Polarizing filter

[marcmccalmont]

Rotating a polarizing filter changes the effect and darkness so how do you compensate for exposure since it is a variable?
Marc

[AJSJones]

? re-meter throught the filter when it's at the intended angle - mostly you'll get one reasonable value for the filter factor. However, dark blue skies and highly reflective surfaces will give a bigger number. For non-polarized (e.g. non-reflected ) highlights the factor will be ~ constant, so you'll have to decide how much shadow detail you need in the areas most darkened by the filter and compensate accordingly.

[Brian Ellis]

See Adams "The Negative" p. 114: "Because of the change in values as the polarizer is rotated, photographers frequently assume the exposure factor must be increased as the degree of polarizaion increases. This is not the case! When used at the non-polarizing angle the polarizer acts as a neutral density filter with a factor of about 2.5; at maximum polarization the same factor applies. If we increased the factor as the degree of polarization increased, the non-polarized areas would be overexposed."

However, he goes on to say that ". . . when the polarizer is used to remove reflection from a large surface such as a body of water, the details of the underwater surface are revealed. These values may be quite low and we may then wish to recalculate exposure to ensure adequate detail therein." I take this sentence in conjunction with the previous paragraph to mean that while the factor of 2.5 remains constant, if reflections are removed by the polarizer and as a result dark detail is revealed then you might wish to base exposure on that detail and not on the brighter reflective surface.

[marcmccalmont]

I see a change from -1 2/3 to -2 2/3 stops as I rotate the polarizer. Perhaps then the -2.5 would be a good baseline.
Thanks
Marc

[David A. Goldfarb]

A filter factor of 2.5X is about 1-1/4 stops. Generally I add 1-1/2 stops for a polarizer.

[65Galaxie]

I have to admit I don't use a polarizer much but it makes more sense to open up 2 stops even though I have only noticed about a 1.5 stop difference. The reason it works is because the foreground is usually a little dark compared to the sky. Using a polarizer will keep the sky dark yet lighten up the foreground for more even exposure. Most of our exposures are with the foreground in shade and the sky full light. Kind of like a graduated neutral density filter. Personally, I can usually tell when a polarizer is used in that the sky with wide angle lenses doesn't have an even density.

[Robert Hughes]

I used one of my old Polaroid polarizer filters yesterday - it has a brownish/orangish tint, similar to an 85 filter combined with a polarizer. When I cover my incident meter with it in full sun, it knocks down the level about 2 stops, and I shot my TriX 320 at an effective ASA of 100; the negatives look properly exposed, and the combination orange and polarizer filter worked well in full sun and partly cloudy sky.

[Alan Davenport]

The variable effect of polarizers only happens with polarized light -- and that's what you want to change. Polarizers also have a neutral density component, and that's what you need to compensate for with the filter factor. That factor doesn't change with rotation, and is what you'll see if you look through a polarizer at a subject with no polarization (i.e., polarization even distributed around 360 degrees.)

Most polarizers have a filter factor around 1 2/3 to 2 stops. If you want to accurately measure the factor for an individual filter, find a subject for which the meter reading does not change when you rotate the polarizer; then the difference between the filter and no filter will be the correct factor for that polarizer.