Assuming from discussions above, that a +/- 40% of beam diameter is an acceptable focus tolerance standard:
The Glowforge’s focused beam spot diameter of .008" (per Dan) would permit a spot size range of 0.0048" - 0.0112".
Assuming even more - that the collimated beam diameter is 0.250" (which Google assures me is pretty much standard), the focal range that gives us the acceptable tolerance is +/- 0.0264".
You can see that at the 0.083" mark (which seems to be the focus error at .500"), the spot size is a whopping 0.0180". An error of 125%.
To get within the suggested +/-40% to have the beam to be considered focused on .500" material at a focused height of 0.417", the collimated beam would need to be 0.085" in diameter.
(The numbers look much worse for GF if we expand the beam diameter, so I am giving them the benefit of the doubt.)
I really wish @dan would weigh in on this. Preferably with something other than “it is within specifications” or “we can’t disclose that because its super secret”.
If it isn’t capable of performing to the original specs, I certainly don’t think anyone is going to freak out. On the other hand, continuing to be misled if it doesn’t…
(Can you tell I really want to understand this?)
EDITED TO ADD:
Here’s the full view of the scaled drawing above, for perspective:
