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Polarisation and Cross-Polarisation
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stevekale



Joined: 11 May 2011
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Location: London, UK

PostPosted: Thu Feb 02, 2012 9:19 am    Post subject: Polarisation and Cross-Polarisation Reply with quote

Howdy.

I've been reading a number of great threads regarding the above topic and want to explore this further as I find many images lit with heavily diffused light generally a bit dull.

I understand that I can use linear polarised sheet in front of my flash units and have ordered a small (8.5"x5") sheet of the below film which was recommended in one of the threads:

http://www.edmundoptics.com/products/displayproduct.cfm?productid=1912

I understand also that I can use a circular polariser in front of my macro lens (rather than needing a linear polariser).

In the case of my Nikon objective (plan 10x 0.25) I was thinking of making a slip-on cylinder out of flocking material with a small rectangle of Techspec film on the front as a rotatable filter.

I also read with interest AndrewC's experiments with a cylinder of polarizing sheet with diffusion material around it.

http://www.photomacrography.net/forum/viewtopic.php?t=11399&highlight=cross+polarization

Hopefully my sheet will be big enough to dabble into that.

I was wondering if anyone had any comments/guidance on the above and whether there was more reading that would be recommended?

thx

Steve
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Pau
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PostPosted: Thu Feb 02, 2012 11:37 am    Post subject: Reply with quote

With a microscope objective the analyzer (second polarizer) is much better placed behind the objective because the light rays are much more parallel. Placing it in front of the objective, apart of losing useful working distance is likely going to introduce spherical aberration and will be more prone to glare.

A dedicated microscope analizer is in principle the best option, but also the Edmund's Techspec linear glass polarizer is excellent, I use it in my DIC microscope after the recommendation of the fellow forum member Chris S. and it works great (but it isn't cheap). The laminated film is optically inferior, it will work but it may degrade a bit the optical quality of the objective.

You can use a circular polarizer, even it is recommended for use with the camera photometer (here AF systems don't matter) but I do not find any problem with this linear one compared with a circular.

If you are able to rotate the flashes you do not really need to rotate the analyzer, but it may be useful.

Perhaps you can find some utility in the discussions here:
http://www.photomacrography.net/forum/viewtopic.php?t=10821
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stevekale



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PostPosted: Thu Feb 02, 2012 12:19 pm    Post subject: Reply with quote

Pau

That's helpful. Given my setup is the Nikon objective attached to one of either these lens:

(1) Canon 100mm Macro f2.8L
(2) Nikon Nikkor 135mm AI f2.8
(3) Canon 70-200 f2.8L

it should be easy to insert a 52mm circular polariser between lens (or step down ring) and the 52-25 objective adaptor. Would this be ok?

I hadn't thought much about the flash polariser setup other than thinking I would simply mask the end of a snoot with the Techspec laminated film. The ability to rotate the film would be good but will make things much more complicated to implement I suspect.


S
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Pau
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PostPosted: Thu Feb 02, 2012 12:45 pm    Post subject: Reply with quote

stevekale wrote:

it should be easy to insert a 52mm circular polariser between lens (or step down ring) and the 52-25 objective adaptor. Would this be ok?

I hadn't thought much about the flash polariser setup other than thinking I would simply mask the end of a snoot with the Techspec laminated film. The ability to rotate the film would be good but will make things much more complicated to implement I suspect.

- yes, it will be easy and would work, but be aware: many (if not most) photographic pol filters induce an strong color cast when crossed and the extintion ratio is not so high compared with microscope ones that are designed to work crossed. It may be more of less important in function of the subject and your exigence. If you have the filter test it, if not, go for a microscope analyzer or the Techspec.

- if you use more than one flash or light source, you will need to be able to rotate the polarizers, at least a bit, to cross them with the analyzer. If you can do it, you will find an adequate position of the analyzer to match your polarizers and fix it inside the adapter.
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Last edited by Pau on Thu Feb 02, 2012 12:54 pm; edited 1 time in total
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AndrewC



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PostPosted: Thu Feb 02, 2012 12:48 pm    Post subject: Reply with quote

One suggestion - when you get a sheet of the stuff it is useful to mark the direction of polarisation with a permanent marker for future ref, and when you cut pieces off do the same. It saves some head scratching later Smile
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stevekale



Joined: 11 May 2011
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PostPosted: Thu Feb 02, 2012 1:18 pm    Post subject: Reply with quote

AndrewC wrote:
One suggestion - when you get a sheet of the stuff it is useful to mark the direction of polarisation with a permanent marker for future ref, and when you cut pieces off do the same. It saves some head scratching later Smile


Good suggestion!
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stevekale



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PostPosted: Thu Feb 02, 2012 1:36 pm    Post subject: Reply with quote

Pau wrote:
If you have the filter test it, if not, go for a microscope analyzer or the Techspec.


I dug around and don't have a 52mm cpf. Every microscope analyser I have seen is hugely expensive. Perhaps I am looking at the wrong thing though.

The Edmunds' Techspec linear glass polariser is unmounted which means I have to grapple with how to fit it....I note that Chris S found a filter holder for Mitutoyo setup but I don't see a Nikon equivalent.

Edmunds sells mounted ones here

http://www.edmundoptics.com/products/displayproduct.cfm?productid=1396

They're not Techspec but are they poor quality?

Surely a good quality circular polariser (e.g. B+W) would not introduce a strong colour cast else it would not be tolerated by pro photographers?


Pau wrote:
- if you use more than one flash or light source, you will need to be able to rotate the polarizers, at least a bit, to cross them with the analyzer. If you can do it, you will find an adequate position of the analyzer to match your polarizers and fix it inside the adapter.


Will have to think about this one a bit...not easy to add a rotatable filter holder to the end of a makeshift snoot taped onto a portable flash unit.

ADDITION:

I wonder if Chris S's statement from here http://photomacrography.net/forum/viewtopic.php?t=12880 blows the notion of a simple insertion of a filter holder in any event?

"Also, I think an important test for a decollimation lens is how it performs with the objective spatially separated from it. This spreads out the light waves a bit, which tends to use less optimal portions of the decollimation lens. This is important if one wishes to use a filter, beam splitter, or other apparatus between objective and tube lens. With the Nikon lens used to decollimate, I could not get useful images with a polarizing filter adapter in place. But with this tube lens setup, I can."
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rjlittlefield
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PostPosted: Thu Feb 02, 2012 2:17 pm    Post subject: Reply with quote

I suspect you'll be OK with an ordinary photo polarizer as long as you're just using it to cut specular reflections. See for example http://www.photomacrography.net/forum/viewtopic.php?t=3945. The polarizers used for that exercise go intense blue when crossed, and the extinction is not nearly complete. But those effects were swamped by the nonpolarized components, so for cutting reflections they worked OK.

I have one different concern about sticking a polarizer close behind the Nikon objective. When I first played with that objective, I made my own adapter and used a filter ring as spacer just behind it. In my haste, I left the filter itself in the ring, thinking there was no need to remove it. Bad idea. With the filter in place, images were very low contrast, apparently due to light bouncing back and forth between the filter, the objective, and other parts of the adapter. Removing the filter solved the problem. In your setup, be sure to use a high quality coated filter and to baffle or flock behind the objective to eliminate shiny surfaces.

It's maybe worth noting that Nikon makes a polarizer that's designed to go just behind the objective, and it appears to be mounted at a slight angle. We've speculated that's related to the issue of reflections between the filter and the objective, but we don't really know for sure.

--Rik
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stevekale



Joined: 11 May 2011
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PostPosted: Thu Feb 02, 2012 2:37 pm    Post subject: Reply with quote

Thanks Rik. Actually the thread you linked to was one I had read some time ago but couldn't find today. Thanks, I will read it again. I've covered the rear of my M52-RMS adaptor with flocking from Protostar so hopefully that will help.

This was the cpf I had in mind (it doesn't cost what is shown here):

https://www.schneideroptics.com/ecommerce/CatalogItemDetail.aspx?CID=606&IID=5631
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Chris S.
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PostPosted: Fri Feb 03, 2012 2:44 am    Post subject: Reply with quote

Steve,

Just saw your thread. Will bat out some thoughts, but am tired, so may not make much sense. I just--at 4:00 am--came upstairs from a marathon session in the basement with the Bratcam and a friend who really gets it. Any day that includes high-mag photography, metalwork, lasers, electronics, first-surface mirrors, and a long-time friend who is enthusiastic about these things (and knows fabrication techniques that I did not) is pretty much guaranteed to be a great one. But exhausting.

1) To be clear, you only need a circular polarizer if you are using autofocus or autoexposure. If you're working in manual, you can use a linear polarizer.

2) I agree with everything Pau said, except maybe to clarify that in my work with one Olympus analyzer and one Edmund Optics Techspec unmounted linear polarizer, they seem pretty much equivalent. So I'm not sure that there is any theoretical benefit to the analyzer.

3) I'd be wary of buying a regular photographic polarizer for x-pol. Note that I said "wary"--not "totally unwilling." Rik and one or two others have reported good results with photographic polarizers; I have tried a B&W and Tiffen and found them much poorer than my Olympus analyzer and Techspec polarizer. We are dealing with pretty small samples here, so any conclusions would be sketchy. Perhaps there are better and worse models of photographic polarizer, even within brands, or perhaps it is somewhat setup dependent. Or perhaps different observers, or their techniques, lead to different conclusions? If you already had a circ pol, I'd encourage you to go ahead and try it. But if you need to purchase something, that's where I'd get wary--I'm not rich enough to buy things twice.

My specific concerns with the photographic polarizers start with the mechanics: The rotational mounts of the units I've tried have not been robust enough to hold an objective with the precision I want. But note that I'm hanging optics off of them that range from 2x to 100x. The higher magnification objectives are pickier. Also, I don't like any sag or wobble, and am using a horizontal rig. Those more tolerant of movement, using less magnification, or using a vertical rig might feel differently.

Further, the extinction of the photographic polarizers I've tried has been far less than with the Olympus analyzer and Techpec polarizer. And color neutrality of the photographic polarizers was poor. With the Olympus and Techspec, it is dead neutral.

4) I'm wary of mounting a polarizer between an objective and a telephoto lens or zoom lens, and comfortable about mounting one between an objective and official Mitutyo tube lens. This is because of the distance added by the polarizer and any mounting accessories, the optical spreading that can occur across this distance, and the tendency of this spreading to bring into play less well-corrected portions of the converging lens. I'd emphasize that I don't have the empirical basis to say more than "I'm wary." My wariness is based on just two samples--my 200mm macro and my 200mm Mitutoyo tube lens. The increased distance drasticaly reduced quality with the macro, and does not bother the tube lens at all. How extensible is this to other lenses? I don't know. And note that Rik's experience has been positive.

5) I think it would be very easy to assemble a behind-the-lens polarizer mount for a Nikon objective, and even make it adaptable for RMS, Mitty, and other mounts. Just use a short SM1 (one-inch diameter, internally-threaded) tube from Thorlabs, a Thorlabs SM1-Nikon objective adapter screwed into the front, and whatever Thorlabs adapter you need to fit your other parts, screwed into the back. Get an Edmund Optics Techspec unmounted 25.4-inch (first check the clear aperture of the SM1 tube to make sure my memory is correct, and get a different size if I'm wrong). Get two of the Thorlabs' threaded inserts that screw into the SM1 tube and hold an optic. Add an O-ring from the hardware store for compression, to keep things tight. Voila, instant polarizer holder. This is what I did for my RMS-mount polarizer. If you get reflections off the polarizer and to angle it, remove one of the internal holding rings, add a couple of shims to angle the polarizer, and re-tighten the holding ring. Simple!

6) The cheaper, mounted polarizers from Edmunds have poorer specifications. I haven't tried them, but suspect the difference would be noticeable to a picky user. In my mind's eye, I see them as being similar to the photographic polarizers I've tried. But this is just a guess.

7) The Edmund Techspec film is great stuff for putting over your lights, and as Andrew said, you want to mark it for direction. But you probably don't want to put it in your optical path, as Pau said. The polarization characteristics are similar to the Techspec unmounted polarizers, so I suspect that the thin film that does the polarization may be the same. But in the polarizers, it is mounted between pieces of very good optical glass. In the film, it is mounted between, iirc, acetate sheets, which are fine for flashes, but will likely degrade an image if placed in the optical path.

Sorry for the rambling post. Have probably comitted quite a few errors--but maybe there is still some value in it.

Cheers,

--Chris
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stevekale



Joined: 11 May 2011
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PostPosted: Fri Feb 03, 2012 3:37 am    Post subject: Reply with quote

Hi Chris

4am? Wow! That reminds me of when I was trying to design a better way of gamut compression in B&W printing.

Quote:
3) I'd be wary of buying a regular photographic polarizer for x-pol. Note that I said "wary"--not "totally unwilling."


I think what I might do is have a play around with one of the Canon lenses for which I already have the correctly sized circular polarizer. (I just don't have one for the 52mm Nikkor AI lens.)

Quote:
4) I'm wary of mounting a polarizer between an objective and a telephoto lens or zoom lens, and comfortable about mounting one between an objective and official Mitutyo tube lens............The increased distance drasticaly reduced quality with the macro, and does not bother the tube lens at all.


The expense of your tube lens setup isn't something I'm up for. I have no intention of going beyond 10x. So my quest is really about starting on the right foot as I venture into lighting at a photomacrography scale. I've not been that impressed with studio, heavily diffused lighting but, as Rik notes, want to bat away those specular highlights. The sheet of Techspec was cheap to try. I was hoping the camera end would be easy to try as well.

Quote:
5) I think it would be very easy to assemble a behind-the-lens polarizer mount for a Nikon objective, and even make it adaptable for RMS, Mitty, and other mounts. Just use a short SM1 (one-inch diameter, internally-threaded) tube from Thorlabs, a Thorlabs SM1-Nikon objective adapter screwed into the front, and whatever Thorlabs adapter you need to fit your other parts, screwed into the back. Get an Edmund Optics Techspec unmounted 25.4-inch (first check the clear aperture of the SM1 tube to make sure my memory is correct, and get a different size if I'm wrong). Get two of the Thorlabs' threaded inserts that screw into the SM1 tube and hold an optic. Add an O-ring from the hardware store for compression, to keep things tight. Voila, instant polarizer holder. This is what I did for my RMS-mount polarizer. If you get reflections off the polarizer and to angle it, remove one of the internal holding rings, add a couple of shims to angle the polarizer, and re-tighten the holding ring. Simple!


Thanks for this. Presumably the shorter the better. One question and a note. How do you manage rotation of the polariser? (Partially unscrewing the mount would hardly be ideal.) I guess trying this (and incurring its cost) requires faith that your point 4) is based on limited samples!

Regards

Steve
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stevekale



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PostPosted: Fri Feb 03, 2012 4:07 am    Post subject: Reply with quote

BTW I do have one question regarding Andrew's experiment here

http://www.photomacrography.net/forum/viewtopic.php?t=11399&highlight=cross+polarization

The polarising film is coiled tightly against the diffusion sheet. Presumably light hitting the diffusion sheet doesn't get much of a chance to scatter before being polarised. Given the snoot apertures are still large relative to the image, I can't see how the diffusion sheet would make a big difference.
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ChrisR
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PostPosted: Fri Feb 03, 2012 5:01 am    Post subject: Reply with quote

Jinfinance sell irises in M42 male/female mounts, or with various objective threads one side and M42 on the back.

These have scope for polarisers I believe.
As an Iris adjuster there's a slot in the outer barrel, for the chrome handle, which sweeps about 90ยบ.
The slot is 2mm wide.
The ring it screws into, is 2.7mm thick.
If the slot were enlarged in length and width, the rotating ring could be slid out through it, flipped round and slid back in (assuming a plain plane polariser) to cover 180.

Plenty of possibilities here.

(and yes, I do know how to put irises back together Wink )




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rjlittlefield
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PostPosted: Fri Feb 03, 2012 10:26 am    Post subject: Reply with quote

stevekale wrote:
BTW I do have one question regarding Andrew's experiment here

http://www.photomacrography.net/forum/viewtopic.php?t=11399&highlight=cross+polarization

The polarising film is coiled tightly against the diffusion sheet. Presumably light hitting the diffusion sheet doesn't get much of a chance to scatter before being polarised. Given the snoot apertures are still large relative to the image, I can't see how the diffusion sheet would make a big difference.

Separation between the polarizing film and anything else shouldn't matter much. That's because polarizing film doesn't change the direction of propagation of a light ray, only the orientation of the electric & magnetic fields that create the ray. The diffuser's job is to scramble the ray directions. Once the ray directions are scrambled, it doesn't matter whether rays hit the polarizer after a short or a long distance. Either way they keep right on going in straight lines and end up hitting the subject with the same distribution.

How much the diffusion sheet matters at all depends on how diffusing it is and how uniformly it's illuminated. My best guess, given separation between the snoot and the diffusion sheet, is that the diffusion sheet roughly doubles the angular size of the illumination source (bright spot on diffuser versus opening in snoot). The illumination source will also have a softer edge with the diffusion sheet in place, since the snooted flash will shine a soft-edged spot of light onto the sheet.

--Rik
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Chris S.
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PostPosted: Fri Feb 03, 2012 1:33 pm    Post subject: Reply with quote

Steve, here is another quickly-typed response, as we go back into marathon mode here.

stevekale wrote:
I think what I might do is have a play around with one of the Canon lenses for which I already have the correctly sized circular polarizer. (I just don't have one for the 52mm Nikkor AI lens.)

This makes plenty of sense. And maybe you can add step-up/step-down rings to adapt this polarizer to whatever lenses you want--at the risk of adding still more distance between the objective and the converging lens. But as it's a minimal-cost approach using a polarizer you already have, it seems like the place to start.

Quote:
One question and a note. How do you manage rotation of the polariser? (Partially unscrewing the mount would hardly be ideal.)

I haven't settled on an ideal answer to this, despite hours and hours of head-scratching and looking at potential parts. Right now, I do indeed partially unscrew the filter mount a partial turn. It is inelegant, but is surprisingly workable, even at 100x (a slight refocusing is neccessary, and with the higher magnifications, a bit of horizonatal sag is introduced, so I need to lower my subject a tad.

As a potential "ideal" solution, Edmund optics now offers a C-mount Polarizer Holder that is rotatable. As far as I know, this wasn't available when I put my two polarizing rigs (one for RMS, the other for Mitutoyo) together, else I'd have tried it. I don't think it would be hard to get from C-mount to just about anything else you might need, though you might need to combine Edmund Optics, Thorlabs, and maybe Newport thread adapters.

Note that there are many rotatable filter holders available, but most do not have threaded, stationary front and back mounts. This product from Edmund Optics does appear to have these.

I would have purchased it already, despite the cost of the adapter ($129) and necessity of buying yet another Techspec unmounted filter (approximately the same money--ugh), but am sketching up something that I want to fabricate, which will do more things.

Best,

--Chris
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