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# Do we need circular polarizers for mirrorless?

Started 7 months ago | Questions thread
Re: Do we need circular polarizers for mirrorless?

iljitsch wrote:

Erik Kaffehr wrote:

I think the circular in circular polarizers means you rotate the filter for which direction you are isolating?

A circular polarizer is a polarizing filter combined with a depolarizer behind. So, the light coming out on the sensor side is not polarized.

In practice that is what it accomplishes, but the light is actually circular polarized, which is... weird.

If you take a circular polarizer and look through it in a mirror (from the "in front of the lens" direction) then you'll see yourself except that the polarizer you're holding in front of your eye is black. This is because the mirror reverses the circular polarization (clockwise to counter clockwise or the other way around) and now the circular part of the polarizer won't allow the light through.

Um, no.

A circular polarizing filter (CPL) consists of a linear polarizer (LP) bonded to a quarter-wave plate (QWP).

In a CPL, the LP's polarization plane makes a θ=45° angle with the fast and slow axes of the QWP.

The light that is reflected back by the mirror passes two times through the QWP.

This doubles the optical path length — and birefringent fast/slow phase shift effect — and makes the QWP, in effect, a half-wave plate (HWP)

A HWP doesn't transform linearly polarized light into circularly polarized light: its effect on an incident, linearly polarized light is to rotate its polarization plane by an angle 2θ, where θ is the angle formed by the polarization plane of said incident light and the HWP material's fast and slow birefringence axes.

The light passing through he "LP → QWP → mirror → QWP" path will thus be a linearly polarized light whose plane of polarization makes a 2θ = 2*45° = 90° angle with the polarization plane defined by the CPL's LP component.

The reflected light will therefore be blocked not by the circularizing component (QWP), but, rather, by the LP component of the CPL filter.

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