Polarizing filter designed for autofocus cameras; cuts reflections and boosts contrast without interfering with AF systems.
Technical Details
The construction combines a linear polarizer with a quarter-wave plate made of birefringent material. The extinction ratio is typically 1:500 to 1:1000, corresponding to a polarization efficiency of 99.8-99.9%. High-quality filters use multi-coating with up to 16 layers to minimize internal reflections to below 0.5%. The rotating mount allows for 360° adjustment of the polarization angle, with maximum effect achieved at 90° to the incident polarized light.
Variants differ primarily in glass quality: standard filters use optical glass with λ/10 flatness, while premium versions achieve λ/20 or better. Slim versions with 3-4mm mounts avoid vignetting with wide-angle lenses below 24mm.
History & Development
Polaroid introduced the first synthetic polarizing filter in 1932, based on Edwin Land's Polaroid material. Circular polarizers emerged in 1987 in response to autofocus and metering problems with linear polarizers in SLR cameras featuring semi-transparent mirrors. Hoya and B+W established themselves as the main manufacturers of professional versions.
Digitalization brought new requirements: IR-cut filters in DSLR sensors interact differently with polarized light, leading to more precise manufacturing tolerances.
Practical Use in Film
Cinematographer Roger Deakins extensively used polarizers in "Skyfall" (2012) for the glass facade scenes to reduce reflections controllably without digital post-processing. For water surfaces, 45° polarization allows visibility down to 2-3 meters depth, as used in "Life of Pi" (2012) for underwater shots.
The filter enhances sky contrast by up to 2 stops at 90° to the sun, but reduces light by 1.3-1.7 stops. Rotation during shooting creates continuous brightness changes for special effects.
Comparison & Alternatives
Linear polarizers completely block autofocus and TTL metering in SLR cameras, but function identically in mirrorless systems. ND filters reduce light neutrally without a polarizing effect; however, variable ND filters are based on crossed polarizers and create similar issues.
Digital de-reflection via algorithms achieves comparable results in static scenes but fails with moving reflections or real-time streaming. Polarizers remain indispensable for sky drama and water photography.