auto focus in digital cameras any different that auto focus in film cameras?

Discussion in 'Digital Photography' started by Guest, Nov 3, 2005.

  1. Guest

    Guest Guest

    Is there any basic difference in the autofocussing technology in
    digital cameras vs film cameras? I have an Olympus 750 and it seems
    like it takes longer to achieve autofocus than its cheaper brother a
    stylus point and zoom shoot film camera.. Is the digital camera just
    trying to do a better job, does the autofocus have to work harder in a
    digital camera or is the difference that I perceive not really there?

    Especially in low light this problem is aggravating and its not helped
    by the fact that the manual focus mode is not very easy to use. Boy
    I'd love to be able to grab a ring and turn it to get focus. As it is
    its a lot of button and menu button selection. I guess that's the
    price you pay for a low end digital camera.

    tks
    Don
     
    Guest, Nov 3, 2005
    #1
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  2. Google helped me find this:

    Point-and-shoot cameras can be slow because they use the contrast
    detection method:
    http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/5700/AUTO-FOCUS/Auto-Focus.html

    Some cameras (e.g. Nikon 8400) have an additional sensor to speed things
    up.

    DSLRs use the phase detection method which is quicker:
    http://www.a1.nl/phomepag/markerink/af_expla.htm


    http://en.wikipedia.org/wiki/Autofocus

    David
     
    David J Taylor, Nov 3, 2005
    #2
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  3. Digital SLRs use the same autofocus as their film counterparts. Digital P&S
    cameras use an abomination known as Contrast Detect. It is universally
    known for being crappy and slow, especially in low light. I can't speak for
    your particular camera, but there are some tricks that will sometimes help
    Contrast Detect cameras find focus. Most importantly, attempt to focus on
    something that has high contrast. Human faces have low contrast, and that
    is what we largely focus on. Try clothing instead, although make sure you
    find something the same distance away. Also, some cameras use an algorithm
    that is tied to contrast in a verticle direction. For some circumstances,
    rotating the camera 90 degrees will allow it to work.

    Cheers!
    Andrew
     
    Andrew Crabtree, Nov 3, 2005
    #3
  4. Guest

    Malcolm Guest

    Especially in low light this problem is aggravating and its not helped

    I too have a C750. In low light, I switch to manual focus, estimate the
    distance and set to that distance. Because of the great depth of field of
    these cameras this method works very well. I don't try and focus by looking
    at the screen. The resolution is just too low.

    Malcolm
     
    Malcolm, Nov 3, 2005
    #4
  5. Guest

    Guest Guest

    tks everyone.
    I'll just have to set a "program" in for manual focus and use that and
    I agree Malcolm looking at the viewfinder/screen does not help.

    I wonder if the film Olympus PS Stylus uses something different. It
    doesn't seem as susceptible to the problem.

    Maybe it doesnt' try/work as hard.

    Don
     
    Guest, Nov 4, 2005
    #5
  6. Most P&S digital cameras use contrast measuring methods using images
    read from the main sensor, because it's cheap (no extra hardware).

    Film P&S cameras don't have this hardware, so have to use some
    additional hardware for autofocus, and typically choose active infrared
    rangefinding. It's fast and works in the dark. A few digital P&S
    cameras use this too.

    Both film and digital SLRs tend to use phase detection, which is fast
    and works for a wide variety of lenses.

    Dave
     
    Dave Martindale, Nov 5, 2005
    #6
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