Partially Cross-Polarized Flash Photography...

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twebster
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Partially Cross-Polarized Flash Photography...

Post by twebster »

Hi y'all :D

Perhaps some of you have seen the image, below, that I posted earlier in the macro gallery. I normally don't mind some glare on my subjects to establish that they are, by nature, shiny. However, I really didn't like rectangular glare of the flash head that was reflected in the ant and there were just too many glare spots in the bait...

Image

At the beginning of the week I had ordered some polarizing sheet material from SurpluShed (http://www.surplusshed.com/pages/item/m1581.html). I cut a small piece from a sheet and placed it over the flash head. I attached a circular polarizing filter to my macro lens. I set the polarizing filter on the lens so that the polarizing angle was 90° to the polarizing angle of the sheet material on the flash. The image, below, is of a Wein flash slave. The dome is a solid casting of epoxy over the electrical components of the slave. The left hand image was made with the partially cross-polarized filters, the right hand image was made without the polarizers. The yellow arrows point to direct reflections of the flash head in the unpolarized image.

Image

As you can see, there is quite a bit of difference between the images. I was able to take the setup out today and I tried to photograph some ants. I think I missed the focus by just a hair because I was having to shoot in deep shade and I could not see well to focus. This was the only ant I could find today...

Image

I left the image uncropped. This is the full field of view as recorded by the cmos sensor in the Canon EOS D30. The image magnification is 2:1 (2x life size). I attached a 2x teleconverter to the 90mm Tamron macro lens to achieve 2:1. Now that I think about it, the slight softness may be due to diffraction. I left the lens set to f. 16 and adding the teleconverter yielded an aperture of f. 32 :!: :D

The glare from the flash head was reduced to mere pin points. This is probably due to the fact that the polarizing sheet material and the circular polarizing filter on the lens are not perfect polarizing filters or my filter alignments were off a bit. Be that as it may, I think this is worth exploring more fully. I will set up a more controlled experiment and write an article on my results.

Best regards to all as always, :D
Tom Webster
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Think about this...maybe Murphy is an optimist!!!

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Post by Guest »

Thanks for exploring and sharing Tom. Unpacked and nicely settled in by now I trust.

Regards
Gunn

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twebster
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Post by twebster »

Hi ya' Ben :D

Settled in but not quite unpacked. Still a few more cartons to unload. :D

I've been experimenting with the crossed-polarized flash some more. This method is very effective at reducing glare from the flash but doesn't always reduce glare from the ambient light. This is because the sheet polarizer material attached to the flash head cannot be easily rotated. This kind of fixes the postion of the polarizing filter on the camera lens. You can't turn the polarizer on the lens to eliminate ambient glare and then turn the polarizing material on the flash head to eliminate flash glare.

Since only a small portion of the flash head is actually illuminating a macro subject I am mounting a 49mm polarizing filter in a rotating mount to the center of the flash head. I'm making my prototype filter holder out of cardboard. I should be able to turn the polarizing filter on the lens to eliminate ambient glare and then turn the polarizing filter on the flash to 90° in relation to the polarizing angle of the filter on the lens to eliminate flash glare.

The experiment continues... :D
Tom Webster
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teva
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Post by teva »

looking at images, i find this very interesting, so i'm trying to test that myself. I'm still looking for polarizing sheet, which is hard to find here.

Some newbie questions...
Does it matter if circular polarizing filter is placed before or after the macro lens?
How did you mount it on the flash? Is it close to the flash light, or you moved it away a bit. Does this even matter? I use flash diffuser made out of plastic bottle, which is about 7cm away from the flash. So if i tape polarizing sheet over the diffuser, would that still work?

I use Panasonic FZ5

tnx

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twebster
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Post by twebster »

Hi ya' Teva :D

I tape a small piece of polarizing material over the flash tube/reflector on my flash. Now, you don't want to use a diffuser with this setup. Diffusing the flash defeats the purpose of the polarizing filter. The circular polarizing filter screws into the filter threads on your camera. Circular polarizers are made to be used on autofocus cameras. The circular polarizers will not interfere with the camera's autofocus and exposure metering. Linear polarizers will not work properly on autofocus cameras. You want to have a diopter lens closest to the camera lens so you would attach the diopter lens to the camera lens first and then screw the polarizer on the diopter lens.

I'll post some images later this week of my setup and it will be more clear how and where to attach the filters. Also, if you send me your address and pay for the shipping I will mail you a piece of polarizing material. However, you may want to wait for an article I am going to write about this method. I have come up with a method to attach a circular polarizer to the flash head and thisa will allow for finer tuning of the polarizing angles.

Stay tuned... :D
Tom Webster
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My bad...

Post by twebster »

Hi y'all :D

All this time I've been saying that I'm using cross polarized lighting. My bad :!: :( I borrowed this term from microscopy in which a totally different concept of polarized light is applied. Actually, the polarizing filter on the camera and the polarizing filter on the flash must have their respective polarizing angles parallel. Setting the polarizing angles to 90° in respect to each filter will not allow any light from the flash to pass to the film or ccd/cmos sensor. I will update this concept when I write my article. Sorry if this caused any confusion. :(

Best regards to all as always, :D
Tom Webster
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Post by rjlittlefield »

Tom, not to panic -- I think you've had it right all along.

Specular reflections retain the polarization of the illumination, while diffuse reflections from nonmetallic surfaces become nonpolarized regardless of the illumination. What happens is that the diffuse reflection scrambles the direction of polarization.

What you're doing is to put the polarizers at 90 degrees ("crossed") so that all of the light from specular reflection is blocked, while only 1/2 of the light from diffuse nonmetallic reflections is blocked.

Think of looking into a mirror. You want the polarizers crossed so that they are black and the camera doesn't see the flash directly.

You're right that the microscopy community uses the term "crossed polarizers" to describe a different setup with transmission lighting, but it's still appropriate for what you're doing with reflection lighting.

BTW, assuming perfect polarizers, the total light loss should be 2 f-stops. You lose half the light (1 f-stop) in the first polarizer, and another half (the second f-stop) in the second polarizer. This applies only to the diffuse non-metallic surfaces, of course. For specular reflections and most reflections off metallic surfaces, the loss is almost total. (But of course that's the idea. :D ) For real polarizers, the loss will be somewhat more than 2 f-stops, but probably not much.

--Rik

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unethical

Post by Guest »

looking at images, i find this very interesting, so i'm trying to test that myself. I'm still looking for polarizing sheet, which is hard to find here.
Is that not a bit unethical ;-) of yourself?

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teva
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Re: unethical

Post by teva »

Anonymous wrote: Is that not a bit unethical ;-) of yourself?
I don't get it? What could be unethical here?

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Edwin Bont
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Post by Edwin Bont »

Another way to reduce highlights is to use indirect flash.
It works great on shiny beetles and ants.
See: http://www.photomacrography1.net/forum/ ... php?t=2537
It might help others too.

Sergio Zalewska
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Post by Sergio Zalewska »

Perhaps you are interested in this article:

http://www.naturescapes.net/042004/wh0404.htm

rjlittlefield
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Post by rjlittlefield »

Sergio Zalewska wrote:Perhaps you are interested in this article:
http://www.naturescapes.net/042004/wh0404.htm
The link is to Wil Hershberger's article "Taming those Annoying Highlights: Cross-Polarization Flash Macro Photography". It's very readable, has a good level of technical detail, and contains some interesting info that complements Tom's excellent work. (I'm gonna have to look more closely at those Wimberley flash brackets...)

Sergio, thanks for posting this link!

--Rik

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