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The past days I've been trying to understand how AutoFocus(AF) works on photographic cameras. There is a statement that says AF systems are polarisation sensitive. This means that they can only work with circularly polarized light.

Trying to understand why, I came accross this article which states that beam splitters are polarisation sensitive. Is this true?

I cannot think of a way that the reflection of a linearly polarised light would have any problem or why a reflected circular polarized light would be OK.

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Ever heard of Fresnel equations? – Ondřej Černotík May 11 '13 at 15:26
@OndřejČernotík: I have heard indeed... What I haven't yet heard, is if an already polarized light(linearly or circularly) is somehow changed when it interacts with a beam splitter/mirror. – Thanos May 11 '13 at 15:34
That depends on the precise polarization state. If the light is polarized linearly but in a combination of parallel and perpendicular orientations with respect to the boundary then you need to apply Fresnel equations for each of the components and combine them again after the interaction. So the state can indeed be affected. – Ondřej Černotík May 11 '13 at 15:57
@OndřejČernotík: You are right on that. The thing is that I cannot understand, why/how the polarization affect the AutoFocus system. Could be the censor? – Thanos May 11 '13 at 16:03
I think that if you used a linearly polarizing filter, you could have a situation in which all light is transmitted through the beam splitter, reflecting no light on the AF sensor. When there is no light on the sensor, the system cannot, of course, focus the lens. – Ondřej Černotík May 11 '13 at 16:15

Reflection polarizes light. A reflected ray becomes linearly polarized perpendicular to the plane containing the incident and reflected rays. This is why polarized sunglasses are effective for reducing glare. The autofocus may not be working as expected because much of the scene is polarized light.

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