If you are using air tools 8 inches should be lots of room for engraving, I'm not sure what is adequate for h&c engraving or other jewelery work, depend on the tools used and the size of your hands, I'd guess.
Cheers
Don
8 inches is pretty much the standard for a scope with a .5 power objective added. Most scope used for engraving will start with a 4 inch working distance and are also too powerful for your needs. The barlow lens, (.5) will cut this in half and increase dist.. to 8in
8 inches is a lot of room. You have to take into account how tall you are too. The longer the working distance under the scope the more "stretched out" you may be to actually work (hands lower relative to your eyes). Not a problem for a 6'4" guy - bigger problem for a 5'4" girl....
it all depends upon what and how you intend to work. i think sam made a post where he did h&c with a scope. for me that would never work. for h&c i only do this in a standing [position. as tira mentioned, one size does'nt a;ways fit all. no matter what, your arrangement must allow you to work in a comfortable and relaxed manner. this means scope, vise and stool should be adjustable to get you into the sweet spot for engraving. 8" will be more than enough for you to work.
If you buy a scope from a dealer that sells to the jewelry or engraving trade it will almost certainly come with an objective lens to provide adequate working distance. 0.5x is the standard. If you buy from someone else check to see if it has one. If not, order one for it.
A standard stereozoom engraving/stonesetting microscope configuration is: 10x eyepieces, 0.5x objective, and zoom range of 0.7x - 4.5x. Different objectives like 0.63x or 0.4x will increase/decrease magnification and working distance.
FYI, a 0.5x objective lens reduces the power of the scope by 50%. Without an objective lens the scope would have way too much power and not enough working distance for engraving or stone setting. The 0.5x will provide about 6" of working distance on most microscopes.
I bought my scope from the microscopestore.com It was their top of the line and paid about 1200 for it complete. When we were dealing and it came with the .05 Barlow on the engravers deluxe setup I got him to throw into the deal 20x eye pieces so I could look at neat crap like bugs and mineral specimens. Really good folks to do business with. I had a problem with left tube. I had carefully cleaned the lens, all I could reach without disassembly to know avail. I called him and he sent me a newer model of the Ohnamo. Unfortunately had the same spot and lens seemed a little browner when blocking the other eye. Trip to ophthalmologist said cataract forming small and to soon to do anything about but it would have to be dealt with at some point. Now having problems with double vision when tired. One is brighter than the other and has a tiny brown spot. Don't drive much now. But the point is the scope purchase helped me find a medical issue at their expense. when I called them to tell them it was my eyes not their product they did not want it back but were simply glade it helped find the real culprate. Fred