What? 3+stops? No. Why is there consensus that between 1.3-1.7 is the standard filter factor for circular polarizers? TTL metering automatically compensates and it ain't 3 stops or more. Dialing in 1.3-1.7 stops more exposure on a incident meter works for me, too. If you couple two polarizers together, it is possible to get a variable ND filter effect by rotating one. Not sure what you're advising makes much sense practically.
Originally Posted by Bob-D659
The OP should probably do some research on how polarizers work.
You don't want to meter through the filter, and also catch the effect of the filter.
The filter is used to block light. Selectively.
If you don't want the parts that get darker as a result to get darker, don't use that filter.
So only use the fixed filter factor of the polarizer. The one that compensates for the overall, non-selective blocking of light.
So no TTL, through-the-filter metering to include the effect of the filter. If you think that the polarizer makes things look too dark, ask yourself: why are you using that filter?
Some scenes, esp backlit ones, can have a huge amount of polarized light from glare off of objects when the sun is close to the horizon. You can't easily use an incident meter, and the glare makes the reflected meter think the scene is much brighter than reality. Somewhat similar to taking photos in bright sun with lots of fresh snow where you meter and overexpose by a couple of stops. Rotating the pol filter for max extinction completely changes the lighting balance compared to the neutral density type light loss at minimum extinction. As you found this can give you somewhat underexposed negatives, all depends on what you are shooting.
Try metering a similar scene thru your pol filter with various angles of rotation from minimum to maximum extinction. I think you will see some major changes in the expose the meter calculates.
As an example, using a pol filter to remove the glare from the surface of a pond so you can photograph the bottom, you can't use incident metering as there can be substantial light loss from the light traveling thru the water in both directions. Besides, most incident light meters are rated as waterproof. :) Your only real option in this situation is ttl metering after adjusting the pol filter to remove the glare from the surface, metering thru the filter will be close, but it is awkward to do at best.
"Try metering a similar scene thru your pol filter with various angles of rotation from minimum to maximum extinction. I think you will see some major changes in the expose the meter calculates."
C'mon, 1.5-1.7 stops isn't exactly "major." Not sure what you're doing to necessitate a +3 stop exp. compensation with a circular polarizer.
It's really simple. When you meter through the lens with a polarizing filter, the readings will change as you rotate the filter. However this is just the meter being fooled by the polarizer. Don't go by that. Filter factors for a polarizing filter are usually 1.3 to 1.7 stops. I use 1.5 stops and I'm happy with that. Take your meter reading without the polarizer (or use a hand held meter) and add 1.5 stops more exposure. That's all you need to know, and it works every time. The amount of polarization you dial in doesn't matter. The filter factor is constant. To the OP, since you already have a circular polarizer, you can use it with anything and it works exactly the same as a linear polarizer.
thanks guys! I have been shooting recently with the pol filter on and exposing at about 1.8 stops over. It might be the dierction of my light, but anything from 1.3-1.7 just looks too under. The sunlight here is very harsh so it could even be a hemispherical thing, but I think the light demands basically a 2 stop comp factor.
I think there is another factor that needs to be considered in the discussion and that is the type of in-camera metering pattern. A spot meter and an averaging meter that meters a much larger area will each be affected differently. As with most photographic calculations, the ranges suggested are only a beginning point and the owner will need to experiement with each camera to see what factor is needed for each camera/meter used.
I have a circular 62mm Nikon for my Bronica that is an averaging meter in the AEII and the suggested compensation factor is pretty close but using the same filter, by holding it in front of the lens, on my Leica CL with a spot meter the factor is more variable de[ending on where and how I use the spot meter. If I meter a grey card it comes close to the Bronica but if metering a scene, it can be as far of as an 1/2 stop.
The circular polarizer was developed because of the AF and not the in-camera light meters just to add some clarity.
It was developed to be usable on a camera having any built-in system that uses a beam splitter or other reflecting surfaces, be it for AF or metering purpose.
Originally Posted by BrianL
(Though some of such cameras are not affected by polarized light, and do not require a circular polarizer.)