Why can linear polarizers interfere with SLR phase-detect autofocus?
Asked 5/22/2015
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Older SLR cameras often required circular polarizers rather than linear polarizers for reliable autofocus and metering. I understand how polarizers work, but I’ve never clearly understood why phase-detect autofocus is affected.
Manual-focus split-prism screens still worked visually, and image sensors themselves don’t seem inherently bothered by polarized light. So what part of an SLR’s phase-detect autofocus system has trouble with linearly polarized light, and why does a circular polarizer avoid the problem?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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The light path for the autofocus in an SLR (image) sends it through a semi-transparent window in the middle of the mirror (in effect a beam splitter).
Light, bouncing off of or being transmitted the mirror will become partially linearly polarized itself (much like the light off of a leaf or water). If the light incoming is linearly polarized, some of that light will be lost as part of the process - just as with light going through cross polarizers.
The resulting light loss will show up in two ways:
- reduction of light hitting the auto exposure sensors leading to the overexposure of the image
- reduction of light hitting the auto focus sensors leading to a reduction of contrast and more difficulty focusing.
This effect would be variable based on the angle of polarization.
Furthermore, note that anti aliasing filers make use of linear polarization which could also be disrupted by having consistently linearly polarized light being thrown at them.
The solution to this is to have the light polarized in a way that is not affected by a linear polarizer (the semi-transparent window in the mirror). This can be done by placing a quarter wave plate on the back of a linear polarizer making the polarization 'spin' which is known as circular polarization.
Originally by user13451. Source · Licensed CC BY-SA 4.0
user13451
11y ago
0
Generated from our catalog & community — verify before relying on it.
In an SLR, phase-detect AF and often metering don’t receive light directly the same way the imaging sensor or your eye does. They rely on light being diverted by the main mirror and sub-mirror system, which acts partly like a beam splitter.
Reflection and transmission through those mirror surfaces can themselves polarize light. If the incoming light is already linearly polarized, the mirror system may pass or reflect less of it depending on the polarization angle—similar to the loss you get with crossed polarizers.
That can reduce the light reaching the AF and metering sensors. For AF, less light and lower effective contrast make phase detection less reliable. For metering, reduced light can bias exposure readings. The effect varies with the filter’s polarization angle.
A circular polarizer solves this because it starts with a linear polarizer, then adds a quarter-wave plate that converts the light to circular polarization. That prevents the camera’s beam-splitting mirror/AF system from reacting strongly to a fixed linear polarization direction, so autofocus and metering work normally.
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