Linear v Circular polarizers

Started Feb 2, 2013 | Discussions thread
Klarno Veteran Member • Posts: 4,141
Re: Linear v Circular polarizers

A linear polarizer is typically made with a polarizing film made by 3M or some other big materials company.

A circular polarizer is, as you know, a linear polarizer that has had a quarter wave plate added between the polarizing film and the lens. The quarter wave plate is made of a birefringent material similar to iceland spar calcite or the materials used to make antialiasing filters. In this use case, it converts linearly polarized light into circularly polarized light.

Linearly polarized light, of course, has different reflective properties off of translucent objects depending on axis (one of the reasons we use them is to cut reflections off of such objects). Because of this, linear polarizers can sometimes cause metering inaccuracy on autofocus DSLRs and probably also AF problems on Sony's SLTs, both of which use semisilvered mirrors.

What determines the effect of the polarizer is the linear polarizing film that is used. What the quarter wave plate does, in fact, is convert the light that has already been passed through a linear polarization filter into light that no longer exhibits linear polarization. The photons that have passed through the linear polarizer still form the same image.

The only disadvantage to a CPL over a LPL is, if anything, the tiniest touch of price. A CPL and LPL in the same product line made by the same company should have an identical effect on the end result (because the only thing that matters to that end is the polaroid film used). The circular polarizer is more versatile because it can be fitted without any problems on a DSLR or SLT.

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