Beacon Camera Club

I assume that you are trying to darken a blue sky to emphasise the clouds Peter. The blue is caused by Rayleigh scattering which results in the light also being strongly polarised. Scattering is greatest at 90 degs away from the sun direction, so this is the direction you will need to point the lens to get the strongest effect. You then just need to turn the polarizer until the sky gets to as dark as you can make it. Note, however, that the effects of a circular polarizer on a blue sky is far more subtle than that of a linear polarizer. This is because the circular polarisation will never be completely orthogonal to the (blue) scattered light which is mainly linearly polarized.

Linear polarizer filters are rarely used these days as they can be the cause of metering and focusing errors with DSLRs (although I have never found them to cause noticeable problems). Circular polarizers don't cause these issues, but they are significantly more expensive.

Apologies if I've merely repeated what you already know.

Thanks Peter. The difference between linear and circular polarizers in photographic terms is actually more complicated than I described. A circular polarizing filter is a linear polarizer followed by another element (a quarter wave plate) which converts the linear polarization to circular polarization. This gets round the problem of any attenuation of light that is reflected off the DSLR mirror with linearly polarization. Given that the darkening of the blue sky is associated with the linear polarizing element in both cases (LPL and CPL), the impact on the final image should be almost identical for a linear and circular polarizer. However, it is likely that you will see more profound effects on the viewfinder image as you turn a linear polarizer due to this being a reflection from the mirror.

With mirrorless, there should be very little difference in what you see using CPL and LPL.