calculate exposure compensation for CIR-POL

[wlaukaitis]

Hello!

How do I figure out how many stops I need ( + or - ) for my circular polarizing filter?

If I take a shot at F8 at 1/100 with out the filter, then put the filter on that same shot is F8 at 1/30 (using Av mode). That is a little slow I think, adn leaves me with little room to work with. I do know that cir-pol reduce by up to 2 stops but I want to know is it .33, 1 or 1.66?

How can this be done?

first post BTW

[zona5101]

Hi and welcome.

Why not let the camera meter make the exposure adjustment? You can then slide your desired combination of fstop shutter up or down or even increase your iso to get you to a workable exposure setting.

...to more directly answer your question, you do the math and determine the # of stops between reads with and without...ie your example 1/30 to 1/100 is 1 2/3 stops
(30 to 60 is one; 60 to 120 is two; 60 to 80 is 1/3 to 1/100 is 2/3)

[OsmosisStudios]

Your camera will calculate proper exposure by itself. Remember: a stop is doubling or halving your shutterspeed.

standard stops are:
30s, 15s, 8s, 4s, 2s, 1s, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000.

[LeeR]

Typically a polarizing filter reduces light by 1 1/2 to 2 stops, but, as has already been pointed out, your camera should make that adjustment for you. Isn't that why you bought a Circular Polarizer? The old polarizers did not work with the meters on some digital cameras but the new circular version works, to the best of my knowledge, with all digital cameras. This means no compensation is required; just slap the filter on the front of your camera and shoot like you normally would.
If you need to know how much light you are losing with your filter, just measure a scene without the filter and then again with. The difference will be the amount of light lost by the filter. This can actually be a good thing. I have on occasion used a polarizer in lieu of an ND filter when I needed a longer shutter speed.

[kirbinster]

As others have said, in a slightly different way, the light meter in your camera is behind the lens. Thus, any filter you put on the lens will change the amount of light hitting the meter and force the camera to make adjustments itself - no need for you to do anything.