At work, we use this microscope for inspection. Overall, it's fantastic, although photographing what you see through the eyepiece is extremely challenging without an attached camera. Removing the main viewing assembly was the best way to decide if we could build an adapter ourselves, but it also revealed these glorious barrel cams:
This mechanism adjusts zoom in the microscope - as one of two knobs is twisted, a spur gear on the inside of the assembly is turned. This gear drives the two barrel cams via two other spur gears, driving the optics inside each eyepiece up and down. You can see the gears a little bit in this video.
So, why use complicated cams instead of a simpler mechanism? Take a close look at the rate at which the four cam followers move - the rate of change isn't linear, so a barrel cam is by far the easiest analog way to control the position of the optics. Something similar could be done digitally, but this is far cheaper.
I've broken down what I think is happening on the inside of these eyepieces in SolidWorks, seen in the GIF below. Each optic is held in an assembly that can slide up and down inside, maybe on some small rails. There's also a threaded hole in each of the optical assemblies so the cam follower can be installed after the barrel cam itself is slid over the eyepiece assembly.
This is the sort of stuff that really excites me. Cams are such neat, simple mechanisms that can accomplish complicated tasks, like moving things at a non-linear rate. They're used all over the place, and for good reason. One of my favorite cam applications is described in detail here. Very cool stuff.
As usual, if you're more familiar with this sort of thing than I and can provide more insight into how this specific mechanism works, please share! I'm particularly interested in learning more about how the lenses slide up and down inside the eyepiece.