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Category: New Questions

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Photography Question 
Charisse Baldoria

member since: 6/25/2001
 

Eliminating Glare and Reflections


I just got my first polarizer (linear). I noticed that, when I tried taking a picture of an aquarium, my flash unit's reflection appeared in many of the images. My polarizer's supposed to eliminate that, isn't it? Could it be just because the polarizer wasn't turned the right way, or could there be another reason it didn't work? I use a Canon F-1.

4/13/2002 1:54:33 PM

 
John A. Lind
BetterPhoto Member

member since: 9/27/2001
  Charisse,

To eliminate the glare from the flash bouncing off of the aquarium glass back to the camera using polarizers, it requires two polarizers. The first is mounted on the lighting and polarizes the light from the flash. The other is on the lens. This is the method used when copying flat artwork and sometimes when photographing jewelry, silverware, or other shiny metal items for advertising. Sheet polarizers to do this with (for the lighting) are expensive, consequently they're typically found only in studios that do this type of work frequently. In addition, with a typical shoe mounted flash you have to "modeling light" to help adjust the polarizer on the lens.

Two much less expensive and easier solutions are:
(a) Using the flash off-camera, at about a 30-45 degree angle to the aquarium so that its light isn't bouncing off of the aquarium glass right back at the camera. This requires some type of trigger cord between the camera and flash unit, plus some method of holding or mounting the flash apart from the camera (typically a light stand or tripod).

(b) Leaving the flash on the camera and aiming it camera at the aquarium at an angle to the glass instead of "head on." This will also keep the flash reflection from bouncing straight back into the camera lens.

-- John

4/14/2002 1:00:54 AM

 
Charisse Baldoria

member since: 6/25/2001
  Thanks, John, I really appreciate it. I see. So this type of polarizer isn't exactly glare/reflection-proof, is it? What exactly does it do then? Does it just lessen the glare compared to if there were no polarizer? Do u know if a circular polarizer is also usable by a fully manual cam like mine (Canon F-1). and would having that be much better?

4/14/2002 1:12:31 AM

 
John A. Lind
BetterPhoto Member

member since: 9/27/2001
  No, it's not. No polarizer is, circular or linear.

First, about circular vs. linear:
A circular won't do anything for you on the Canon F-1 that a linear can do. Actually a circular polarizer *is* a linear with a "quarter wave plate" cemented on its back side to keep it from interfering with auto-focus and metering systems mounted behind SLR mirrors. Your SLR, like two of mine, doesn't use this method (behind mirror light sensors), so a circular does nothing more than a linear would.

Next, about your specific glare problem:
Polarizers work by filtering out polarized light; it's the reason they can be turned to block light with a specifc polarization orientation. Reflected light can become polarized in a specific orientation, but only under certain conditions. The reflective surface must be smooth and electrically non-conductive. Metal surfaces won't polarize light, but glass ones will. The second condition is the angle of reflection, and the exact angle (or small range of them) depends on the reflective material's "index of refraction." This is different for each material. For glass, it works out to about a 45 degree angle. Water is similar, but not exactly the same. Sky is different. At exactly 90 degrees, or "head on," no reflective surface will polarize the light it reflects. Because of this second "angle" condition, no matter how you orient the polarizer, and no matter what type of reflective surface it is, you cannot use a polarizer on the camera lens to eliminate the reflected glare from an on-camera flash. This is why I suggested remoting the flash off-camera so the light reflected by the glass is bounced in a direction other than back to the lens, or shooting at an angle (with flash still on camera) which will essentially do the same thing; the light from the flash won't reflect directly back at the camera lens.

I will omit the finer details about how using two polarizers works (one on the studio light and one on the lens). The lights must still be at an angle, typically on light stands. The basic principle being used is polarizing all the light emitted by the strobe(s) before it's reflected from the artwork being copied.

Explaining the finer details about exactly how and why all this works the way it does requires a pile of diagrams and a complete web page or two. A truly in-depth one easily occupies a chapter in a university Physics text in the section on light and optics. :-)

-- John

4/14/2002 2:30:18 AM

 

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