This is my current M42 extension tube setup for studio macro photography. This is my main setup for macro lenses, bellows lenses, and finite microscope objectives on APS-C sensor bodies.
For For full frame sensor cameras I also have a 52mm based studio setup that I use for for infinity corrected microscope setups, Scanner-Nikkor ED, etc. See this link for the M52 full frame setup: https://www.closeuphotography.com/52mm-setup
In designing this setup I was looking for mainly for rigidity, configurability, at a reasonable cost. I based this setup on a M42 x 1 threaded tube that I pulled from an industrial automated line-scan setup. This tube has an OD of 51mm, and wall thickness of 6mm. To give you an idea of how heavy-duty that is, M42 tubes you find on Ebay are about 45-46mm and maybe 2mm wall thickness.
The Thorlabs C2RC clamps are designed for 2 inch, or 50.9mm tubes.
If you are thinking of setting up my system and need a QR plate, I recommend buying the lowest cost unit online. I paid $10 for my plate on Amazon, Ebay has lots of low-cost choices also. Look for clamps like the no-name version on the far left, it will give you unrestricted clamp movement.
Some plates tend to restrict the amount of travel for your clamps. Look at the image about, the left blue circle on the left side is over a Wimberly plate. Not only does the plate cost 5x the amazon plate but you lose maybe 3/4 of an inch for this legacy flash bracket that no photographer on earth even uses any longer! I recommend to skip paying the extra $40-80 for the Wimberly or RRS plate and buy a new lens with the money you save.
Same goes for the QR clamps I used in this setup, they are RRS knock offs made in China, and they work perfectly. RRS clamps are $70 each, thats $140 plus shipping, so maybe, $160 total? Save the $147 and buy the amazon clamps for $13, with free shipping!
2 x Thorlabs C2RC 2 inch clamp. $37 each.
2 x Generic brand QR clamps, from Amazon. $13 a pair.
1 x Generic QR plate, from Amazon. $10.
1 x Raf camera 50mm extension tube
1 x Raf camera 110mm extension tube
Important Update, January 2018
Raf camera (www.RAFcamera.com) has made a set of extension tubes available in 50mm, 75mm and 110mm lengths with the perfect dimensions to fit the Thorlabs C2RC clamps! I have the first set of these tubes and they work beautifully.
Photos of the tubes will be posted soon.
RAF 50mm extension tube:
RAF 75mm extension tube:
RAF 110mm extension tube:
Raf also built us a M42 variable extension tube for my set-up. This tube makes it possible to dial in the perfect magnification rate, framing or infinity focus with a tube lens with zero flex. These variable tubes I have from Raf are very well made and have a perfect fit. Raf made one with both external male and internal female M42 threads and one with M42 external male and M52 internal female threads to use with a tube lens for work with a infinity corrected microscope objective.
50mm long M42 x 1 variable extension tube:
50mm long M42 x 1 variable extension tube with M52 x 0.75 internal female thread to fit the Sigma LSA 200mm tube lens.
Photos coming soon.
C2RC clamp at Thorlabs: https://www.thorlabs.com/thorproduct.cfm?partnumber=C2RC/M