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How does autofocus work ?

Discussion in 'Photography Forums' started by HMHB, May 17, 2005.

  1. HMHB

    HMHB
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    It's something we all take for granted and moan about when it doesn't seem to be perfect, but I have no idea how autofocus actually works !
    Can anyone enlighten me or point me in the direction of some good links to read up about this, as I'm sure that the more I know about how it works, the more success I will have with autofocus
     
  2. dolph

    dolph
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    I have to admit my knowledge in this area is scant...

    So many things to know about...

    What is Servo
    What is AI etc etc 9 point?...
     
  3. dolph

    dolph
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    Well at least this thread prompted me to look into Servo vs One-Shot - quite simple really. One-shot for stationary objects, servo for moving....
     
  4. Geordie Jester

    Geordie Jester
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  5. seany

    seany
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    The 9 point af is when you want an off centre subject in focus so you select the point that the subject is in


    This is off a screen print, and cropped twice so ignore the res. Put it shows the 9 selectable af points and the one i used
     

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  6. SeaneyC

    SeaneyC
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    Multi point AF (most cameras implement some kind of multi point point AF, anywhere from 3 - 45 points) works in 2 ways, you can either let your camera choose the AF point, or select the AF point yourself. Whilst being initially quite sceptical about how the camera would know what you want to focus at, it's surprising how often it gets it right. I only just got a camera with multipoint AF, so i'm still working out which modes i prefer to use for different types of photo.
     
  7. HMHB

    HMHB
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    The thing that baffles me is how the autofocus actually does it - you know how does it know how far away an object is so that it can focus on it ?
     
  8. seany

    seany
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  9. seany

    seany
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    I'd not let the camera select the af point, af it looks for the most contrast in subjects and that not always the subject you're after. If you're using lenses that have a wide aperture you really need to select the af point because of the shallow depth of field.
     
  10. tomson

    tomson
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    it does fall over though - when trying to pick out a face in a crowd for example - but then thats hardly surprising. I always have my cameras fixed on the central AF point these days - seems to serve me well, just have to focus and recompose for off centre subjects.

    I read somewhere a while back that the centre point is usually the most accurate - anyone know if that still holds true?
     
  11. seany

    seany
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    I've read that often to tom. I also lock the focus, and recompose. I never leave it on auto select
     
  12. dolph

    dolph
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    I assume what you mean by this is what I often do with my A80 - point camera straight at subject - hal press shutter, recompose and fully press to take shot....?
     
  13. tomson

    tomson
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    that's the one - only i work in manual mode so add an extra stage in there for setting correct exposure & aperture.
     
  14. ancientgeek

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    The autofocus doesn't know how far away the object is, although the lens may tell the camera what the distance is after autofocus is complete. In the same way that you can focus manually, but you'd then look at the lens scale to see what the distance is.

    Simply by moving the lens back and forth until the image contrast on a small row of sensors is maximum, the lens can be focussed. Add some more clever programming and you can track moving subjects by predicting the changes in focus needed based on the movements needed in the preceding fraction of a second, leaving only final correction to be completed.

    When the lens is badly out of focus to start with, you may catch the camera setting off adjusting focus in the wrong direction; it may have to hit the end of its range to realise it's going the wrong way. Especially noticeable with long telephoto, or macro lenses with wide focussing ranges. On the other hand, if it starts nearly in focus, it will immediately see an increase in blurriness and reverse focussing direction.
     
  15. condyk

    condyk
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    I remember once I dropped a point and shoot camera I had and from the corner of my eye I thought I saw a very tiny person in orange shorts and t-shirt running away. They shot under the lounge door before I had chance to look properly. I dunno, maybe it was just a bug or something, and I had just been to the pub that day, but from that moment on I always had problems with the autofocus on that camera. Even now when I shake my 300d I hear a faint rattle as if someone is actually living inside. Anyone else experience that?



    :rotfl: :rotfl:
     
  16. TRambler

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    According to Canon, this is not the case - the camera determines the amount the lens has to move to achieve focus, and then moves the lens straight to the correct point. It only moves back and forth when it can't achieve focus (due to low contrast areas), or to correct for the motor overstepping.
     
  17. HotblackDesiato

    HotblackDesiato
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    I too leave generally leave the centre sensor active, lock AF and exposure then recompose BUT it's worth noting i've been caught out with soft shots at wider apertures. By recomposing you're effectively altering the distance to the now off centre subject. Where i think that'll be a problem i'll select an off centre focusing point.
     

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