Polarising Filters???

[Allen Mead]

Can someone tell what the difference is between linear and circular filters?

Daft question, but I'd like to know.

[David L]

A polarising filter allows light polarised only in one direction to pass the filter, of course..

Linear polarising filters are the simple, original version of the filter.. think of the linear filter like a very fine set of prison bars that only let light polarised to align with the bars through, in the case of the prison bars, that would be vertically.. Light not aligned with the bars does not pass.. If you rotate the filter, the alignment changes, and the direction of polarised light allowed to pass changes..

Linear filters do not allow auto focus operation..

I cannot remember quite how circular polarising filters work.. I do seem to remember, however, that the "bars" in these filters are circular, around the filter, as opposed to across it like the linear and it is this that lets circular polarisers allow auto focus operation.. They are directional in so far as you cannot mount them back to front and still have them work and so they have to be facing the right way (hence the slip in versions for long prime lenses have a face forward arrow)..

A little confusing as I have forgotten the technical explaination for how they work..

I once had a great article from an american magazine source that described all this to a tee.. But, as you do, I lost it in a throw out session a year or so ago..

There must a web article explaining all, perhaps, if you find it, you can post a link..?

Regards

[Allen Mead]

Hi David, thanks for the explanation!

[neil_davidson2]

David's description is spot on with regards to the linear filter, but if I recall correctly a circular filter is a linear filter with an extra plate that rescatters some of the light again. This is why they are directional, you don't want the scatter plate in front of the linear polariser or you'd effectively get the same result as a linear polariser by iteself.

The reason for this is that autofocus cameras use beam-splitters to pick up some of the light going through the camera and redirect it to the autofocus and metering systems. If the light has been linearly polarised then there may not be enough light at the beam-splitters to allow the systems to work correctly, rescattering some of the light gets around this problem.