Nikon Measuring Microscope objectives, also known as Measurescope Objectives, Toolmakers objectives, TM objectives, or just MM objectives, in this case the MM 10X. The official name of this lens at Nikon is the EDF20100 TM OBJECTIVE LENS 10X.
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 10X Key Features
Ultra Long Working Distance: A long working distance will make lighting easier and the MM objectives are in a class of their own, the MM 10X has a extra long working distance of 49.5mm. The LWD Mitutoyo Plan APO 10X has a working distance only 33mm and Nikons CFI Plan Achro 10X NA 0.25 has a WD of just 10.5mm. Older Nikon CFI achromat 10X objectives have about 7mm 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 10X MM objective is consistent from center to corner but there are some 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 0.20. The nominal aperture is f/2.27 and the effective aperture is f/25.
Overall rating: Recommended
The MM 10X and MM 10X -A are the only objectives of this series with set-screws on the front barrel. When I first saw this lens for sale I thought the Ebay seller had modified the lens but they are made this way.
The MM 10X is not as easy to find as the 3X and 5X MM objectives but they do pop up from time to time in used condition on Ebay for reasonable price if you are patient. I have a an extra 10X that is available so let me know if you are interested in buying one I help if you don't buy it from me. The MM 10X is not as common as the 1X, 3X or 5X but a lot more common than the 20X, 50X and the MM 100X has to be the most rare on the used market.
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 three MM 10X objectives I own all have different length front barrels and one has a slightly different rear barrel also. The front barrel gets progressively longer on the newer units and the older unit has a machined rear shaft above the mounting threads. I do not see any difference in image quality between the three. The MM 5X units that I own also show small variations between units.
The MM 10X 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 10X-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 10X part number is EDF20102 TM objective lens 10x. 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 MM objectives: http://www.nikon.com/products/industrial-metrology/lineup/measuring_instruments/measuring_microscope/accessory/
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/