Nikon Measuring Microscope objectives, also known as Measurescope Objectives, Toolmakers objectives, TM objectives, or just MM objectives, in this case the MM 3X. The official name at Nikon is the EDF20030 TM OBJECTIVE LENS 3X.
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 never seen one on the used market!
MM 3X Key Features
Ultra Long Working Distance: the MM 3X gives you 75mm of WD to light your subject, the Mitutoyo M Plan APO 2X and 4X give you only 34mm of WD, Nikons 2X and 4X objectives, even less.
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. You can turn off scaling adjustment when using stacking software with a telecentric lens.
Image Quality: The MM objectives are consistent center and corner performance. They are not the sharpest in the center due to the conservative NA but they so make a nice image.
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 0.09. The nominal aperture is f/4.17 and the effective aperture is f/16.7
Overall rating: highly recommended
MM objectives are easy to find in used condition on Ebay for reasonable prices if you are patient. I also have some extra tested samples that are available so let me know if you are interested in buying one I help if you don't buy it from me.
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 3X was the standard objective on at least the MM-10, MM-11, MM-22, MM-200 and MM-400/800 series 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 3X-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 current MM 3X part number is EDF20031 TM objective lens 3x. 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/