How do cameras focus on reflected images?

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  • Thread starter Thread starter GeorgeV
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    Cameras Focus Images
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Discussion Overview

The discussion revolves around how modern cameras focus on reflected images in mirrors, particularly addressing the mechanisms behind autofocus systems in cameras and the perception of light from reflections.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant recalls that the image in a mirror lies beyond the mirror and questions how a camera sensor can focus on this reflected image.
  • Another participant suggests that automatic cameras use optical edge detecting algorithms, where sharp edges indicate that the image is in focus.
  • A participant elaborates that the camera sensor does not perceive the mirror as a flat plane of light because the light rays diverge similarly to how they would if a spatially-reversed copy of the object were placed behind the mirror.
  • Further clarification is provided that edge detection serves as a fine-tuning step in autofocus, following an initial approximation using an infrared beam.

Areas of Agreement / Disagreement

Participants present multiple viewpoints regarding the focusing mechanisms of cameras, particularly in relation to edge detection and the perception of light from mirrors. There is no consensus on a singular explanation, as different aspects of the autofocus process are discussed.

Contextual Notes

Some assumptions about the nature of light and reflections are not fully explored, and the discussion does not resolve the complexities of how cameras interpret reflected images.

GeorgeV
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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.
 
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Automatic cameras use optical edge detecting algorithms. When the edges are sharp the image is assumed to be in focus.
 
GeorgeV said:
Why does the sensor not see the mirror as a flat plane of light with varying degrees of intensity?

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.
 
Doug Huffman said:
Automatic cameras use optical edge detecting algorithms. When the edges are sharp the image is assumed to be in focus.
Makes sense. Thanks.
 
Doug Huffman said:
Automatic cameras use optical edge detecting algorithms. When the edges are sharp the image is assumed to be in focus.

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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