[bdery]

A polarizing filter cannot increase light from a reflection. It might reduce other reflections, making the one in question more visible, but it does not increase light from any reflection.

That's correct, I apologize if I wasn't clear about this. I confess I was struggling to find the right words, being originally French-speaking.

[maltfalc]

There may be some misunderstanding of polarization filter function here. Light can be polarized by reflecting off a surface at low angle - about 33 degrees for water and glass. A polarizing filter can reduce light rays in a particular plane. When oriented with the polarized light from a reflection, it will reduce the reflection. A polarizing filter cannot increase light from a reflection. It might reduce other reflections, making the one in question more visible, but it does not increase light from any reflection.

if the filter is aligned with the polarization of the reflection most of the light from the reflection will pass through the filter, but half of the unpolarized light will be blocked, so the overall brightness of your subject will drop about a stop, but not the reflection, which will increase the apparent brightness of the reflection.

[devorama]

Hopefully a picture is worth 1000 words.




These are three shots I took to illustrate the principle. The top shot is with no polarizer. This is our baseline. I put some rocks in the bowl so you can tell how much the reflection obscures what's below the water.

The second photo shows what you can do if you align the polarized filter to allow the maximum amount of polarized light. Notice the exposure time is 1.5 stops longer. This must be done because the polarizer is blocking a lot of the unpolarized light from the rest of the picture as well. This 1.5 stop exposure compensation will allow a comparable overall exposure.

The last shot shows how the polarized filter is more typically used. Here is is turned 90 degrees to block the polarized light from the direct reflection. Again, a 1.5 stop longer exposure is used.

While the difference between the 1st and 3rd shot is most striking, there is a definite difference when you use a polarizer to maximize the reflections, as seen in picture 2. The reflection of the shade around the bulb is brighter, and obscures the rocks below more than in the first shot.

Hope this helps!

[Anvh]

Thank you very much and looking and thinking about it, it seems maltfalc is at the right end.

[Ron Boggs]

Devorama's photos illustrate the variability of full reflection to no reflection obtainable with a circular polarizer. Really nicely done Devorama!

Found something interesting in a manual for Pentax polarizers. The manual was printed in Nov. '95 in Japan. There is a chart listing "Polarizing Reflection Angle". For the surface of glass or ceramic ware the angle is "about 57 degrees"; water surface is "about 53 degrees"; surface of glossy paper is "about 58 degrees"; surface of polished wood is "about 55 degrees."

Kind of funny that they actually list "about" rather than precise figures. Must have been too many digits? The manual includes a small diagram of what they mean by this Polarizing Reflection Angle and contrast it with the Incidence Angle in the diagram. Not sure if this manual is in the Pentax database?

For me, just put it on the lens and play with the amount of effect you want.

[icywindow]

Thanks everyone for your responses on this! I will make sure to pack a CP for such a purpose. In particular, thank you devorama for the illustrative photos. Interesting that the CP eats about a stop and a third of light. I suppose I would have expected that, since it's a darker filter.