"Video AF" in digital cameras?

Discussion in 'Digital Photography' started by Nostrobino, Aug 8, 2004.

  1. Nostrobino

    Nostrobino Guest

    All of my Minolta digital cameras use "video AF" according to the specs
    given in the manuals. How is this different in principle from autofocus
    methods used in 35mm cameras?

    I'm asking because I've had a lot of unexplainable autofocus misses with
    these digital cameras, where such misses were never (or very, very rarely)
    experienced with 35mm AF cameras, either SLR or point-and-shoot. The misses
    appear to be related to the use of wide-area AF, and seem (so far, fingers
    crossed) to disappear when spot AF is used instead. But wide-area AF is the
    default in these cameras.

    Indeed, the very popular little X-series Minoltas have no provision for
    switching to spot AF. Fortunately, they don't seem to need it either--I've
    had no AF misses with my Xt and Xg cameras that I can recall.

    It does seem to vary according to camera model. The S404 and S414 are
    somewhat notorious for AF misses according to other users (and I have one of
    each and they both sometimes do it), though as mentioned, switching to spot
    AF apparently fixes this. At least one user has complained about AF misses
    with the 7i, but mine has given no problem with this even in wide-area AF.
    My F300 in wide-area AF mode in one case gave a bizarre result: a vase way
    over to the side, well outside the indicated AF zone, was in perfect
    focus--while the girl across the table from me, in the center of the
    picture, was out of focus.

    I have seen similar complaints about high-end Olympus cameras (like the
    C-5060) which have wide-area AF, so I presume this is not a rare phenomenon
    and occurs across many if not all makes.
     
    Nostrobino, Aug 8, 2004
    #1
    1. Advertising

  2. "Nostrobino" <> wrote in news:wquRc.894$ZC7.551
    @newssvr19.news.prodigy.com:

    > All of my Minolta digital cameras use "video AF" according to the specs
    > given in the manuals. How is this different in principle from autofocus
    > methods used in 35mm cameras?


    I assume that Minolta means that the video output from the CCD
    is used for auto focus. That is the most common auto focus method
    for non SLR digital cameras, i.e. cameras with live preview.

    > I'm asking because I've had a lot of unexplainable autofocus misses


    Yepp - that is a problem.

    Cameras without live preview, e.g. film based ones, must have a
    mechnism that is specially built for auto focus. This mechanism is,
    depending on price range for the camera, very cleverly built. It is
    often fast and accurate.

    Now - for cameras with live preview you have another option. You can
    use the data from the live preview for meassuring in the auto focus
    system. This has some advantages.

    1. It is (almost) for free, i.e. you save money.
    2. You focus exactly on the sensor, i.e. no need for
    expensive aligment between the auto focus sensor and
    the picture sensor. The camera becomes more robust.
    If you drop it, it will probably still focus correctly.
    3. You don't obscure the optical path to the sensor.

    But it also has some disadvantages.

    1. It is slow. Specially built small sensors are much faster.
    2. It is too simple. Specially built auto focus sensors can
    be made to detect phase differences, by using some prisms
    or sensors in several planes etc.

    The result of both the speed issue and the too simple construction
    is that it is generally less precise. It needs rather much light
    (as the sensor gets slower when it is darker) and also a rather
    good pattern with high contrast, often also aligned along some
    axis, e.g. vertical.


    /Roland
     
    Roland Karlsson, Aug 8, 2004
    #2
    1. Advertising

  3. Nostrobino

    Nostrobino Guest

    "Roland Karlsson" <> wrote in message
    news:Xns953FEFD938388klotjohan@130.133.1.4...
    > "Nostrobino" <> wrote in news:wquRc.894$ZC7.551
    > @newssvr19.news.prodigy.com:
    >
    > > All of my Minolta digital cameras use "video AF" according to the specs
    > > given in the manuals. How is this different in principle from autofocus
    > > methods used in 35mm cameras?

    >
    > I assume that Minolta means that the video output from the CCD
    > is used for auto focus. That is the most common auto focus method
    > for non SLR digital cameras, i.e. cameras with live preview.
    >
    > > I'm asking because I've had a lot of unexplainable autofocus misses

    >
    > Yepp - that is a problem.
    >
    > Cameras without live preview, e.g. film based ones, must have a
    > mechnism that is specially built for auto focus. This mechanism is,
    > depending on price range for the camera, very cleverly built. It is
    > often fast and accurate.
    >
    > Now - for cameras with live preview you have another option. You can
    > use the data from the live preview for meassuring in the auto focus
    > system. This has some advantages.
    >
    > 1. It is (almost) for free, i.e. you save money.
    > 2. You focus exactly on the sensor, i.e. no need for
    > expensive aligment between the auto focus sensor and
    > the picture sensor. The camera becomes more robust.
    > If you drop it, it will probably still focus correctly.
    > 3. You don't obscure the optical path to the sensor.
    >
    > But it also has some disadvantages.
    >
    > 1. It is slow. Specially built small sensors are much faster.
    > 2. It is too simple. Specially built auto focus sensors can
    > be made to detect phase differences, by using some prisms
    > or sensors in several planes etc.
    >
    > The result of both the speed issue and the too simple construction
    > is that it is generally less precise. It needs rather much light
    > (as the sensor gets slower when it is darker) and also a rather
    > good pattern with high contrast, often also aligned along some
    > axis, e.g. vertical.


    Yes; I've noticed that when I try to test whether the camera is focusing
    properly or not--by shooting objects with clear, crisp areas of contrast,
    lettering etc., so that I can judge the sharpness after taking the shot--the
    AF usually gives perfect results. But the same camera often fails to give
    sharp focus on people pictures. The S404 gave me a lot of mildly
    out-of-focus shots in bright sunlight, always with people in the middle
    distance as the subjects. Shots of houses, boats etc. were perfectly sharp.
    Maybe such subjects have more edge acutance than people and are easier for
    video AF to focus on for that reason?

    What is surprising in very, very low light is that the camera often SIGNALS
    that it has established focus, even though on the LCD monitor it is plainly
    way out of focus. That seems strange.

    Thanks for the info.

    N.
     
    Nostrobino, Aug 9, 2004
    #3
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. zxcvar
    Replies:
    12
    Views:
    922
    (Pete Cresswell)
    Jan 4, 2004
  2. Steven C \(Doktersteve\)

    Which is better? digital cameras or older crappy cameras that use film?

    Steven C \(Doktersteve\), Jan 23, 2004, in forum: Digital Photography
    Replies:
    22
    Views:
    961
  3. Dan Jacobson

    turning traditional cameras into digital cameras

    Dan Jacobson, Oct 30, 2004, in forum: Digital Photography
    Replies:
    9
    Views:
    366
    Phil Wheeler
    Oct 31, 2004
  4. wagwheel

    Cameras--Cameras--Cameras

    wagwheel, Mar 31, 2007, in forum: Digital Photography
    Replies:
    4
    Views:
    459
    Mark B.
    Apr 1, 2007
  5. wagwheel

    Cameras--Cameras--Cameras

    wagwheel, Apr 1, 2007, in forum: Digital Photography
    Replies:
    1
    Views:
    440
    Ken Lucke
    Apr 1, 2007
Loading...

Share This Page