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According to Wikipedia, autofocus systems contain linearly polarizing elements.

What role does the polarizing element play in the AF system?

I have a basic understanding of "phase detection" AF, and I know how looking at light arriving under different angles can help determine the correct focus.

I am also interested in links/references to technical articles explaining this point.

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  • Is this for research? Do you need that kind of reference? Or just some reference?
    – null
    Sep 5 '16 at 20:14
  • @null It is to satisfy my curiosity. But I can probably read highly technical articles/books as well. And I am curious enough to want to read up on how AF works in great detail.
    – Szabolcs
    Sep 5 '16 at 20:15
  • Despite the video title, you may be interested in Old SLR Manual Focusing Demo.
    – user
    Sep 5 '16 at 20:41
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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.

light paths in a SLR camera

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  • Why is the mirror sensitive to polarization?
    – coneslayer
    Sep 5 '16 at 23:37
  • So that part of it can pass through and the rest of it can be reflected towards the focusing screen/prism/light meter/viewfinder.
    – Michael C
    Sep 6 '16 at 5:26
  • This is the coolest image I've seen today. Sep 6 '16 at 8:12
  • @coneslayer, it's not really that the mirror is sensitive to polarized light, rather a mirror causes the light (reflected and refracted) to become (partially) polarized. See Brewster's angle on Wikipedia.
    – Octopus
    Sep 6 '16 at 9:12
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    @MichaelClark Thanks for the followup note. The answer to coneslayer is, then, "because a relatively inexpensive, high quality splitter will have some polarization sensitivity." Sep 6 '16 at 12:31
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When polarized light strikes a surface that is conductor of electricity a phase change is induced. The first surface reflex mirror of the SLR is made by vacuum plating aluminum on flat glass. The aluminum based mirror is an excellent conductor of electricity.

Should a linear polarizer be mounted on the camera lens, the phase change after the mirror strike will be +180⁰. Now the focus mechanism of many SLR’s is dependent on detecting a phase change. The phase change induced by the mirror can frustrate this detection. The countermeasure is a circular polarizer.

This filter consists of two filters sandwiched together. The first is a linear polarizer that does the deed. The second is a retarder that de-polarizes. Thus the focusing mechanism is presented with light that is not polarized. The result is the focusing mechanism is not distressed.

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    this is incorrect, the "phase detection autofocus" does not detect or use the light wave phase - it is a simple geometric optics mechanism. polarized light disrupts the camera automatics simply because reflecting a polarized light (off the camera mirror) can attenuate the incoming light. after all, one reason why polarizers are used is to hide the reflections.
    – szulat
    Sep 6 '16 at 8:25
  • Light reflecting off the reflex mirror as well as those that pass through a semitransparent mirror will be partially linearly polarized. Should a linear polarizing screen be mounted on the camera lens, rays that arrive at exposure and focusing detectors will likely be reduced in intensity just as if they transversed cross polarizers. All this is a variable effect based on the orientation of the mounted linear polarizer. Best if we mount a circular polarizing screen. However, I actually have never seen an ill effect from mounting a linear; maybe it’s all not data, just lore. Sep 6 '16 at 16:35

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