Actually there's no 'average' distance, it's very specific, as the beam must be in focus to plus/minus a small distance. Lens focus length determines the plus/minus range...
Yes the different lenses change the distance, dramatically. The beam begins from the lens center, at the mirrors above the lens. Lens size determines the focus distance. From the lens center the beam emanates outwards in a cone shape. The larger the lens, the farther the focus distance, which also increases the 'cone size', aka the working area. As focus distance increases, so does the minimum beam spot size, and the larger the beam spot, the less power density-- so while longer lenses increase working area size, the compromise is you have less power to work with, and to a certain extent, detail suffers. Shorter lenses decrease working area, but power and detail increases. Always a compromise
One common lens I DON'T have is a 100 (I'm talking working area with these numbers), but I do have two 220's, two 150's and a 70 -! The 220's and 150's are different lenses as to physical size. The small 220 focuses at over 17" from lens to focus point, the larger one is around 14-3/4"... the smaller 150 focuses about 11", the larger around 9-3/4"-- my little 70mm focuses at only 4-3/8" from lens to work, and only has a 2-3/4" work area, but the power increase over the 150 is amazing! It'll take aluminum out real quick, steel almost as quick. Of course, there's a compromise or two; the beam spot is so small that a tighter hatch is necessary, and between the extra hatch lines and heat, slag buildup can be a problem...
As to .001" deep in anything, it depends on the engraving area... a 1/4" x 1" rectangle, to get a clean .001 deep, anywhere from 30 seconds to minutes, many variables as to lenses and what one considers 'clean'
-- just a while ago I engraved "1911 A1" into an old Ranger slide, I made it resemble a single tool-cut pass, text height was .188", I ran it 40 passes of 2 cross hatches: two 'digger' cross-hatches and 2 'cleanup' cross-hatches, followed by another 20 cleanup only passes, for 200 passes total- Sounds like a lot but they're FAST passes, the whole run took less than 4 minutes. Yeah, that's more time than tool engraving, but (A) Setup is MUCH quicker, (B) I can get another job working while it's running, and (C) it takes time to sharpen dull carbide tools... One thing I'll never have to do is sharpen a laser beam!
Here's a silly-putty impression of the engraving, it's an honest, and clean .003-.004" or more deep in hardened steel...
a1.jpg