Something a little different here. A microscope at work. Obviously caught my attention due to the blue logo on it...
This is the Carl Zeiss Stemi 2000 stereo microscope. This is similar to binoculars in that there are two optical paths, one for each eye. They're deliberately angled so you get a stereo image as opposed to two identical images. The fun part here is there is an optional camera attachment, and you can get adapters for common camera mounts. I will be using it with EOS mount.
A detented knob lets you select magnifications from 0.65x to 5x at approximately 3 steps per doubling. It seems to be parfocal so you don't need to refocus when you change magnification.
This fitted lens modifies the magnification and working distance. With this 0.3x option lens fitted, the working distance is a massive 285 mm. Without it, it is only 92mm.
Relating the set magnification to effective photographic magnification turned out to be rather convenient. With the 0.3x lens fitted, the optical magnification was half that of the scope value. e.g. 5x setting gave 2.5x optical. Without the 0.3x lens fitted, this increases to about 8x magnification.
Focusing is dependant on the mount. You need to move the whole scope relative to the subject to focus. Photographically there is no aperture adjustment. What you see is what you get, with limited depth of field. I suspect the microscope is providing diffraction limited images to the camera.
I don't know how big the image circle is. It easily covers the whole of an APS-C sensor.
Here is one resized full sample taken without the reducing option lens fitted. The image is of a chip as may be found in tiny electronic devices, and shows the solder balls.
The scope can show longitudinal chromatic aberrations as can be seen above with the purple and green fringing, although that is a high contrast test so worst case. It is still visible in lower contrast scenes. Lateral chromatic aberration doesn't appear to be an issue, unless you fit the 0.3x option lens. I don't recall if it was a single or multiple element assembly, but it is a single assembly that fits under the two optical paths. Relative to the optical axis, it is not perpendicular to either of them. Whatever the mechanism, this gives noticeable blue/red lateral CA when in place. This is only really significant viewing photos, and not through the eyepieces.
To recap, for comparison with photographic macro lenses, this can give up to 8x magnification at a working distance of 92mm. With an optional 0.3x lens, you can get up to 2.5x magnification at 285mm working distance. I think if you want to do extreme macro, this might be a possible consideration over more extreme photographic lenses, providing you can work without adjustable aperture.
One side note: photos of the microscope here were taken with the Zeiss 2/50 makro planar.