First, some basic information. If you want to look at micro minerals, you do not want to buy one of the common biological microscopes intended for use at high magnifications with transmitted light. What you do want is a stereo microscope intended for use with reflected light. You want the kind of thing that is used for looking at bugs and flowers, or perhaps micro electronics.
Surplus microscopes from electronics manufacturers and research laboratories are just what you want to get your hands on. There are lots of things to consider when buying a used microscope, but in particular when buying a stereo microscope, insist that you be able to return it if it is not in proper optical alignment.
The following are some notes on microscopes that I own or have some in-depth experience with.
Two people I know have SZ series scopes and swear by them.
The SZX-12 is very well regarded, with a 12.86:1 zoom range. 800 lines per millimeter resolution is claimed.
Joe, who has the SZX-12 uses it with 10x eyepieces and the 1.6x objective. This gives him a range of magnification from 16 to 144 (my math says it should go to 192x). He sometimes wishes he had the 10x, but overall likes that setup very well. He has tried higher power eyepieces, but feels that the crucial thing is to get the magnification you need by using an objective that gives it to you. Higher magnification eyepieces he feels tend to give "useless magnification".
Also Joe has a turret that would allow him to have two objectives mounted and handily switch between them. He does not use it because it interferes with his fiber optic illuminator. Whenever he would switch objectives, he would have to rearrange the fibers and finally he just gave up on it.
The SZX-9 offers a 9.05:1 zoom range (0.63 to 5.7x). New prices are $7000 to $8000.
The SZX-16 is the current high end model. It resolves 900 lines (with an N.A. of 0.3). The zoom range is 16.4 to 1.
The SZX-10 is the current "less expensive" model. The marketing literature says it is "designed for routine research". With an N.A. of 0.2 it resolves 600 line pairs per millimeter.
The SZX-7 is the current "budget" model, with an N.A. of 0.1
There is also an older but perfectly fine SZH series which seem to have optics of the same quality as the SZX series, but some different mechanical features. In particular, the eyepieces cannot be tilted up and down to different angles. My friend Jerry got a good deal ($4500) on one of these.
The SZ6045 is a very nice microscope, but not in the same class as the SZX series. Zoom is 1 to 6.3x. I have seen used copies for $1000 or $1200. Refurbished copies with a "pedigree" for $2200. Here is a product brochure
Note that Leica picked up the old American Optical and Baush and Lomb lines and these inferior scopes may now be being sold under the Leica label, beware.
The "Cycloptic" designation refers to the fact that a single large common objective lens forms a single image which is viewed by the pair of eyepieces. This seems to be the usual arrangement with modern stereo microscopes, but was somewhat unique at the time (1957). The objective lens provides a fixed and generous 4 inch working distance. Truly old stereo microscopes were in reality a pair of independent microscopes mounted side by side, each with its own objective and with a view converging on a common subject.
Also note that the turret that is rotated to obtain different mangifications holds a set of Galilean telescopes (or Galilean lens systems). These get used in both forward and reverse directions for different effect.
Various "turrets" were supplied to allow different magnification choices. No optics in the turret at all yields 15 power (so a user might expect this to be the clearest view, since the fewest optical surfaces are involved).
The "J" turret offers 7, 15, and 25 power.
The "K" turret offers 10, 15, and 20 power.
The "M" turret offers 7, 10, 15, 20, and 25 power.
The above powers are with a 10x eyepiece.
The J and K turrets have 4 positions (two are identical at 15x). The M turret has 6 positions (with two identical at 15x).
The astute reader will have figured out what is happening. A J or K turret has one set of optics, used either forward and reversed, and a clear path (and it doesn't matter which way you look through a hole). The M turret has two lens systems, each of which can be used forward or reverse for different effect, and a clear path.
Bausch and Lomb makes a whole series of scopes, from economy student models to higher end models. I have been disappointed with all of the Bausch and Lomb optics I have had a chance to look through, despite their excellent reputation, which I think is overhyped.
Wild makes excellent scopes. Apparently Meiji microscopes can be good, and I am sure there are others that I simply haven't had a chance to get my hands on.
One issue that crops up is the question of tube length. The most commonly encountered situation is an objective that provides a real image at what is refered to as a 160mm tube length. (This is refered to as the intermediate image) The eyepiece is simply a means of studying this real image, and most eyepieces yield a virtual image within the microscope, not a real image behind it. The other arrangement (I have only encountered it in the AO cycloptic microscope) is an objective that places the image at infinity. This of course requires a compatible eyepiece. Some people say this doesn't really make a difference, but my experiences with the cycloptic say otherwise.
I may be muddying the water here somewhat, so if some real expert is reading this and feels in a mood to educate and correct me, please get in touch.
There is another situation, that as far as I know never comes up with the relatively low magnification optical systems in stero microscopes. This is the issue of compensating eyepieces. Some optical systems use an objective which does not provide complete color correction. These systems require an eyepiece which corrects the problems introduced by the objective. Such eyepieces are called "compensating eyepieces" and are often labelled in some special way, often with a "K". This might be something to be aware of if you consider purchasing some used eyepieces. An objective which handles its own color correction issues without relying on help from the eyepiece is called an achromatic objective and is the only thing you can expect to encounter in the stereo microscope world.
Tom's Mineralogy Info / tom@mmto.org