At work, we are using a Leica microscope with an 85mm OD M series lens. I could not find anyone selling a ring light large enough to fit this, so I just designed one myself.
Lens that this fits: http://spectraservices2.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=10450030
Attached are schematic and gerber data to build the boards yourself. When printing the mount, my computer decided to install windows updates and reboot before it was completed, so the skirt below the PCB is incomplete in this picture. It turns out that might be better in our application due to the limited clearance between the sample and the lens.
I grouped the lights into quadrants that I can turn on and off for different lighting methods. Video of this feature can be found here: http://youtu.be/Adb1nxOA1uM
Hope that someone finds this helpful. You can also use this PCB with a smaller USB microscope: http://www.thingiverse.com/thing:105056
printed at 0.3mm layers with 20% infill. No skirt, because it would barely fit on my printbed.