Why separate AF sensors in DSLRs ?

Discussion in 'Digital Photography' started by Alfred Molon, Sep 4, 2004.

  1. Alfred Molon

    Alfred Molon Guest

    I understand that these currently allow faster focusing speeds, but they
    also require a careful alignment / calibration with the CCD and the
    lens, to avoid focusing problems.

    I think these separate sensors are a design which originates from the
    old film SLR days in which the film frame could not be used for AF.

    But a CCD can be used for AF, so you could remove these separate
    anachronistic sensors. The advantage would be lower cost and less strict
    tolerances of the CCD / lens combination (production would be cheaper
    because an additional calibration step would be avoided).

    By the way, the new 7MP Casio Exilim has a shutter release delay of just
    10 milliseconds
    http://www.dpreview.com/news/0408/04082504casio_p700.asp

    so short shutter delays are possible even with compact cameras with no
    separate AF sensors.
     
    Alfred Molon, Sep 4, 2004
    #1
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  2. Those 10 ms does probably not include focussing time.

    BTW - DSLRs cannot use the CCD for live preview. And therefore
    it cannot use the CCD for focussing.


    /Rolands
     
    Roland Karlsson, Sep 4, 2004
    #2
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  3. Alfred Molon

    Alfred Molon Guest

    Where did you read this ?
    Oh well, just lock up the mirror.
     
    Alfred Molon, Sep 4, 2004
    #3
  4. Alfred Molon

    Chris Brown Guest

    It can do contrast detection AF. It can't do phase detection AF, which is
    what's used in DSLRs, and is generally superior.

    Furthermore, the sensor is behind the mirror and the shutter. Flipping the
    mirror up (and thus making the viewfinder go dark) to autofocus would not be
    workable.
     
    Chris Brown, Sep 5, 2004
    #4
  5. But the shutter and mirror are in the way. Since the shutter is used to
    determine exposure time, and also protects the CCD during lens changing,
    some more thought would be needed on how to reconcile these needs. The
    mirror is needed to give a the view through the reflex viewfinder. I
    have never yet seen an LCD finder on a compact camera which is in the
    same league as a good reflex optical viewfinder.

    I have thought for some time that it would be useful to have a "mirror
    up/shutter open" facility for direct focussing (through a TV output)
    when doing photomicrography or photomacrography. I'm not going to hold
    my breath though, this is probably a minority wish.
    The dpreview page seems to be simply reproducing the manufacturer's
    publicity release. It is very doubtful if any AF camera can AF and
    expose in 10ms; even the Canon EOS 1nRS (6ms lag time) only achieves
    this by pre-focusing in RS mode.
     
    David Littlewood, Sep 5, 2004
    #5
  6. Yes, partially; the current DSLRs are based on film bodies.
    Part of the cost would be having to use a completely different type of
    CCD. The high quality ones used in DSLRs don't support real-time
    readout, for viewing *or* for focusing. The ones that *do* support
    those features, aren't as good. I don't know if this is inherent in
    the technology (the extra pathways for real-time readout take up
    enough space to impact the sensor well size, or something) or just a
    question of what's available *today*, though.
     
    David Dyer-Bennet, Sep 5, 2004
    #6
  7. Alfred Molon

    MarkH Guest

    Where did you read that it is possible to focus a lens in less than 10ms?

    When a manufacturer claims a 10ms lag time we can be pretty sure that a
    figure that fast must be under optimal conditions. i.e. from a half press
    to a full press of the shutter, when the camera has already prefocused and
    set the exposure.

    I checked the press release on DPReview that you provided a link to, there
    is no mention of a 10ms focus time, I can’t think of any reason to believe
    that the camera’s focus time is included in the 10ms figure quoted. I am
    also doubtful that the lens could be focused that fast.
    The mirror is in the way, the shutter is in the way, the sensor does not
    generate a live preview.

    One of the advantages of a D-SLR is faster focusing, no one apart from you
    is keen on taking away the advantages and adding some disadvantages.
     
    MarkH, Sep 5, 2004
    #7
  8. Alfred Molon

    Guest Guest

    then how do you know what it is focusing on?
     
    Guest, Sep 5, 2004
    #8
  9. It's a shame when cousins marry.
     
    Randall Ainsworth, Sep 5, 2004
    #9
  10. Alfred Molon

    Jeremy Nixon Guest

    The key development would be to make a sensor that doesn't have to give
    anything up to do both live feed and imaging. But still, focus using
    the sensor would be limited to contrast detection, which won't be either
    as fast or as accurate -- and that's one reason why SLRs focus better
    and faster than digicams.
     
    Jeremy Nixon, Sep 5, 2004
    #10
  11. Alfred Molon

    Alfred Molon Guest

    The Olympus 8080 has a phase detection sensor on its body, thereby
    offering both autofocus mechanisms (contrast and phase).
     
    Alfred Molon, Sep 5, 2004
    #11
  12. Alfred Molon

    Alfred Molon Guest

    The compact Olympus 8080 has a phase detection sensor on its body.
     
    Alfred Molon, Sep 5, 2004
    #12
  13. Nowhere. I wrote probably, i.e I don't think it is
    possible. Show me the reference that it _can_ focus
    in 10 ms and I believe you.
    Then the view in the finder will diasapper and you
    have to use the LCD or en EVF for it to still be
    possible to see anything. Better with a real EVF then;
    then you will get rid of the mirror.

    BTW - the sensor is not made for continous view,
    so you have exchange that also. Currently, the
    best quality ones does not allow for continous
    view and therefore you have to decrease the picture
    quality.

    BTW - have you not already started this thread
    a multiple of times before? And got all those
    answers already?


    /Roland
     
    Roland Karlsson, Sep 5, 2004
    #13
  14. David Littlewood wrote:
    []
    Although I rejected the camera for other reasons, the Minolta A2's EVF
    with its VGA resolution display may be an indication of the way things are
    moving. It certainly wasn't as good as an optical, but it was much better
    than others I have seen. Now if it could get up to XGA standards.....

    David
     
    David J Taylor, Sep 5, 2004
    #14
  15. Hmmm ... maybe shall take a look at one. I have not seen an EVF that
    I liked yet. Would be interesting to see if this is good enough.

    But - even if you use an EVF you still have the problem with
    the inferiority of contrast detection vs phase detection. Maybe
    you shall have a combination of both - an external phase detector
    for speed and then contrast detection at the sensor for accuracy.


    /Roland
     
    Roland Karlsson, Sep 5, 2004
    #15
  16. Alfred Molon

    Chris Brown Guest

    OK, so you lock up the mirror, and put the shutter in bulb mode, and let a
    constant stream of dust fall on to the (charged) sensor, and maybe you can
    use it for contrast detection autofocus, but what's the point when it can't
    stream video? You wouldn't know what the camera is pointing at!
     
    Chris Brown, Sep 5, 2004
    #16
  17. Alfred Molon

    Chris Brown Guest

    And the 20D has 9 phase detection sensors, offering superb autofocus
    performance without the need to resort to contrast detetction AF, with its
    associated problems.

    Methinks you have a solution looking for a problem.
     
    Chris Brown, Sep 5, 2004
    #17
  18. Sorry; I should have said "DSLR", I manage OK with fixed-lens compacts
    but there are advantages in using a lens-removable SLR.

    David
     
    David Littlewood, Sep 5, 2004
    #18
  19. Alfred Molon

    Alfred Molon Guest

    Then the question would be why you need nine - perhaps one or two are
    not enough. Any BTW the current DSLR design makes the assumption that
    the CCD is a passive element like film, which can't be used for focusing
    or exposure setting. If the CCD can be used for these tasks too,
    performance can only improve.
    It appears that there are focusing problems with some DSLR-lens
    combinations:
    http://www.colorfoto.de/sixcms/media.php/258/0904Autofokus_cofo.pdf

    (very interesting article in German only however)

    Basically the German photography magazine Colorfoto ran a series of
    tests with Canon 10D and Nikon D70 digital cameras and several (six
    to be precise) lenses.

    It turned out that with some lenses the cameras didn't focus
    properly. There is a long explanation about this, but in essence the
    main problem is that some lenses don't communicate well with the body
    (especially older ones) and old lenses might be worn out and not
    capable to focus accurately.

    Tamron recommends to use its new line of lenses specifically designed
    for digital cameras (more accurate, precise focus, lower tolerances,
    better communication with the body). Nikon recommends to use its
    newer Nikkor lenses and admits that there may be problems with older
    Nikkor lenses or other third party lenses.

    The main problem is that the CCD is not used for focusing as is the
    case in compact digital cameras such as the 5050, 5060 or 8080 for
    instance. DSLRs have separate autofocus sensors, which while allowing
    faster focusing, can suffer from misalignment issues with the CCD or
    lens.
     
    Alfred Molon, Sep 5, 2004
    #19
  20. I already know the problem: photomicrography. It's a minority interest
    though.

    David
     
    David Littlewood, Sep 5, 2004
    #20
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