What’s the difference between linear and circular polarizing filters in photography?

Question

I’m choosing a polarizing filter and am confused about linear vs circular polarizers. Some explanations say a circular polarizer is basically a linear polarizer plus another optical layer, while others describe circular polarization in terms of phase shift and quarter-wave plates.

For practical photography, what difference does this make in the final image? Do linear and circular polarizers produce different photographic results, or is the main difference how they interact with the camera’s autofocus and metering systems?

user15871 · Answer

A 'circular' polarizer is just a linear polarizer with a quarter-wave plate behind it that repolarizes the light in a circular kind of way. But the polarized light removed by passing through the linear polarizer is gone, and thus is not reintroduced by the "re-polarization" of the remaining light.

A quarter-wave plate is not another linear polarizer. It is arranged 45° with respect to the linear axis of the polarizer. As the polarizer is rotated to alter the effect of the filter with regard to light from a particular direction, the quarter-wave plate is attached (bonded) to the linear polarizer and rotated as well. The quarter-wave plate is always turned 45° with respect to the polarizer's linear axis.

The polarization of the light by the quarter-wave plate allows phase detection autofocus (PDAF) systems and light meters to function properly. PDAF systems often fail when trying to focus light that has passed through a linear polarizer alone. Light meters placed past the viewscreen in reflex¹ cameras are also affected by linearly polarized light.

There is no effect of the quarter-wave plate on the appearance of the photo compared to a linear polarizer without a quarter-wave plate. Neither film nor digital cameras record in which direction light is polarized when it strikes the sensor or film. The linear polarizer allows light polarized in one direction to pass through it. The light that is not allowed to pass through is gone. The quarter-wave plate can only act on the light that has been allowed to pass through the linear polarizer in front of it. It can't recreate the light that the linear polarizer blocked. So the light striking the film or sensor is the exact same light either with or without a quarter-wave plate behind the linear polarizer (other than the minimal amount of light lost due to transmission through a refractive medium as happens with every lens element in the optical path).

Two linear polarizer filters arranged 90° out-of-phase with one another will (theoretically) block all light from passing through (with or without a quarter-wave plate behind the second one). So-called 'Variable Neutral Density' filters are actually two stacked polarizers. As one is rotated with respect to the other, the varying angle between them blocks more or less of the light striking them.

_{¹ 'Reflex' is the 'R' in SLR and DSLR and refers to the reflex mirror used to divert light from the lens to the viewfinder.}

What’s the difference between linear and circular polarizing filters in photography?

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