Yeah me neither. Just wanted to point out that it was still there somewhere.
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This was just posted in another thread talking about a page that you’re shown when it’s time to enter your address (https://community.glowforge.com/t/11-x-19-5-actual-work-area/):
The link in the thread has a pdf from November, and in the pdf there’s a page talking about autofocus:
3D Autofocus The precision laser beam on your Glowforge automatically focuses on both flat and curved surfaces.
Done? The software uses a low power visible laser and 5 megapixel camera to measure the height of your material to the thickness of a human hair, then focuses precisely so every cut is as quick and clean as it is powerful. Single-point autofocus works perfectly for flat material.
To Do: Multipoint autofocus, which can continuously change focus to print on curved surfaces.
Seems it may have been re-termed as “multipoint autofocus” instead of “continuous autofocus” and the marketing “curved, uneven, or irregular” might be reduced to just "curved surfaces. It looks the there will be some implementation of it, which is good.
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I think you are both correct, but are talking about slightly different things.
The statement is that it can focus on materials up to 0.5”. It does not say that the focal point can move 0.5”—which is what you are claiming they said. So, you are right that the lens travel is restricted.
But there is also the question of depth of field. If the beam is focused sufficiently at 0.5” even though it is .04 to .08 away from optimal, then they are stating the truth, just as much as you are stating truth that the lens travel is only 0.42.
My range of possible errors is based on the supposition that they may have the lowest point also slightly above the bed.
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Yes just redefine what to focus means. Like everything else with Glowforge it is only not a lie if you redefine English.
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Sorry guys, I just focused one of my lenses to infinity and caused a small incident in Andromeda. 
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THREAD RECAP:
(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.
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Only thing I would argue is the assumption about flatness of material and cut success being directly related to focus accuracy or resolution. Support’s comments are related to automatic settings when using Proofgrade only. These settings seem to be extra conservative to just barely cut through and minimize char and kerf. If the focus is different than company testing for these specific thicknesses (approx 1/8 & 1/4) then the cut may not be successful. Focus can be perfect or wrong as long as it is the same as used for their tests.
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I’ll concede that it may not always be the only factor, or even one of.
But given the case where the failure to cut through the material is attributed to it not being completely flat (it certainly seems that is typically what support implies, along with the lens/mirrors/windows being dirty), I would say that if minor variations in focus can (and usually do) have an impact on how well the conservative settings for Proofgrade perform, then I think it still holds that the focus is a very important factor.
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I believe the commonly accepted answer in the laser world as to “depth of focus” is a 40% spread on either side of the focal point. If I remember correctly, with a 50mm lens and the spot size that has been mentioned on this forum, that number is about 0.1”. I believe @palmercr also came up with the same numbers.
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Depth of focus is a different metric. The fact remains that for materials 0 to 0.4" thick you can focus on the surface. For 0.5" materials you can’t. How much affect that will have depends on the DOF but focal range does not include DOF.
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Good thing you don’t live in the States. English is redefined here everyday. Your head would likely explode 
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I don’t have a GF yet, how does it actually autofocus? is it camera related, does it have a proximity sensor, does it use the actual laser beam used for phase-shift or time of flight measurements?
It has a separate visible laser on the “print head” from which it projects a red dot at an angle, then it takes a picture of it with the camera and calculates the height from the offset.
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You can see captures from the head images when it is measuring the material height here.
I think the precision of the measurement will depend on the material it is measuring. Obviously, transparent materials will be difficult to measure, as well as reflective materials.
The pictures shown in the link are of unfinished baltic birch.
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It might be messy but dusting the top of clear or reflective work material with a fine powder will help the focusing laser to “see”. It won’t last long, as the air assist fan will blow it away, but till then…
Alternatively applying a thin layer of something akin to rice paper or tracing paper (as thin as possible, laser safe, & opaque) would also do the trick and not blow away if a spray on tack adhesive was used.
At the machine shop we used pressurized sprayers to apply a fog/mist proprietary Swiss oil on the top of the sheets of stainless (due to reflectivity) and aluminum (to reduce surface splatter from the pierce welding to the sheet surface).
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Has anyone actually verified that focus height 0 (i.e. bottom of travel) is positioned at the top of the crumb tray? Or that entering 0 positions the lens at that position? Or that multiples of 0.7mm yields exactly the expected number of steps up?
If the accepted beam spread for being in focus equates to 0.1”, as @jbmanning5 commented, then even if the surface to 0.417” is exact above-the-crumb-tray distance (+/- 0.35mm) the surface at 0.5” is within that focus range.
No language redefinition needed.
This may not be the answer you want to hear, but it is within spec at that point.
I’m not understanding the point being made with dof. The depth of field remains the same in either case. With the old spec of 0.5in of focus lens travel, a lens motion of 0.5in would have given you Zdof+0.5 of total effective focusable range. The new spec, the effective focusable range is max 0.5in.
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I have verified that when you enter 0 it moves 0 steps and for each increment adds two half steps up to 30 at about .4" and doesn’t move between .4 and .5". So it cannot be said it can focus at .5" because it is still focused at .4
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I’m not sure where the .1" figure comes from, but it seems excessively large.
Can someone* do a quick experiment and score a line in Proofgrade acrylic at the correct focal height, and another one with the same settings but focused .1" higher? I’d be interested to see the difference.
* I’d do it myself, but my results would not be representative as my 'forge is not currently in a ‘factory’ configuration. 
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Here’s an experiment I did with 1/2" acrylic from Inventables. This was done in Oct 2017, before the software allowed .500" focus.
This is the inverse of the experiment I mentioned in the post above. Here, I am moving the focus from .300" below the surface of the material to .077" below.
But, the difference in .100" of focus change is very definitive, nonetheless.
What is difficult to see in this picture, but is there if you look closely, is the change in kerf. Look at the top cuts verse the bottom cuts.
I still have these pieces somewhere. I’ll see if I can get some better pictures of the changes to the kerf.
An interesting side note is that you can clearly see the over-burn at the start and stop of each cut when the laser is at full power before the head begins to move.
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