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Thread: How does Auto Focus Function work in a D/SLR

  1. #1

    Question How does Auto Focus Function work in a D/SLR

    Was just curious to know how does the AF (Auto Focus) work in a DSLR or a SLR camera.

    Anybody...

  2. #2

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    Passive AF
    No beam emission. Image analysis performed on image. Camera drives lens till best focus achieved.

    Active AF
    Sound or IR emmission. Based on bounced wave, the camera performs some calculation on distance and then adjusts lens position.

    Most cameras use first method.

  3. #3
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    an example of the active infrared af would be the mamiya 645afd... medium format slr. expensive

  4. #4

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    For Passive AF (didn't know this term exist) most of the time focus is determined by the contrast of the scene. Therefore it is easier for the camera to lock focus for subjects of high contrast.

  5. #5

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    Quote Originally Posted by Ah Pao
    For Passive AF (didn't know this term exist) most of the time focus is determined by the contrast of the scene. Therefore it is easier for the camera to lock focus for subjects of high contrast.
    That's for a digi cam, where the system processor runs a constrast detection on the 'live preview' image from the sensor chips.

    SLR/DSLR use seperate focusing imaging chips that work differently. I don't think any would have enough resolution to do a contrast test anyway. They use other tricks. Ones that work much better than the contrast detection used in Digicams.

  6. #6

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    just do a google search lah, sure can find one. I found out that way too, something about taking sensor readings from 2 sides of the lens than comparing or something, like a rangefinder. can't rem now.

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    Quote Originally Posted by cqrv
    Was just curious to know how does the AF (Auto Focus) work in a DSLR or a SLR camera.

    Anybody...
    i hope you won't regret asking this one:

    The techinology that most SLRs use to focus is called Phase Detection, or Phase Difference Detection.

    It works by first diverting of some of the image to the AF system. The AF system then divides the light into pairs (or phases) that are compared with each other for differences. The system is passive bcoz it has no moving parts and does no emit any light towards the subject.

    Where i got the info from:

    http://photonotes.org/other/ai-servo.html

    Actually would appreciate if anyone has a link that explains phase-detection (wrt to AF) in more detail. Quite hard to find.

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    Quote Originally Posted by ST1100
    i hope you won't regret asking this one:

    The techinology that most SLRs use to focus is called Phase Detection, or Phase Difference Detection.

    It works by first diverting of some of the image to the AF system. The AF system then divides the light into pairs (or phases) that are compared with each other for differences. The system is passive bcoz it has no moving parts and does no emit any light towards the subject.

    Where i got the info from:

    http://photonotes.org/other/ai-servo.html

    Actually would appreciate if anyone has a link that explains phase-detection (wrt to AF) in more detail. Quite hard to find.
    If I am not wrong, the term phase difference refers to a difference in the phase of the curve that is plotted for the light image directed to the AF sensor. That is if the light from the image is resolved into a sine curve, the same image which is split into 2 by the camera, must reproduce the same curve with the curve having troughs and peaks at the same points on the x-axis for both curves since they come from the same first image taken by the lens.

    If the same image is split into two and compared, and when phase of the equation does not match, the image is considered out of focus. The camera then instructs the lens to alter the focus until the incoming light from the image produces curves that are perfectly matching in its phase.

    I think it is like that lar........ I might be completely wrong... someone please verify also....

  9. #9

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    Hmmmm...

    I think I have at least an idea. And in most of the cases, Passive AF is what I have used. Hence the issue of having sufficient contrast in the subject.

    Thanks buddies.

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