L-R, Mitutoyo MT-1 lens, Nikon MXA20696 Tube lens, and the Thorlabs/Nikon ITL200 lens
Nikon MXA20696
The Nikon MXA20696 tube lens, also called a 2nd Objective Lens Unit by Nikon, is compatible with CFI60-2, CFI60 and CF&IC infinity-corrected objectives. CFI=Chromatic Aberration Free Infinity; CF=Chromatic Aberration Free. This lens and the ITL200, both made by Nikon, look very similar, but the similarities are only cosmetic, the optics are very different. They are designed to be used with different objectives, the ITL200 is designed for use with APO, and Super Fluor objectives. The MXA20696 introduces lateral chromatic aberrations to APO corrected objective images so I would not recommend it for APO or S Fluor objectives, instead I would recommend the ITL200 as it is a much highly corrected tube lens.
A detailed review of the ITL200 lens is available here on Closeuphotography.com; https://www.closeuphotography.com/thorlabs-itl200/
WHAT IS A TUBE LENS?
Tube lenses are a required part of a infinity-corrected optical system. To use an infinity-corrected objective for photography you must have a tube lens in addition to the main objective or the system will not focus. Infinity-corrected objectives are identified with the infinite mark (∞) on the side of the lens barrel.
Microscope manufacturers design their systems to provide the magnification engraved on the housing when they are used with a tube lens of a specific focal length. 250 mm for Qioptiq Mag.x, Mitutoyo, Thorlabs, Nikon, and Leica objectives, use a 200 mm tube lens.
With infinity-corrected objectives you can change the tube lens focal length to create different magnification ratios at the camera sensor without compromising the axial color correction. To calculate the system magnification for different tube lens and objective combinations, you can use this simple formula; Effective magnification = Marked magnification x the new tube lens focal length/the system tube lens focal length.
Setting Up The MXA20696 Tube Lens
I have tested this lens quite a bit and I find that I prefer the performance of the lens in reverse of Nikon's recommended set-up with the threads facing the sensor and the smaller end facing the objective.
You can see a comparison of both set-ups below in the Forward or Reverse section below. With the lens in reverse I setup the lens so that focus was on infinity so I used more extension than Nikon recommended using, something like 240-245 mm. You can always test the lens with with 151.2 and infinity focus to see what position you prefer. In any case I recommend 100-200 mm between the objective and tube lens.
The Nikon MXA20696, and Thorlabs ITL200 also made by Nikon, were originally designed to fit Nikon microscopes so they use an uncommon M38 x 0.5 threaded mount. Thorlabs sells a tube lens adapter called the SM2A20 that has a female M38 mm thread and male 52 mm SM2 threads on both sides. This will thread right into my 52 mm studio set-up which is based on the Thorlabs SM2 system. SM2 male threads are compatible with standard 52mm female threads. Thorlabs makes an adapter that will fit the MXA20696 lens, called the SM2A20, that is designed to fit 2 inch SM2 threaded parts ( also threads into 52 mm extension tubes). Since my 52 mm studio set-up is based on SM2 threaded extension tubes, it was an easy decision for me.
The Thorlabs SM2A20 adapter has the tube lens mounting direction marked on the barrel but In testing I found that the performance of the MXA20696 is significantly better when used in reverse of the direction on the barrel. See the set-up section below for more details.
Thorlabs products are lab-grade quality and reasonably priced. I do use lots of Chinese made gear but 30% of the low cost parts I buy from China are either mis-threaded and incompatible or just flat out improperly machined so I prefer to go with better quality gear whenever I can.
Mounted on my 52mm studio set-up with the SM2A20 and tube lens is 100% rigid and flex-free.
FORWARD OR REVERSE?
After spending a lot of time testing the Mituyoyo MT-1, and Thorlabs/Nikon ITL200 I have found that for best image quality the lens should be used in reverse with the lens flange towards the sensor. I know this is the opposite of what Mitutoyo, Thorlabs, and Nikon recommend but you are free to follow their instructions but be prepared reduced image quality compared to other tube lenses or this lens in reverse.
To compare the image quality with the MXA20696 in normal and reverse I used the lens with the Mitutoyo 5x M Plan APO lens on a Sony A6300 on a Nikon MM-11 vertical stand with two godox TT-350 flash units and X1T wireless trigger for consistency and the sharpest results possible.
For more details on the test set-up see the TEST SET-UP DETAILS section below.
Click on any image below to launch a new window with a larger version. You can right click, or two-finger click with a Mac and select the open in a new tab or new window options.
Above is the un-cropped image made at 5x showing the crop areas highlighted in blue.
Click on any image below to launch a new window with a larger version. You can right click, or two-finger click with a Mac and select the open in a new tab or new window options.
Normally mounted the MXA20696 has a issue with lateral chromatic aberrations and softness in the center. Look at the word "metal" and you can see a red glow or fringing that disappears on the reverse mount image. Even the reverse mount image isn't perfect but its a lot better than the normal mount version.
Click on any image below to launch a new window with a larger version. You can right click, or two-finger click with a Mac and select the open in a new tab or new window options.
The situation is similar here, CAs and softness in the normal image, the reverse mount image showing better sharpness and a lot less CAs.
Click on any image below to launch a new window with a larger version. You can right click, or two-finger click with a Mac and select the open in a new tab or new window options.
Click on any image below to launch a new window with a larger version. You can right click, or two-finger click with a Mac and select the open in a new tab or new window options.
TUBE LENSES COMPARED AT 100%
Thorlabs ITL200 Tube Lens
Nikon MXA20696 Tube Lens
Mitutoyo MT-1 Tube Lens reverse mounted
Raynox DCR-150 / Model CM-2000 1.5X / +4.8 diopter / 208mm Lens
The center crop area compared at 100% view in Photoshop.
Click on any image below to launch a new window with a larger 2500 pixel version. You can right click, or two-finger click with a Mac and select the open in a new tab or new window options.
All of the lenses here are very sharp in the center with the Raynox trailing slightly behind the other three. The Thorlabs and Raynox handle chromatic aberrations best.
All of these images in this test were processed with the settings in Photoshop with all correction turned off.
The off-center crop area compared at 100% view in Photoshop.
Click on any image below to launch a new window with a larger 2500 pixel version. You can right click, or two-finger click with a Mac and select the open in a new tab or new window options.
Off-center all of the lenses excellent detail and sharpness. The Nikon has a higher amount of lateral CAs.
The corner crop area compared at 100% view in Photoshop.
Click on any image below to launch a new window with a larger 2500 pixel version. You can right click, or two-finger click with a Mac and select the open in a new tab or new window options.
In the corner area the ITL200 is best, with the Mitutoyo second sharpest. The Thorlabs and Raynox have the best CA control, the Nikon has the worst issue with CAs here and its also the softest image of the 4.
TEST CONCLUSION
The center sharpness of the MXA20696 is pretty good but there are problems with CAs off center and the corners are not good at all, and this is only a APS-C sensor. I was told that the Thorlabs ITL200, made by Nikon, is designed for APO and S fluor objectives and it really shows with excellent CA control and much more consistent image quality from edge to edge over the entire APS-C frame. The ITL200 is more expensive than the MVA20696 but I think its worth it.
A detailed review of the ITL200 lens is available here on Closeuphotography.com; https://www.closeuphotography.com/thorlabs-itl200/
TECHNICAL SPECS
Nikon MXA20696 2nd Objective Lens Unit - Tube Lens
Part Number: MXA20696
Type: 2nd Objective Lens Unit - Tube Lens
Focal length: 200 mm
Optical Design: Achromat
Lens mount: M38 x 0.50 mm
Manufacturer: Nikon
Correction: Visible light
AVAILABILITY AND COST
The MXA20696 sells on Edmund Optics site, new for $275 USD. I would recommend spending the extra money for the Thorlabs ITL200 that is actually made by Nikon. The lens used for this test was purchased on Ebay, new old-stock in-the-box for $89. I still think that is too much money.
PROS AND CONS
What I really like:
Center sharpness is good
What I dislike:
Chromatic aberration
Image quality when mounted normally
Very small sensor coverage
THE BOTTOM LINE
I would not recommend the MXA20696 tube lens. I would recommend the Raynox +4.8 on the low end and if you have more in the budget I would recommend the Thorlabs/Nikon ITl200.
A detailed review of the ITL200 lens is available here on Closeuphotography.com; https://www.closeuphotography.com/thorlabs-itl200/
There are a few other excellent tube lenses in other focal lengths that I have tested as part of a tube lens test. For more details the tube lens test is available here on Closeuphotography.com: www.closeuphotography.com/tube-lens-test
TEST SET-UP DETAILS
Camera: Sony α6300, model # ILCE-6300, also known as the A6300
Sensor size: 23.5 × 15.6 mm. APS-C. 28.21 mm diagonal. 3.92 micron sensor pitch
Flash: Godox TT350s wireless flash x 2 with one Godox X1s 2.4G wireless flash transmitter
Vertical stand: Nikon MM-11 with a Nikon focus block
A series of images was shot with each lens in 10 micron steps, and the sharpest image was chosen at Photoshop at 100% view. The single RAW file was processed in PS CC with all noise reduction and lens correction turned off, all settings were zeroed out (true zero) and the same settings were used for all of the images.
LINKS FOR MORE INFO
The MVA20696 on the Edmund Optic site:
https://www.edmundoptics.com/p/nikon-200mm-tube-lens/15374/
The Tube Lens test page on Closeuphotography.com:
www.closeuphotography.com/tube-lens-test