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Mitutoyo G Plan Apo 50x

 
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Lou Jost



Joined: 04 Sep 2015
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PostPosted: Tue Jan 14, 2020 10:36 am    Post subject: Mitutoyo G Plan Apo 50x Reply with quote



Mitutoyo M Plan Apo objectives are famous as being among the best objectives for photography, but Mitutoyo also has other lesser-known objective series, including some that have never been tested anywhere on the internet. These include specialized infinity-corrected objectives, and also finite telecentric toolmakers' objectives similar to Nikon's MM series. I'll deal with the Mitutoyo telecentric toolmakers' objectives later; here I want to present the first internet test of a Mitutoyo G Plan Apo 50x objective.

For my work these odd objectives are very interesting. One of my main tasks is to photograph microstructures of orchid flowers under water or oil. These thick media can cause aberrations for normal lenses and objectives. I've written a few posts here about my solution for high resolution photography at low to moderate magnifications. However, when high magnification is needed, aberrations are much more severe, and a better solution is required.

This is where Mitutoyo's G Plan Apo objective series, and Nikon's little-known LCD Plan series, could be useful. These objectives are made to shoot through thick glass. Mitutoyo's G Plan Apo objectives are corrected for 3.5mm of glass, while Nikon's LCD Plan objectives are usually corrected for thinner glass. Either of these objective series should also be able to image through an appropriate amount of water, so I have been on the lookout for them on eBay for a long time. The G Plan Apo objectives are especially attractive because they have long working distances and are apo corrected. They are prohibitively expensive new (Edmund Scientific sells the 50x G Plan Apo 0.50 for $4380) but after a long wait one of them came up on eBay and I negotiated a price of $600 from a seller with not very good feedback (96.5%). The objective seems to be in perfect condition. I added a black filter ring to take a 3.5mm glass filter, so it can also be used in air.



I am using it with a full frame camera and a favorite FF tube lens, the Nikkor-Q 135mm f/3.5, which pushes the objective down to 34x. There is no vignetting at all on the FF sensor. A 90mm tube lens did cause very minor vignetting. In the picture above, I have removed a diffuser made of a cylinder of tracing paper. I am sure better diffusion would improve the image.

I think the optical path through 3.5mm of glass is equal to the optical path through 4mm depth of water (4mm = 3.5mm * 1.51/1.33, with the conversion factor being the ratio of the indices of refraction of the media). So I shot a wafer through exactly 4mm of water. To measure the depth of water precisely, I first focused on the surface of the wafer with no water, then moved the objective 4mm above that, and added water drop by drop until the dirt on the surface of the water was in focus. I then refocused on the wafer; this required moving the objective 2.961mm down towards the wafer. I then made a normal stack. The stack log showed that the objective is telecentric as an added bonus.

Here is the image of the wafer under 4mm of water, with no sharpening and no contrast adjustment, though I adjusted the brightness:


There is very slight pincushion distortion, easily corrected.

Here is a 100% crop from the edge; not too great, but reasonable considering the magnification. It looks decent at 50%. There is color fringing on blown highlights, not present in the center of the main image, but not horrible and probably avoidable with more diffused lighting:


Later I will try it with oil instead of water; that should give better results, since its depth will be almost exactly equal to the 3.5mm of glass that the objective expects. But overall I am happy with this; it is a breakthrough for my work. Better results can be obtained by water-dipping objectives, but these have much smaller working distances and are far from telecentric, so they can't be stitched and they often hit some part of the flower I am photographing.
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abpho



Joined: 17 Aug 2011
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PostPosted: Tue Jan 14, 2020 3:54 pm    Post subject: Reply with quote

Good for you. Bet it feels great to finally get what you have been looking for. I anticipate your future findings.
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Lou Jost



Joined: 04 Sep 2015
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PostPosted: Tue Jan 14, 2020 4:03 pm    Post subject: Reply with quote

Now the hard part of this is to figure out how to get exactly 4mm of water above the flower part of interest......Maybe time to make a glass boat under the Mitutoyo at exactly the right distance.
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Scarodactyl



Joined: 14 Apr 2018
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PostPosted: Tue Jan 14, 2020 4:11 pm    Post subject: Reply with quote

I love the work you're doing on this (and hope to apply some of it to photographing inclusions in gems at some point).
Have you tested how it compares to using a normal M plan apo through 4mm water? I know the answer is 'better' but I'm kind of curious how extreme the effect is.
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Lou Jost



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PostPosted: Tue Jan 14, 2020 4:25 pm    Post subject: Reply with quote

Sacrodactyl, thanks, I hope these techniques are useful for others.

I don't have a comparable normal Mitutoyo M Plan; the 10x is my highest-mag normal Mitu. But at NA =0.50 any objective that is not specially corrected would be mush. Even this one is mushy if the water depth is off by a millimeter or two.
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mawyatt



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PostPosted: Tue Jan 14, 2020 5:11 pm    Post subject: Reply with quote

Lou,

That's a clever solution to your problem!!

I recall you mentioning that water evaporation was also an issue. If so, could you add a drop of oil to the water that floats to "seal" it somewhat and reduce the evaporation rate? Since this would be a very thin layer I wouldn't think it wouldn't have much optical effect for near orthogonal rays.

BTW do you like the Nikon 135 Q better than the Raynox 250, Zeiss 135 or Vivitar Komine 135 for a tube lens with the Mitutoyos?

Best,
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rjlittlefield
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PostPosted: Tue Jan 14, 2020 5:29 pm    Post subject: Reply with quote

That looks like a great start!

A couple of thoughts...

In addition to the spherical aberration caused by planar medium, any non-planarity will add other aberrations.

Even 1/2 lambda variation in thickness across the aperture could cause a lot of murkiness in fine detail.

I have not tried to figure out the impact of fluid meniscus, but it worries me.

One big advantage of the glass boat is that it takes liquid free surface out of play.

However, the window of the boat had better be perfectly planar also, or you'll have the same problem all the time.

The skeptic in me suggests sacrificing a couple mm of working distance and using something like https://www.edmundoptics.com/f/lambda10-fused-silica-windows/12713/ .

Edit: Oops, brain glitch! Using a thick window would cut into water layer thickness, not just the objective's working distance. So I guess you have to go with a thin window and just be careful to pick a planar one.

Similar issue applies for your tests shooting through a glass plate. It would not take much irregularity in that glass you're shooting through, to account for the level of blur that is shown in your images here.

--Rik
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Lou Jost



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PostPosted: Tue Jan 14, 2020 5:54 pm    Post subject: Reply with quote

Mike, for full frame I do now prefer the Nikon 135mm. Thanks for the idea about the oil, that would work. Evaporation is more of a problem when using alcohol.

Rik, thanks, I add soap to the water to reduce the meniscus. I do use the lambda/10 Edmund window as a filter when shooting this objective dry. But I have no way to ensure perfect parallelism; I think that is genuinely beyond home-brewed solutions.
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rjlittlefield
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PostPosted: Tue Jan 14, 2020 6:12 pm    Post subject: Reply with quote

Lou Jost wrote:
I do use the lambda/10 Edmund window as a filter when shooting this objective dry. But I have no way to ensure perfect parallelism; I think that is genuinely beyond home-brewed solutions.

I have not tried to make a quantitative model, but my gut feeling is that it's not terribly important that the window is perfectly perpendicular to the optical axis, as long as it's planar. Using the lambda/10 window should take care of most of the problem.

Soap in the water is a very clever idea.

--Rik
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Lou Jost



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PostPosted: Tue Jan 14, 2020 8:21 pm    Post subject: Reply with quote

Here is my first orchid with this technique. 34x on FF. Considerably processed.


A crop of this:


It was nice to see traces of the vacuoles inside the cells.
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Lou Jost



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PostPosted: Wed Jan 15, 2020 8:27 am    Post subject: Reply with quote

I just compared this with a stack from a 20x 0.50 water dipping objective, and in spite of teh lower magnification, the image is considerably better than that of the G Plan Apo. This could be because it is almost impossible to accurately adjust the water depth to 4.0 mm.

For specimens in oil, though, the G Plan seems to be the only option. I do not know of an "oil-dipping" objective. And oil gives much better results; plants are mostly water and their surfaces almost disappear in water, while the different refractive index of oil reveals the cells' surfaces better.
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Pau
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PostPosted: Wed Jan 15, 2020 9:35 am    Post subject: Reply with quote

Lou Jost wrote:
For specimens in oil, though, the G Plan seems to be the only option. I do not know of an "oil-dipping" objective ...


In fact any oil immersion objective excepted the ones with very high NA can act as an "oil dipping" objective because immersion oil has a RI very close to glass (time ago they were named "homogeneous immersion, HI" by some makers.
Unfortunately this doesn't make them useful for your specific application as the working distance is usually very very small. Maybe some models designed for seeing through Petri plates glass with inverted microscopes could do in some cases.
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