Nikon Measuring Microscope objectives are also known as Measurescope Objectives, Toolmakers objectives, TM objectives, or just MM objectives, in this case the MM 5X. The official name at Nikon is the EDF20050 TM OBJECTIVE LENS 5X. These are industrial grade objectives are used in manufacturing, research and development, quality control, and inspection with measuring microscopes for making non-contact measurements of parts and electrical components.
Nikon 5X Measuring Microscope objective Specs
Name: EDF20050 TM OBJECTIVE LENS 5X
Type: finite correction (no tube lens is needed)
Focal length: 42.3mm
Cover glass thickness: none
Mount: Nikon M26 x 0.75 'BD' thread
Field number or image circle: the official field number is 21.5mm but they will cover a Sony APS-C 28mm diagonal sensor.
Coverage: APS-C sensor
Working distance: 64mm
Typical Ebay price: $200-300.
There are a few important advantages of a measuring microscope objective over typical microscope objective for macro photography.
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 5X has a huge working distance of 64mm. The LWD Mitutoyo M Plan APO 5X has a working distance only 34mm. Nikons TU, LU, Plan Fluor 5x all have just 23mm WD! More working distance will give you a lot more lighting flexibility so the MM 5X has a serious advantage here.
Shock-proof and Dust-proof Design: These are the only objectives that I know of that are designated as shock-proof.
Telecentric design: with this design perspective error is eliminated so objects farther away appear to be the same size as closer objects. This allows you to turn off scaling correction with focus-stacking software.
Image Quality: Good consistent center and corner performance.
Mounting threads: standard Nikon M26 x 0.75 'BD' thread. You can find adapters on Ebay to convert the M26 to common threads for easy installation.
Easy installation: This lens has a finite optical design (the image is formed only by the objective), so no tube lens is needed to install this objective as you would be with infinity-corrected optics. You only need a M26 adapter, an extension tube or bellows, and a body mount adapter.
Coverage: up to APS-C or DX DSLR sensors with a diagonal of 26-28mm.
Overall rating: Recommended
The Nikon MM 5X is a great choice for someone that wants to photograph at 5X with just extension tubes (or bellows) and and does not want to go with infinity-corrected lens that requires a tube lens .
At 5X magnification the infinity-corrected Mitutoyo M Plan APO 5X, sells for 2-3 times the cost of the MM 5X with about half the working distance but with better CA correction. If you own a Canon DSLR you can pick up a MPE-65, a great lens, for about 3X or 4X the cost and about 1 inch less working distance and a big light blocking lens barrel.
The 3X and 5X MM objectives are easy to find in used condition on Ebay for reasonable prices if you are patient. I have a few extra tested samples that are available so let me know if you are interested in buying one, I can help if you don't buy one from me. The MM 5X is seems to be the most common of all the MM objectives (the MM 100X has to be the most rare).
These objectives are corrected for chromatic aberration in the lens so they do not require an eyepiece or tube lens for correction for use in photography. They are achromats, not apochromats, so they do show some Longitudinal Chromatic Aberration or LoCAs which is normal for this type of objective and they are don't cause any big issues with photography especially when focus stacking.
All of the Nikon MM objectives are finite designs so installation is simple, you just need 3 additional parts, all of which can be purchased on Ebay for less than $20 each.
1. Nikon MM 5X objective - M26 x 0.75 mount.
2. Objective adapter - 26mm x 0.75 > 42mm x 1. This adapter converts the nikon M26 mount to a standard M42 mount.
3. Extension tube - 42mm mount. Extension tube (+ adapters) length of 150mm gives me 5X magnification on my Nikon D500 DSLR.
4. Camera mount adapter - 42mm female mount to male camera lens mount.
Installation notes: all of the three items in the installation diagram are listed as 42mm but you should know that the objective adapter is actually 42mm x 1mm better known as M42 thread mount, the extension tube and camera mount adapter are actually 42mm x 0.75 threads better known as T-mount threads but luckily the M42 threaded adapter screws into the T-mount tube all the way in and even bottoms out so they work together okay even though they shouldn't technically but its not surprising due to widely varying manufacturing tolerances.
The three 5X objectives that I own show small cosmetic variations on the rear barrel above the mounting threads. The middle unit also has a slightly longer front barrel. The MM 10X objectives have even bigger differences between units. These three objectives do not show any difference in optical performance.
Old vs. New
The MM 5X is an optional objective 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 are almost always the old style parts. In other parts of the world I have seen the new 1X-A, 3X-A, 5X-A, 10X-A, and 20X-A for sale on the used market. The new current MM 5X-A part number is EDF20052 TM objective lens 5x. 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 to meet the lead and arsenic-free EU regulations. Nikons industrial division Rayfact has moved over to 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/