Why can a linear polarizer interfere with SLR phase-detect autofocus?
Asked 9/5/2016
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I’ve read that some autofocus systems are affected by polarized light, and I’m trying to understand the role of polarization here.
For SLR phase-detect autofocus, why can a linear polarizing filter cause problems? Is there actually a polarizing element inside the AF system, or is the issue related to the camera’s mirror path? I’m looking for a technical explanation of what part of the AF system is sensitive to polarization and why circular polarizers are usually recommended instead of linear ones.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
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No, AF systems do not have polarizing elements.
Wikipedia only says that the light used by AF passes through a polarizing element, namely the mirror.
In SLR cameras light can either go directly to the sensor/film or can be redirected to the viewfinder and AF system using the moving mirror.
Polarized light reflecting of the mirror might be filtered our (depending on the angle, remember how polarizers are used to remove unwanted reflections) which disrupts the camera autofocus sensor.
Mirrorless cameras, compact cameras or smartphones generally do not use mirrors (except for weirdos like Zenfone Zoom) and do not suffer from this limitation.
Originally by user32811. Source · Licensed CC BY-SA 4.0
user32811
9y ago
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Generated from our catalog & community — verify before relying on it.
SLR phase-detect AF systems generally do not contain a dedicated polarizing element. The issue is mainly the mirror path used to send light to the AF sensor.
In an SLR, light is split by the reflex/sub-mirror system: some goes to the viewfinder and some to the AF module. Reflections from the mirror can affect polarized light, and if the incoming light has already been strongly linearly polarized by a filter, the AF sensor may receive reduced or altered light in a way that can disrupt phase-detection.
That’s why linear polarizers have historically caused AF/metering problems on some SLRs.
A circular polarizer avoids this by combining a linear polarizer with a retarding plate, so the light reaching the camera’s AF/metering system is no longer strongly linearly polarized in a problematic way.
This is mostly an SLR concern because mirrorless cameras, compacts, and phones do not use the same mirror-based AF light path.
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