal point. The smallest hole is 0.3 mm, with the scorched ring bringing the mark to 0.4 mm, although the blast radius depends on the material and laser current setting.
A closer look at the focal point target from the beam alignment tests:
The hole is slightly off-center because I hadn’t yet tweaked the red dot laser.
The hole came from two pulses, one at the autofocus level and another with the platform 10 mm lower. Comfortingly, the beam hit the same spot, with the slightly defocused beam chewing the rim a little larger, showing that the beam path is perpendicular to the platform motion.
So far, so good!