Nikon Measuring Microscope objectives, also known as Measurescope Objectives, Toolmakers objectives, TM objectives, or just MM objectives, in this case the MM 50X. The official name at Nikon is the EDF20500 TM OBJECTIVE LENS 50X.
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 50X Key Features
Ultra Long Working Distance: A long working distance will make lighting easier, the MM 50X has a generous 15.1mm working distance. For comparison the LWD Mitutoyo Plan APO 50X has a working distance of 13mm, some high NA 50X objectives have less than 1mm 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 50X MM objective IQ is very good, very sharp in the center with good corner performance and mild CAs. This is probably my go-to 50X objective because it is so easy to set-up and shoot with, even though the Mitutoyo 50X does have a slight edge in image quality thanks to APO correction.
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, DX DSLR sensors or smaller.
NA and Aperture: This lens has a numerical aperture or NA of 0.55. The nominal aperture is f/0.88 and the effective aperture is f/45.
Overall rating: highly recommended
The MM 50X is almost impossible to find unless you want to by one directly from Nikon. I was lucky to find this lens as new old stock in the box at an online retailer: http://www.dealcorner.com/Nikon-Objectives/
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 they probably do not have chromatic aberration correction in the lens itself.
The MM 50X is an optional objective, the MM 5X 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 units where in other parts of the world I have seen the new A version MM objectives like the 1X-A, 3X-A, 5X-A, 10X-A, and 20X-A for sale on the used market but not the 50X. Looks like the 50X is still listed for sale at the Nikon website. 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/