Polarizer for Mitutoyo

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Pizzazz
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Polarizer for Mitutoyo

Post by Pizzazz »

Gang

Has anyone used a polarizer filter on a Mitutoyo objective?

If so, how did you set it up.

I am getting annoying highlights on my subject and I am using a polarized
light source, but need to polarize the light entering my objective.

I have seen an adapter, but it is very pricey, and I do not know if it will fit a
Mitty.

Thanks

Mike

JohnyM
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Post by JohnyM »

Can't you just place it behind the objective?

Pizzazz
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Post by Pizzazz »

Hi

I have not tried, but I have found that light leaving the objective and hitting
glass directly behind it affects the image due to dispersion. I discovered this
when I left my UV filter on my lens.

I am using a Nikon 200mm macro lens as my conversion lens.

I can try to do this but I am not optimistic.

Mike

Macro_Cosmos
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Post by Macro_Cosmos »

One simple method is to simply use an M42 tube, a step-up ring to whatever your CPL diameter is, mount the CPL onto the contraption.

Then use a piece of foam. Cut it into a cylinder shape to insert into the M42 tube. You then dig an appropriately sized hole out to insert the mitty.

Crude but works. Not sure about mounting the CPL between the objective and lens however, I personally think this will greatly degrade image quality since you're using a telephoto lens. With tube lenses such at the DCR-150 from raynox, it should be a lot better.

Again, I'm just giving a crude method. I've only tried fiddling with polarised light once or twice and the results were just kind of alright. Other forum members who are more experienced in this will be happy to help you out.

Pizzazz
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Post by Pizzazz »

Hi

Yeah, that would be a good approach, as I would not want to scratch the
Mitty!

I have something that might work.


Mike

Pau
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Post by Pau »

Macro_Cosmos wrote:Not sure about mounting the CPL between the objective and lens however, I personally think this will greatly degrade image quality since you're using a telephoto lens
In principle filters must be placed behind the microscope objective.
With the exception of 1/4 plates placed in front of very low magnification objectives to control their own reflections with epi illumination (antiflex system in Zeiss nomenclature) all microscopes place the filters at the infinite space between the objective and tube lens -even many finite systems do create an infinite space with telan lenses for this purpose-

This is meant to avoid spherical aberration. Mitutoyo objectives are designed to be used without cover glass, not with a several mm thick glass like many pol filters. Also a filter in front of the objective will cut the useful working distance. The original Mitutoyo microscopes of course also put the polarizer between the objective and tube lens.

Placing the polarizer between the tube lens and the sensor will have much lower deleterious effect than putting it between the objective and the subject just because the angle between light rays is much smaller.
Pau

Lou Jost
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Post by Lou Jost »

I sometimes use polarizers behind the Mitutoyo objectives. I have not encountered any problems caused by this.

rjlittlefield
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Post by rjlittlefield »

Use a high quality coated filter to minimize reflections, and place it as far away from the objective as you can afford to.

In this situation, the big advantage of using a Raynox or one of the purpose-made tube lenses is that they can handle a lot of separation without vignetting. With ordinary telephotos, vignetting often kicks in quickly as you increase separation between objective and telephoto.

--Rik

Chris S.
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Re: Polarizer for Mitutoyo

Post by Chris S. »

Pizzazz wrote:Has anyone used a polarizer filter on a Mitutoyo objective?

If so, how did you set it up.
Mike,

If you look at my post on the Bratcam's tube lens and go five pictures down, you will see the polarizing filter assembly I use with Mitutoyo objectives. You will find a parts list, plus a couple of thoughts on alternative approaches.

This setup works well, and has never caused a problem with reflection. (I do recommend flocking it.) On the downside, it must be screwed in or out up to a quarter-turn to rotate the polarization axis, which slightly alters the field of view. While this is not a big deal, even at 100x, it annoys the perfectionist in me. If making such an assembly today, I'd consider basing it around Edmund Optics 20mm C-Mount Polarizer Holder Stock #90-621, which permits 90 degree rotation. This rotatable filter holder did not exist when I built my setup, else I'd have tried it. Do note that a build around this mount would require different adapters (it's C-mount, not Mitutoyo, but adapters can be obtained from Edmund Optics or Thorlabs) and a different size filter.

Also interesting is Edmund Optics C-Mount Filter/Polarizer Holder Stock #84-346, which would permit the use of a drop-in, rotatable polarizing filter or a blank, depending on need. The capability to insert and remove the polarizer without changing anything else is appealing.

A couple of remaining thoughts: The filter I sourced from Edmund Optics is excellent in terms of extinction and color neutrality--better than my best polarizers for general photography, and equal to one I have that was originally used in an Olympus microscope. I'd recommend this glass highly. (Though I suspect that the part number and naming have changed, so you may have to do some hunting.) But for the non-glass hardware, you might check to see what the current offerings are from Thorlabs. Thorlabs is constantly adding items, and parts competitive with the Edmund ones may now be available from Thorlabs. If so, the price will likely be lower; the quality will probably be very good, if less Tiffany-level than Edmund parts.

Also--and as you may know--when a polarizing filter is used behind a microscope objective, microscopists call it an "analyzer." They reserve the term "polarizer" for a polarizing filter or film placed over the light source for cross-polarization. Coming from a general (non-microscopy) photographic background, my early Web searches on this subject were confounded by this difference in nomenclature; search strings such as "polarizing filter microscope objective" gave little of value. Once forum members taught me to look for information on analyzers, queries turned up better results. (Also, the idea of putting the analyzer behind the lens startled me--but it works great.)

Cheers,

--Chris S.

--fixed typo
Last edited by Chris S. on Tue Oct 30, 2018 3:43 pm, edited 1 time in total.

Macro_Cosmos
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Post by Macro_Cosmos »

I'm seeing some great tips. Going to bookmark this.

One major problem is wobble. All the CPLs I've handled, high quality or otherwise, do have some wobble. This is going to cause different problems.

Simply sandwiching the CPL between two tubes just isn't going to work. It would be awesome if there's an extension tube that allows filters to be inserted, much like many telephoto lenses and some mirrorless adapters.

I'm going to look for a way to get this sorted using thorlabs parts. I want the CPL to be inserted in the optical path, not sandwiched between tubes.

And yeah, having the CPL in front of the lens isn't advised. It will cause a lot of spherical aberrations, contrast and resolution will be greatly lowered as well.
rjlittlefield wrote:Use a high quality coated filter to minimize reflections, and place it as far away from the objective as you can afford to.

In this situation, the big advantage of using a Raynox or one of the purpose-made tube lenses is that they can handle a lot of separation without vignetting. With ordinary telephotos, vignetting often kicks in quickly as you increase separation between objective and telephoto.

--Rik
I have another question, by being as far away from the objective as possible, do you mean:

M (objective)
T (tube lens)
= (extension)
C (camera)
P (Polariser)

1. M === P T===C
2. M ===T ===P C

Basically, is it necessary to have the polariser in front of the tube lens, or can it be just in front of the camera mount if possible? Or maybe it depends on the situation and different setups have different optimal positions? (after or before tube lens)?

MC

Pizzazz
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Post by Pizzazz »

All

Thanks for the excellent advice. I will attempt to fold in as much as possible.
I will also look at the parts Chris S. recommended.

I hope it does not break the bank.

:)

Mike

genera
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Post by genera »

Macro_Cosmos wrote:I have another question, by being as far away from the objective as possible, do you mean:

M (objective)
T (tube lens)
= (extension)
C (camera)
P (Polariser)

1. M === P T===C
2. M ===T ===P C

MC
#1 is common in most microscopes that have provisions for POL
-Gene

Pau
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Post by Pau »

1. is the best optical position as I said because being the light rays parallel it doesn't alter their properties avoiding aberrations, in particular spherical aberration. Be aware that the microscope infinite corrected design was born just because this issue and before being widespread it was used by some makers in their metallurgical microscopes.

Another advantage is due to the fact that the tube lens may not be designed for cross polarization and putting the analyzer behind it in some cases can degrade the polarization state of the light.

I've never experienced issues putting the analyzer just at the rear of the objective. I use vertical setups for macro and I just use a high quality linear polarizer dropped over the objective mount. Because likely you're not going to use the camera autofocus you don't need a circular polarizer as analyzer.

As Chris S. said, camera polarizers are not designed to work crossed and in most cases a microscope polarizer or other high quality models designed to work crossed like the good ones from Edmund do work better.

All that said, if your goal is just to cut some reflections many camera polarizers work well enough, I use them for lower magnification macro without microscope objectives
Pau

rjlittlefield
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Post by rjlittlefield »

Macro_Cosmos wrote:I have another question, by being as far away from the objective as possible, do you mean:

M (objective)
T (tube lens)
= (extension)
C (camera)
P (Polariser)

1. M === P T===C
2. M ===T ===P C

Basically, is it necessary to have the polariser in front of the tube lens, or can it be just in front of the camera mount if possible? Or maybe it depends on the situation and different setups have different optimal positions? (after or before tube lens)?
I meant #1, with the polarizer in infinity space, but as far from the objective as possible.

But to answer your question, #2 will also work. For the reasons noted by Pau, there may be some slight degradation with #2. However, because light rays at the sensor are nearly parallel (large effective f-number), the degradation will be small. The situation then is similar to what happens when people add a clear filter into an adapter that stays mounted on the camera at all times, to protect the sensor from dust.

--Rik

genera
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Post by genera »

Pau wrote:Another advantage is due to the fact that the tube lens may not be designed for cross polarization and putting the analyzer behind it in some cases can degrade the polarization state of the light.
There's no reason to retain polarization after the analyzer. In critical applications a waveplate is added before the tube lens to prevent disruption from strain in the tube lens, prisms, or eyepieces.
I've never experienced issues putting the analyzer just at the rear of the objective. I use vertical setups for macro and I just use a high quality linear polarizer dropped over the objective mount. Because likely your'e not going to use the camera autofocus you don't need a circular polarizer as analyzer.
Olympus doesn't think it's a problem either. Many of their BX2 series scopes have a slider, just above the objectives, that holds a polarizer or a DIC prism and polarizer.
-Gene

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