Nikon Measuring Microscope objectives, also known as Measurescope Objectives, Toolmakers objectives, TM objectives, or just MM objectives, in this case the MM 20X. The official name of this lens at Nikon is the EDF20200 TM OBJECTIVE LENS 20X.
The Nikon MM series objectives offer great image quality and are really simple to set-up and thanks to a long working distance, lighting is really hassle free so they are very easy to use for macro photography. These are my personal favorites, I own and use the MM 3X, 5X, 10X, 20X and 50X. Nikon also makes a MM 100X but I have yet to even see a single sample on the used market!
MM 20X Key Features
Long Working Distance: The MM 20X has a working distance of 20.3mm, that is similar to the Mitutoyo Plan APO 20X. The Nikon CFI LWD 20X has 3.9mm WD and Nikon Super long WD CFI L Plan SLWD 10X offer about 24mm WD.
Shock-proof and Dust-proof Design: Nikons MM series are the only objectives that I have ever heard designated as shock-proof.
Telecentric design: With this lens light rays are parallel to the optical axis in object and image space. This means that magnification is nearly constant over a range of working distances, so perspective error is eliminated so objects farther away appear to be the same size as closer objects.
Image Quality: the 20X MM objective IQ is very sharp in the center with good corner performance and mild CAs.
Standard mount: MM objectives use a standard M26 x 0.75 so adapters are easy to find on Ebay.
Easy installation: This lens has a finite-corrected design (an optical system in which the image is formed only by the objective), so no tube lens is needed to install this lens. You only need a M26 adapter, an extension tube or bellows, and a body mount adapter.
Coverage: APS-C or DX DSLR sensors or smaller.
NA and Aperture: This lens has a numerical aperture or NA of 0.40. The nominal aperture is f/1.19 and the effective aperture is f/25.
Overall rating: highly recommended
The MM 20X is really hard to find on the used market. They do show up on Ebay from time to time but since they are so scarce they usually with a high price, sometimes in the thousands. List price from Nikon was 4480.00 USD a couple of years ago. In this case you would be better off buying a Mitutoyo Plan APO 20X since they are a lot more common on the used market and can be picked up for less than $500 and they have APO correction.
A year or two ago I was the only Ebay bidder on a MM 20X, new in the box, but unfortunately the shipping box showed up empty and re-taped with my address re-written sloppily. The postman didn't notice since it was in a pile of boxes but unfortunately I signed for it and the guy later apologized for the error but he would not take the box back. So I phoned the police and I used the official statement to get a full refund from Paypal in about 60 minutes even though I had received and signed for the empty box. The objective that I now own I luckily found on Ebay mislabeled by a microscope seller as an optical compeditor (not my spelling) for about $200, again I was the only bidder. So these are nice to own if you can get lucky and find one for a resonable price.
These objectives are corrected for chromatic aberration in the lens design so they do not require an eyepiece or tube lens for use for photography. They are achromats, not apochromats, so they do show some Longitudinal Chromatic Aberration or LoCAs which is normal for this type of objective. Note that they are also Mityutoyo and Olympus measuring microscope objectives on the used market but I doubt they have chromatic aberration correction in the lens itself.
The MM 20X is an optional objective, the MM 3X being the standard one for the MM-10, MM-11, MM-22, MM-200 and MM-400/800 series and other Nikon measuring microscopes. The MM objectives on the used market in the US at least are the original old style parts. In other parts of the world I have seen the new current 20X-A marked lens. In other parts of the world I have seen 1X-A, 3X-A, 5X-A, 10X-A, and 20X-A for sale on the used market. The new and current MM 20X part number is EDF20202 TM objective lens 20x. From the outside the newer A type objectives look unchanged, but I have a feeling that it might be that the glass type has been changed since Nikon Rayfact industrial lenses have moved to lead and arsenic-free Eco-Glass some time ago.
Links for more information:
Nikon industrial measuring microscopes: http://www.nikon.com/products/industrial-metrology/lineup/measuring_instruments/measuring_microscope/
Nikon 5X Measuring Objective test: http://coinimaging.com/nik5xmeas.html
The 1X, 3X, 5X, and 10XMM objectives are sold under a private label in Germany under the name Vision & Control MK series http://www.vision-control.com/