The 0.5 barlow lens is the key to great working distance.
This is from my very first post on EEVblog Simon thought it was spam.
I have been looking for a reasonably priced boom mount sterero zoom microscope for a long time. Features that I wanted were a very long working distance (distance from the objective to your work). This is so I could solder or assemble/disassemble under magnification. And very wide fieild of view. Amscope model SM-3BZ provided all of these features.
http://store.amscope.com/sm-3bz.html .
One important thing I learned is that the .5 Barlow lens is what gives the 165mm 6.5" working distance! Your total magnification is your eyepiece mag. x your zoom mag.(.7 to 4.5) x your Barlow mag. So the lowest mag possible is 10 x .7 x .5 = 3.5 mag. At this setting you have 165mm working distance and 57mm field of view. if you now zoom to 4.5 you have 10 x 4.5 x .5 = 22.5 mag. At this setting you have 165mm working distance and 8.9mm field of view. So if you want to maintain that 165mm working distance at higher magnifications you need to get a set of 20x eypieces also. this will double the magnifications above without changing the working distance. Wtih the 20x eyepieces and the .5 barlow you have 165mm working distance and 28.6mm field of view at 7x mag and 4.4mm field of view at 45mag. You could get the same values with 10x eyepieces and no Barlow but your working distance drops to 100mm. If you put the 2x Barlow on your working distance drops to 30mm Ouch!
I asked them to exchange the 2x Barlow for two 20x eyepieces and I would pay the $20.00 US difference. No problem and it came with the 2x Barlow anyway! That give me the ability to go to 180x. at 180x I can see the ripples in each individual pulse puddle of laser marking on a metal crystal can!
Also got the 48 led light ring. Scope is worthless without lighting.
If you mention that you saw the Microscopes on thier Ebay store you get free shipping! So my entire package cost $463.98 US delivered.
The quality is outstanding both mechanicaly and opticaly. Ii is all metal construction and all glass optics. I still cant believe the qulaity at this price.
Keep in mind this is the model I picked for my purposes and you can get much less expensive and much more expensive units from them. I do feel this is the ideal model for serious electronic work with the boom mount and the huge working distance and wide field view. You can get the same models with camera tubes but I take pictures right through the eye piece and i works great for the number of times I need to take a photo.
I give it three thumbs up!