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How do cameras focus on reflected images?

  1. Jan 8, 2015 #1
    I can remember from my distant school physics days that the image in a mirror lies beyond the mirror. But how does a modern camera sensor "know" this. If I face a mirror with my canon camera which has a cmos sensor and usm lens, it autofocuses correctly on the part of the reflection I select. Why does the sensor not see the mirror as a flat plane of light with varying degrees of intensity?. I have done multiple google searches and can't find an answer.
    Any light on this subject appreciated.
  2. jcsd
  3. Jan 9, 2015 #2

    Doug Huffman

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    Gold Member

    Automatic cameras use optical edge detecting algorithms. When the edges are sharp the image is assumed to be in focus.
  4. Jan 9, 2015 #3


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    Staff: Mentor

    The same reason we don't see the mirror as a flat plane of light. The light rays (or waves) arriving at the camera lens or our eye diverge in exactly the same way as if we had removed the mirror and placed a (spatially-reversed) copy of the object in the space behind the mirror.
  5. Jan 11, 2015 #4
    Makes sense. Thanks.
  6. Jan 11, 2015 #5
    Edge detection, to my understanding, is the "fine-tuning" step actually. Cameras often do a quick 'n dirty approximation with an infrared beam, and then follow it up with edge detection.
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