Compensating for Circular polarizer

This is a general question about compensating using a light meter when I have a CP installed on a lens.

I'd like to use manual mode on my 20D and have got hold of a Sekonic L558. If I set for example I want to use F4 and the measured speed comes up as 125th sec, what would I set to to compensate for the CP?

Any advice would be welcome.

Thanks in advance

Rog

I believe that a CP costs you 1-2 stops, so I guess you'd have to compensate accordingly. But, why not just use in-camera metering?

Thanks for the info.......

I just want to see what difference it makes as the 20D does not allow true spot metering.

Rog

In order to discover the proper correction factor for your filter - and the conditions you are shooting in - meter the same scene botyh with the camera (NO filter) and the handheld meter. This is to verify that both are calibrated the same. Then, meter the same scene with the CP filter on the camera and using the camera's meter. The difference in exposure values found in the camera can then be transferred to readings made with the handheld meter. The difference, as stated above, should be in the 1 to 2 stop range.

Also note that the amount of compensation will vary (if you're looking for an exact number) depending on the degree of rotation of the polarizer and the position of the sun relative to the filter.

Thanks Skip, Leo, I'll be out this evening to give it a go...... won't get dark until after 9pm so lots of time to experiment.

Rog

There's no single compensation value for CP.

Depending on what you shoot, you can take out from one to more than 4 stops of light from your scene.

The checking of exposures with a hand-held meter and the camera (with polarizing filter) will only work if you keep the filter turned to exactly the same angle.

Take care,
Harry

Very good advice here from Skip.

Great advice Dimitri.......I've given it a go photographing my daughter in low light and found that the filter on my 60mm lens is 2 stops difference whilst on my 17 40 it's only 1 1/2.......

Rog

I've read that with CPL vs non circular types, CPL's do not affect your light metering. However, in my personal experience, it is causing underexposure by less than one stop or so. So now I try a bit of +EV and check the histogram with better results.

Circular polarisers don't mess up the metering or autofocus. Whilst most people talk about linear polarisers as affecting autofocus on many cameras, they can also mess up the built-in metering as well, leaving you with incorrect meter readings.

However, a circular polariser does have a filter factor - you do lose somewhere between a stop and two stops when using it.

David