You set the focus to the height of your material above where the bed would be if it was there.
The bed of my machine is 1.385" above the floor. That means if I put something in that sits 1.5" above the floor, my focus height is 0.115".
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Something very strange is happening now. There is no tray. When i try to give it a height of 1.92, it says to make the height lower in between 0 and .5 So i change the height to .42 and click print. At which point it calculates and says that without the tray height must be set to 1.5-2. At which point i go to change the height again to find it greyed out and unable to manipulate. When the page is refreshed the height box is still greyed out but just reads “auto” with no numbers. I have repeated this process many times, please help.
If you are using the auto focus, the top surface of the material to be worked on must fall between 1.4" and 1.9" from the metal plate at the bottom of the machine. That is the range that the actual physical surface has to be placed into for the camera to be able to focus correctly.
Get the top face of the object into that range, then use auto focus. If the material is thicker than ½ inch and thinner than 1.4", you are going to have to prop it up on something to bring it into the correct range.
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How do I activate auto focus? It just has “auto” locked into the height slot and greyed out. When I try to print ot tells me print canceled and tells me the restrictions height restrictions for printing both with the tray in and out. The object is propped up on some glowforge boards
Make sure the top surface is in the correct range, then click on Set Focus in the three vertical dots icon, then click in the center of the material, not too close to a cutout.
After you do that, the view will improve…you can shift your design and get better alignment. If you haven’t run the Calibration routine, doing that also improves alignment for your machine.
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The tray is out and you cannot set the height. and when you try to prepare the print it says it cannot and continue and gives the height restrictions.
What material are you choosing? If is is uncertified, you should be able to leave the “auto” locked in place and use the set focu(The set focus tool is in the drop down menu under the three dots in the interface.) If the surface you are engraving is between .5" and .001" inches from the lens, the set focus tool should work very well. Click set focus and place your cursor on the material and click. Watch to make sure that the red focus beam lands on the material and you should be good to go.
I read through this whole thread only to discover the whole process has been replaced by “set focus”… 
maybe newbies_234 can modify the initial text, so beginners know they have it much easier these days? 
Welcome to the forum.
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I don’t pay attention to updates to the GF these days, but I believe you are incorrect. This calculator is meant to be used by people who are removing their tray from the GF and need to figure out how high something has to be to elevate the item they want to laser.
Set focus works great when the item is small enough to use on the regular tray, or even once you’ve raised it high enough without the tray. But once you start to remove your tray to laser bigger stuff, that’s what this is for.
If the GF does all of that these days, then yes this can probably be archived/deleted.
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maybe I just got lucky
I used your great calculator to get close enough, so autofocus could do the rest for me… I didn’t have to enter focus manually.
I suppose a note in your original post would suffice, the information is still valid and useful!