(Time for one of those)
Seems there are several different, but all focus related, sub-threads in this thread:
1. Lens Travel Distance - Housing Limitation*: In the original specifications, available when most of us purchased our units and up until somewhere in the first half of 2017, the lens travel was specified as: “Lens moves internally up and down inside the head by 0.5” (13mm)”. However, the lens housing and carrier are designed in such way that the physical travel is limited to 0.470". The Shipping Verification I received on 2017-08-21 did not mention lens travel limitation.
**NOTE: This item is about the device not meeting the original specification for “travel”. (Yes, I know we all agreed that the specifications were ‘subject to change’ when we bought the thing.) “Focal range” is covered further down on this list. **
2. Lens Travel Distance - Electromechanical Limitation*: As identified by @palmercr, the lens travel appears to also be limited in range by the stepper drive system. Currently, it seems the software only commands the stepper to move the lens within a travel range of 0.417". If you enter any focus value above 0.417" in the software, it appears that it will not travel any further than 0.417" from its 0 position.
3. Lens Travel Resolution Limitation*: Also identified by @palmercr is the limitation to the resolution of the lens travel. The electrical design of the lens motion system only allows for full and half steps. Lens travel during focus operations utilizes the half step mode resulting in a travel distance of 0.0278" for each step. Thus, the minimum lens travel is in increments of 0.0278".
Why is this a concern? Because the current specifications (though not the original) state that the focus is “Exquisitely precise — laser height measurement can measure the surface of the material to within 0.004 inches (0.1mm).” While this statement does not claim the actual focus resolution is that precise, the fact that it is listed under the “Auto Focus” heading could be interpreted that way by a perspective purchaser. (I feel this statement is misleading, at best. Why make a precision claim of 0.004" when the available resolution is limited to 7 times that?)
4. Focal Range: This is where there may be wiggle room for GF, even without redefining commonly held definitions of words in the English language. GF clearly states, and affirms, that the product can focus on materials ranging from 0 to 0.500". What they don’t specify (at least that I can find) are any additional details about their focal claims. @johnse brings up the concept of depth of field (DOF). To define DOF for laser optics, we need to know what the circle of confusion is for the GF optics (when dealing with human vision it relates to the focal range at which our brains still perceive the image to be in focus). Then with some math we could calculate the beam spot size at the focal point, and the variance at 0.083" out of focus. This would tell us the veracity of GF’s claims about the device’s focal capabilities. I, however, doubt we will ever be told this information by GF.
Now, think about this for a moment: Slight warps in material cause failed cuts, all the time. It’s a constant refrain from support that materials must be perfectly flat, and that ‘any’ warp will cause an issue. But, now we’re being told that a lens that is well over 1/16" out of focus (pushing 3/32" at the .500" material height) is well within proper focal range. This guy <- Not buying it.
5. Continuous Auto Focus: As it seems that this is still a claimed future capability, speculation on whether or not it will come to fruition may be a little premature. What use it will be in practice with the device’s limited focal resolution remains to be seen. We’re talking almost 1/32" steps in focal height. With numerous reports how sensitive the device seems to be to material warps well within that range (perhaps those issues are partially explained by the limited focal resolution), it may present a challenge to provide continuous auto focus that doesn’t result in very visible artifacts.
*These apply to my Basic Glowforge. They may (and likely do) apply to others, but they have only been confirmed on my unit. My unit may be a one off, or it may be representative of every production unit. One thing we know for sure is that GF likes to change things without saying anything.