Need Help with Cross Polarization (or Just Polarization)

[Vinc T]

I am setting up a test to try cross polarization for the first time. The first thing I did was cutting a disc out of a very cheap polarizer sheet to put on my speedlight. I tested turning it to see if it worked on my monitor and it sure did.

However, when I tried using it after putting a bowl underneath an LED light, I saw something that I totally didn't expect. I am sure all the highlights on the bowl were from the one LED above (first photo below) but when I turned the polarizer, I killed all highlights but that one strip at the top (second photo below). I tried tilting the bowl and polarizer but I just could not eliminate that highlight.


Polarizer tilted to show the LED above.


Could not get rid of that one reflection.

So, there are two kinds of highlights or reflections that I was seeing?

(Guess I better find out the answer before trying cross polarization!)

 

[grsnovi]

Not sure what your long-term use case is or what about the bottom shot that you didn't like.

Are you using a diffuser on your flash? How is the flash directed? Are you using any bounce reflectors?

Have you tried a good CP filter on the lens?

 

[Lester Lefkowitz]

Not quitre sure of your setup.

"Cross polarization lighting," means there is a polarizer on the light source AND also one on the lens. Then, looking thorough the camera, rotate the lens polarizer until all reflections are gone. Assumiong you have one light source with its own polarizer, such a system should get rid of all specular highlights. In your post, you do not indicate if you also have a polarizer on the lens.

Because that porcelain bowl is so shiny, it may be reflecting some stray light from the LED that does NOT go through the light polarizer. Is there a window nearby? An exit sign?

If more than one light source is used, the polarizer sheets in front of each light must be oriented so that their axes of polarization are all identical.

Interesting factoid: You can't use polarization to eliminate reflections on conductive surfaces (metal).

Lester Lefkowitz, author of The Manual of Close-Up and Macro Photography, Volumes I & II

www.MacroPhotographer.net

 

[Vinc T]

Not sure what your long-term use case is or what about the bottom shot that you didn't like.

Are you using a diffuser on your flash? How is the flash directed? Are you using any bounce reflectors?

Have you tried a good CP filter on the lens?

I am not a working photographer. I am just trying to take photos of some snacks in transparent packages. The light is a continuous desk lamp with no modifiers. No reflectors either.

After reading your comment, I put a 58mm circular polarizer on my lens and tried it again. Same results as shown below.





I could not get rid of that highlight no matter how I turned or tilted my camera or bowl. I therefore is wondering what the difference is between the 2 highlights.

(I don't have time to try cross polarization yet. Will definitely try it may be in a week or two.)

 

[Vinc T]

No modifiers or polarizers on the LED desk lamp. Both the top and bottom highlights were from the desk lamp above the bowl as I have tried turning the lamp off. I was just trying to see how well a cheap polarizing sheet I bought online worked and then I noticed that I could not get rid of the top highlight. Same thing I tried a bit earlier with a CPL on my lens.

Interesting factoid: You can't use polarization to eliminate reflections on conductive surfaces (metal).

Lester Lefkowitz, author of The Manual of Close-Up and Macro Photography, Volumes I & II

www.MacroPhotographer.net

Thanks! I will try to remember that related to metallic surfaces. (I have really bad memory.)

 

[Lester Lefkowitz]

Not sure what your long-term use case is or what about the bottom shot that you didn't like.

Are you using a diffuser on your flash? How is the flash directed? Are you using any bounce reflectors?

Have you tried a good CP filter on the lens?

I am not a working photographer. I am just trying to take photos of some snacks in transparent packages. The light is a continuous desk lamp with no modifiers. No reflectors either.

After reading your comment, I put a 58mm circular polarizer on my lens and tried it again. Same results as shown below.





I could not get rid of that highlight no matter how I turned or tilted my camera or bowl. I therefore is wondering what the difference is between the 2 highlights.

(I don't have time to try cross polarization yet. Will definitely try it may be in a week or two.)

If you don't use a polarizer on the light AND on the lens, you will never get rid of all reflections. That's what is meanty by "cross polarization."

Lester Lefkowitz. www.MacroPhotographer.net

 

[grsnovi]

If you're not going to have your snacks in a white bowl, why show that as an example?

Do you have a shot of the snacks in a package that is causing your trouble?

BTW - I'm not a professional either. I have only used polarizing filters to adjust the sky and/or to remove highlights from wet vegetation.

 

[Paul5566]

To get rid of all reflections using cross polarisation you must have a polariser on your lens and a polariser on the light source. There must be no ambient light as the ambient light will not be polarised. The two polarisers must be polarised at 90 degrees to each other. You must eliminate reflected light from the light source e.g. ceiling walls etc. as the reflected light will lose its polarisation. It can be quite tricky to achieve all the above. With one polariser you can reduce reflections but it will not remove all of them.

A good video here.



--
https://paulstickley.com/insects-and-close-ups
https://www.flickr.com/photos/140647103@N08/

 

[Vinc T]

Thank you everyone for your replies!

I have finally tried cross polarization!

As before, when properly turned, the polarizer in front of the lens got rid of the bottom highlights. I tried late last night covering up the LED desk lamp as well with a polarizer film. When properly turned, the film eliminated the top highlight. So cross polarization worked in this situation.

I was and am still amazed at how one light source shining on one material generates two types of highlights.