The intent is that the relay lens is something close to a 200mm focal length. What I have used thus far is a Canon FD mount 200mm f/4 prime lens from the good old days of film photography. This lens has a 52mm filter thread, so I purchased a 52 to 26mm adapter disk that I thread the Mitutoyo into and it works. (I also have an FD to EOS adapter that mounts the old Canon on my modern EOS camera.
For many years I have read about people using the Raynox 150 as a relay lens for infinite conjugate objectives. I have one of these. I bought it in 2015, so it has been sitting around for 7 years waiting for me to figure out the various adapters which are required. It is my intent today to figure out exactly what I need and to order it.
Note that the Raynox DCR-150 sells right now (this very minute) on Amazon for $70.
There are many ways to do this and plenty of information on line. This article will just add to that wealth of information.
I just tripped over this nice aricle by Allan Walls and am trying to use it as a guide. It is not as good as it could be. Plenty of flash and dazzle, but the list of parts at the end is incomplete (how about a link to the M42 to M43 adapter?). The second article (from PM.net) is pretty amazing.
First, an important note. The threads on a Mitutoyo objective are 26mm by 36tpi. There are other 26mm threads in use (such as 26mm by 0.75mm) but these are not what you want. Indeed the Mitutoyo uses an odd mix of metric and imperial units, but don't blame me. Note that 36 tpi is 0.706mm, very different from 0.75.The back end of the Raynox has 43mm male threads (not 42mm as you might hope), so the first thing to buy is a 43 to 42mm adapter. Some time ago, I purchased a 42mm helicoid that gives variable 12 to 19mm of extension. I'll swear that somewhere I have some 42mm extension tubes (and I probably do). The second thing to buy is a 42mm (aka "M42") to EOS adapter. I thought I had one of these, but I don't seem to. This will take care of the back end of the Raynox, if I can find those extension tubes.
The front end of the Raynox has 49mm female threads (the sort of thing you would thread a filter into). We need to get this to mate up to our 26mm by 36tpi thread on the Mitutoyo, perhaps with some extension so that the back end of the Mitutoyo doesn't scrape against the front element of the Raynox. Note that the spacing here can be anything -- the infinite conjugate design works in our favor here.
I should note here that some people prefer to flip the raynox around. There seems to be some controversy about whether it is best used "forward or reversed". For now I am satisfied just to get all the stuff to set it up to be used forward and that is all that I will talk about.
(1) An EOS to M42 adapter. I have several of these (with focus confirmation chips) in my junk box, so I don't need to buy anything. Something like this would do for $10.
(2) You need some extension tubes (or a bellows perhaps if you have an M42 belows laying around). I went with a cheap set of extension tubes for about $10 like these. Note that I also have an M42 focus heloid (an adjustable 12-19 extension tube) laying around that maybe be either handy or entirely unnecessary.
(2b) A different option would be to buy a set of EOS extension tubes and put them between the camera and the EOS to M42 adapter. Either way works, but consider these from Amazon for $15 (with next day prime shipping). I bought some of these also, just to have them on hand.(3) Next you need an M42 to M43 adapter. These are always tricky because the descriptions rarely tell you whether the M42 threads are male or female and if it is going up or down. You want male M42 threads and female M43 threads (M43 so the Raynox can screw into it). The following on Amazon for $15 looks like the deal:
It says it has the following:Male thread: M42 (42mm 1mm thread pitch) -- goes to the extension tubes Female thread: M43 (43mm 0.75mm thread pitch) -- goes to the RaynoxIndeed the M42 lens standard has a 1mm pitch (look up M42 lens on Wikipedia). I cannot find a specification for the pitch of the Raynox M43 threads anywhere. Putting an M42 lens side by side with the raynox under my microscope, I do get the impression that the Raynox threads are closer than the M42, so they are probably 0.75. So I am gambling the $15 and ordering the adapter from Amazon.
(4) Next you need the Raynox DCR-150 itself.
(5) I am going to use a 49 to 52 filter adapter on the front of the Raynox. This is $8 on Amazon.
(6) Next you need a 52 to 26mm 36tpi adapter. I already have such a thing, and that is why I am ordering the 49 to 52 step up ring. You may be able to figure out other ways to go from the 49mm threads on the Raynox to the 26mm threads on the Mitutoyo. Remember what I noted earlier about the 36tpi thread pitch. I see comments that you can screw (force?) the Mitutoyo into 0.75mm pitch threads. Suit yourself.The following item on Ebay look exactly like what you want for $15.
Tom's Computer Info / tom@mmto.org