Thorlabs ITL200 Tube Lens Test

August 5, 2018

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Thorlabs ITL200

The Thorlabs ITL200 lenses are made by Nikon for use with Fluor and APO objectives. The lens is designed to cover the visible light spectrum and is APO corrected for axial CAs. Thankfully, this is common lens, so used samples are not difficult to find on Ebay, usually selling for about 25% of the cost of a new unit which is about $460 straight from Thorlabs.

In a test last month, I compared over a dozen tube lenses and the performance of the ITL200 was the best of the group. For more info on the test: https://www.closeuphotography.com/tube-lens-test

What is a Tube Lens anyway?

Tube lenses are a required part of an 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. This is 250 mm for Qioptiq Mag.x Objectives, Mitutoyo, Thorlabs, Nikon, and Leica objectives the standard tube lens focal length is 200 mm.

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.

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Tube Lens Test Set-Up

These test images are taken from a tube lens test that I ran last month comparing 18 different telephoto, tube lenses, and close-up lenses. For the tube and close-up lenses I shot these forwards and reversed. For all the lenses I tested without any objective extension and with my normal 75 mm (3 inches) of extension.

All of the comparison images were processed identically in Photoshop at the same time. 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. All of the crops below are from a single file chosen out of a stack of images for crop area. Some tube lenses introduced quite a bit of field curvature. The images were made with the iris wide open. For consistency all of images were made on a MM-11 microscope stand with two godox TT-350 flash units and X1T wireless trigger.

For this comparison I used a favorite 6 inch 150 mm Silicon wafer. This particular wafer was sold by an Ebay seller in France as a new untouched disk. But the disk arrived scratched from edge  to edge, so I made a claim for a refund, and the seller refunded my payment. Now a year later, this is one of my favorite wafers because the scratches are a fantastic way to compare sharpness.

Silicon Wafers are perfect targets for testing a lens. Nothing beats a wafer for flat ultra-fine details; the geometrical patterns are from a few microns (μm) down to the nano-meter (mµ) range. The best part of working with wafers is they will not warp or change shape in the middle of a test, they are perfectly flat and they stay flat.

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.

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Telephoto lenses vs the ITL200 at 100%

Thorlabs ITL200 Tube Lens

Nikkor 200mm f/4 Ai Lens

Asahi  Pentax Tele-Takumar 200mm f/5.6

Carl Zeiss Tele-Tessar T* 200mm f/3.5

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.

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There is not a big difference between the lenses in terms of center sharpness. In my eyes the Zeiss lens may actually be slightly sharper than the ITL200, but there is a big difference in chromatic aberration. Looking at the edges of the blue squares, the ITL200 is the only lens in this group that is free of chromatic aberrations.

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.

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Off-center there is not much difference between the ITL200, the Nikkor, and the Zeiss lens in sharpness, but I can see even more problems with chromatic aberrations with the telephoto lenses.

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.

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The performance of the ITL200 in the corners on an APS-C sensor is very good. The Nikkor is the best of the 3 lenses but it does show a performance drop in the corners compared to the center. The Zeiss has the worst performance in the corner area.

Conclusion Telephoto vs Tube Lens

One thing that the test results make clear is that there isn't much difference in sharpness in the central area between these lenses. Almost all the lenses in the test were sharp at 100% in the center.

What this test really shows is the differences in the amount of aberrations introduced. The ITL200 is an APO corrected lens and it shows with near-perfect lateral chromatic correction and much better than any of the telephoto lenses in this comparison, but note that none of these telephoto lenses are APO corrected. An APO telephoto lens like the Voigtlander APO-Lanthar or Sigma 150 mm would perform much better.

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The Best Tube Lenses at 100%

Thorlabs ITL200 Tube Lens

Mitutoyo MT-1 Tube Lens

Raynox DCR-150 / Model CM-2000 1.5X / +4.8 diopter / 208mm Lens

Sigma LSA Life Size Attachment

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.

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Comparing these 4 lenses at 100%, I think the ITL200 and Raynox look sharpest at 100%, with a slight edge going to the ITL200. The Mitutoyo is slightly soft and suffers from lateral CAs. The Raynox image is much cleaner than the Sigma, thanks to excellent lateral CA control. 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.

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All the lenses are very sharp off-center. The ITL200 and Raynox images show impressive chromatic aberration control here. The Mitutoyo MT-1 crop is sharp but it does the worst job of suppressing lateral chromatic aberrations. 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.

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The ITL200 is easily the best performer in the corners at 100%. Beyond this crop area, the sharpness and CA correction keeps going all the way to the very edge of the frame.

The Mitutoyo MT-1 actually improves in the corners and looks better than the Raynox and Sigma lenses. The Raynox corner quality is off compared to the center, I triple checked the results to confirm. The Raynox introduces some field curvature so the corner frame is different than the center frame. The Sigma has better CA control in the corners than the center, similar to the MT-1.

Each corner crop above was selected independently of the center as I found out that some lenses, like the Raynox, introduced field curvature.

Conclusion

The Thorlabs ITL200 is the best choice for a 200mm focal length tube lens in this test, with close to perfect performance across the entire frame with almost zero aberrations.

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

The ITL200 vs the Nikon MXA20696

The MXA20696 tube lens can be found on the used market and Ebay, I picked up a new old-stock MXA20696 lens in the box for less than $100. You can see this lens in the center of the image below. The MXA20696 looks similar to the ITL200, but the performance is not comparable. A detailed review of the MXA20696 lens is available here on Closeuphotography.com; https://www.closeuphotography.com/nikon-mxa20696-tube-lens-test/

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L-R, Mitutoyo MT-1 Tube Lens, Nikon MXA20696 Lens, and Thorlabs ITL200 Tube Lens.

Setting Up the ITL200 For Photography

This lens was originally designed to fit Nikon microscopes and uses an uncommon M38 x 0.5 threaded mount. An excellent tube lens adapter called the SM2A20 is sold by Thorlabs 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.

Note: You should be aware that the Thorlabs SM2A20 adapter has the incorrect direction marked on the barrel. In testing I found that the performance of the ITL200 is significantly better when used in reverse of the direction on the barrel. See the set-up section below for more details.

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Thorlabs SM2A20 tube lens adapter, ITL200 tube lens, and SM2 extension tube.

The Thorlabs SM2A20 adapter 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.

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 misthreaded and incompatible or just flat out improperly machined, so I prefer to pay more for higher quality gear whenever possible.

Mounted on my 52mm studio set-up, the SM2A20 + ITL200 combination is 100% rigid and flex-free, this combination would be hard to beat, and is a big advantage over other extension methods.

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Important Note on Setting up the ITL200

After spending a lot of time testing the ITL200, it turns out that the Thorlabs official Set-Up instructions are wrong if you are looking for the best image quality.

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The images below show the difference between the Thorlabs set-up, on the left, and the recommended set-up, on the right. With 148 mm spacing, the lens is nowhere near infinity focus. Also note that the tube lens needs to be in used in the opposite direction, large pupil or threads facing the camera.

Another reverse Set-up

There is another set-up for the ITL200 for excellent results. Mounted in reverse with 148 mm of extension from the sensor to the rear of the ITL200 you will get 4.3x, not the full 5x with the Mitutoyo 5x M plan (or with another 5x objective) but the image quality is really very good from edge to edge on an APS-C sensor body.

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.

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TECHNICAL SPECS

Thorlabs ITL200 Tube lens

Part Number: ITL200

Type: Tube Lens

Focal length: 200 mm

Optical Design: Apochromatic

Resolution: Appropriate for use with camera pixel sizes down to 2 µm.

Lens mount: Nikon threads, M38 x 0.5 mm on one side of the lens housing.

Manufacturer: According to Thorlabs, Nikon makes the ITL200 for use with Nikon fluor and APO lenses.

MECHANICAL QUALITY

The ITL200 lens is made at a professional lab level of quality and not made for the consumer lens market so I would be careful when cleaning the lens to avoid damaging the lens coating.

PRICE/PERFORMANCE

The lens is an excellent value on the used market if you need a 200 mm focal length tube lens.

AVAILABILITY AND COST

There is one seller on Ebay that sells mint condition new-old-stock surplus ITL200 lenses for $169. They are not new in the box, but they are in mint condition. This is an excellent value for that price. If you have to buy this lens new for $465, I would look for an alternative maybe. I am testing a Nikon tube lens next week so that might be a good alternative.

PROS AND CONS

What I really like:

Easy installation with the Thorlabs adapter

Very uniform sharpness and contrast from edge to edge on APS-C

APO correction

What I dislike:

M38 Nikon mounting threads

Hard to install without an adapter

Incorrect set-up instructions on the Thorlabs site.

THE BOTTOM LINE

The ITL200 is highly recommended. The APO correction of his lens really makes all the difference when used with the Mitutoyo M Plan APO 5x. I don't know of another 200 mm tube lens that can compare. There are other APO alternatives at other focal lengths but for 200 mm this is the one to get.

LINKS FOR MORE INFO

The ITL200 page on the Thorlabs site:

https://www.thorlabs.com/thorproduct.cfm?partnumber=ITL200

More information on the SM2A20 adapter on the Thorlabs site:

https://www.thorlabs.com/thorproduct.cfm?partnumber=SM2A20

The Tube Lens test page on Closeuphotography.com:

www.closeuphotography.com/tube-lens-test