In black and white work the sky often photographs lighter than it appears, because the blue it emits records strongly on most panchromatic emulsions. The familiar remedy is a coloured contrast filter, which darkens blue by absorbing it. A polarizing filter reaches a similar end by an entirely different mechanism: it selects light by its plane of vibration rather than its wavelength. That distinction is what makes it useful where a coloured filter cannot help, and what governs when it works at all.

How a Polarizer Darkens the Sky

Light scattered by the atmosphere is partially polarized, and the degree of polarization is not uniform across the sky. It reaches a maximum in a band roughly 90 degrees from the direction of the sun, and falls away toward the sun and the anti-solar point. A polarizing filter passes only light vibrating in one plane; rotated so that plane is crossed with the dominant polarization of the skylight, it absorbs much of that scattered component, and the sky records darker.

The strength of the effect therefore depends on aim. A scene photographed with the sun to one side sits in the band of strongest polarization; one shot toward or away from the sun shows little change however the filter is turned. Wikipedia’s summary of the technique notes the useful zone as a band of about 15 to 30 degrees measured from the optimum direction.

Why the Tonal Balance Is Preserved

The advantage over a coloured filter lies in what the polarizer leaves alone. Light diffusely reflected from foliage, rock, skin and most other surfaces is largely unpolarized, so it passes through the filter regardless of orientation. Only the directionally polarized component, the scattered skylight and specular reflections, is selectively removed. The sky darkens while the rendering of greens, browns and flesh tones stays close to neutral.

A red contrast filter, by comparison, darkens blue sky by absorbing blue light wherever it occurs, which also lightens red objects and darkens blue ones across the whole frame. The polarizer shifts no colour relationships, because it does not discriminate by colour. The two tools can be combined, a coloured filter for spectral contrast and a polarizer for glare, since they act on independent properties of the light.

Suppressing Reflections off Water and Glass

The same selectivity removes glare. Light reflected from a non-metallic surface, water, glass, wet leaves, painted surfaces, becomes polarized on reflection, and the polarization is complete at Brewster’s angle, approximately 56 degrees from the vertical for ordinary glass. Near that angle a polarizer crossed to the reflected light can extinguish most of it, revealing detail beneath a water surface or behind a pane. Bare metal does not polarize light on reflection, so highlights on chrome or unpainted steel are not controllable this way.

This control costs light. Because the filter blocks one plane of vibration even from unpolarized sources, it reduces exposure by roughly one to three stops, a filter factor of about 2 to 8 depending on how much of the scene’s light is polarized. Through-the-lens metering accounts for the loss, but the magnitude shifts as the filter is rotated and should be read after the orientation is set.