How to tell if autofocus is spot-on?

Discussion in 'Digital Photography' started by SS, Mar 5, 2006.

  1. SS

    SS Guest

    I would like to know if there is some way I can check my camera is finding a
    pin sharp focus on autofocus but here is the problem: I am looking at
    distant tree branches on my photos, however as I zoom in on PSP details
    starts to go and they look fuzzy - BUT - shortly followed by obvious
    pixellation i.e. I am getting to the limit of the sensor (7MPX) which of
    course causes detail to go, even if focus is spot on. Its a Canon A620 which
    gets excellent reports and has a manual focus system if required but I find
    it very hard to use as you tend to 'overshoot' using the focus buttons so
    never sure if its spot on (is it best to zoom in then focus then zoom out
    for the shot?).
     
    SS, Mar 5, 2006
    #1
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  2. "SS" <> writes:

    > I would like to know if there is some way I can check my camera is finding a
    > pin sharp focus on autofocus but here is the problem: I am looking at
    > distant tree branches on my photos, however as I zoom in on PSP details
    > starts to go and they look fuzzy - BUT - shortly followed by obvious
    > pixellation i.e. I am getting to the limit of the sensor (7MPX) which of
    > course causes detail to go, even if focus is spot on. Its a Canon A620 which


    Make a few test shots of a small object grass or some other surface
    with small details. It should be easy to see if the focus plane lines
    up with the intended target.

    > (is it best to zoom in then focus then zoom out for the shot?).


    No. Zooming often changes focus.

    --
    Måns Rullgård
     
    =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?=, Mar 5, 2006
    #2
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  3. SS

    bmoag Guest

    It isn't much better using a digital SLR which has a dim, non-pentaprism
    viewing system, looking through an F4 (or smaller) lens in less than optimal
    light through a focusing screen that has no critical focus adjustment
    features, e.g. split image.
    This is a real problem with all dSLRs except the very top end. Even then the
    viewing systems and viewing screens are not optimal.
    This is why some sort of heads up, projectied live EVF view of the scene may
    (emphasis on may) be helpful. Olympus has announced such a system and I
    believe it will become mainstream.
    If you can put your camera on a tripod you actually have to bracket focus
    under these circumstances and use as small an f-stop as practical for at
    least one of your shots.
     
    bmoag, Mar 5, 2006
    #3
  4. SS

    Prometheus Guest

    In article <6ADOf.18810$>, bmoag
    <> writes
    >It isn't much better using a digital SLR which has a dim, non-pentaprism
    >viewing system, looking through an F4 (or smaller) lens in less than optimal
    >light through a focusing screen that has no critical focus adjustment
    >features, e.g. split image.
    >This is a real problem with all dSLRs except the very top end. Even then the
    >viewing systems and viewing screens are not optimal.
    >This is why some sort of heads up, projectied live EVF view of the scene may
    >(emphasis on may) be helpful. Olympus has announced such a system and I
    >believe it will become mainstream.


    So a low resolution and slow responding power consuming 'viewfinder'
    will be better than the high resolution and fast responding live
    pre-view on my SLR!

    --
    Ian G8ILZ
     
    Prometheus, Mar 5, 2006
    #4
  5. SS

    Jim Townsend Guest

    SS wrote:

    > I would like to know if there is some way I can check my camera is finding a
    > pin sharp focus on autofocus but here is the problem: I am looking at
    > distant tree branches on my photos, however as I zoom in on PSP details
    > starts to go and they look fuzzy - BUT - shortly followed by obvious
    > pixellation i.e. I am getting to the limit of the sensor (7MPX) which of
    > course causes detail to go, even if focus is spot on. Its a Canon A620 which
    > gets excellent reports and has a manual focus system if required but I find
    > it very hard to use as you tend to 'overshoot' using the focus buttons so
    > never sure if its spot on (is it best to zoom in then focus then zoom out
    > for the shot?).


    Here's a web site dedicated to checking the focus of cameras.

    http://www.photo.net/learn/focustest/

    One thing to consider when 'zooming in' on your images and viewing them on
    your monitor. When zoomed to 100%, you're looking at an image that was
    created by a sensor the size of your little fingernail and then enlarged to
    approximately 3 Feet X 2 Feet. At 200%, it's 6 feet X 4 feet.

    A lot of the 'blur' you see is caused by the fact the lens isn't absolutely
    and totally perfect. Such a lens doesn't exist.
     
    Jim Townsend, Mar 5, 2006
    #5
  6. SS

    Tesco News Guest

    "SS" <> wrote in message
    news:jdCOf.73213$...
    >I would like to know if there is some way I can check my camera is finding
    >a
    > pin sharp focus on autofocus but here is the problem: I am looking at
    > distant tree branches on my photos, however as I zoom in on PSP details
    > starts to go and they look fuzzy - BUT - shortly followed by obvious
    > pixellation i.e. I am getting to the limit of the sensor (7MPX) which of
    > course causes detail to go, even if focus is spot on. Its a Canon A620
    > which
    > gets excellent reports and has a manual focus system if required but I
    > find
    > it very hard to use as you tend to 'overshoot' using the focus buttons so
    > never sure if its spot on (is it best to zoom in then focus then zoom out
    > for the shot?).
    >
    >

    Hi.

    A terrific amount of time is wasted by people worrying about the Resolution
    of their lenses, and the accuracy of the Auto Focus system.

    If your Lens or Camera has a fault, you will soon be aware of it, unless of
    course you can't tell good quality results from bad.

    If you really must test your AF, get yourself outside in nice bright light,
    set a high shutter speed so that your aperture is at is maximum, and take
    photos of a wooden fence or brick wall, from an angle. Stick something to
    the wall or fence for you to focus on. When you examine the results the
    wall or fence nearest to you should be unsharp, and gradually get sharper,
    and then start to become unsharp again. The sharpest point should be at
    your marker.

    Roy G
     
    Tesco News, Mar 5, 2006
    #6
  7. SS

    Rich Guest

    Rich, Mar 6, 2006
    #7
  8. SS

    Ron Hunter Guest

    SS wrote:
    > I would like to know if there is some way I can check my camera is finding a
    > pin sharp focus on autofocus but here is the problem: I am looking at
    > distant tree branches on my photos, however as I zoom in on PSP details
    > starts to go and they look fuzzy - BUT - shortly followed by obvious
    > pixellation i.e. I am getting to the limit of the sensor (7MPX) which of
    > course causes detail to go, even if focus is spot on. Its a Canon A620 which
    > gets excellent reports and has a manual focus system if required but I find
    > it very hard to use as you tend to 'overshoot' using the focus buttons so
    > never sure if its spot on (is it best to zoom in then focus then zoom out
    > for the shot?).
    >
    >

    It is much more likely you are at the resolution limit of your
    lens/sensor set.
     
    Ron Hunter, Mar 6, 2006
    #8
  9. In article <%gKOf.76978$>, Tesco News
    <> writes
    >
    >A terrific amount of time is wasted by people worrying about the Resolution
    >of their lenses, and the accuracy of the Auto Focus system.
    >

    It will all
    become irrelevant soon anyway. ;-)
    http://www.photonics.com/spectra/tech/XQ/ASP/techid.1749/QX/read.htm
    --
    Kennedy
    Yes, Socrates himself is particularly missed;
    A lovely little thinker, but a bugger when he's pissed.
    Python Philosophers (replace 'nospam' with 'kennedym' when replying)
     
    Kennedy McEwen, Mar 6, 2006
    #9
  10. SS

    Tesco News Guest

    "Rich" <> wrote in message
    news:...
    > Buy this:
    >
    > http://www.dslrfocus.com/


    Hi.

    I like that.

    It should keep all the worriers very busy for a very long time.

    Roy G
     
    Tesco News, Mar 7, 2006
    #10
  11. SS

    Colin D Guest

    Tesco News wrote:
    >
    > "SS" <> wrote in message
    > news:jdCOf.73213$...
    > >I would like to know if there is some way I can check my camera is finding
    > >a
    > > pin sharp focus on autofocus but here is the problem: I am looking at
    > > distant tree branches on my photos, however as I zoom in on PSP details
    > > starts to go and they look fuzzy - BUT - shortly followed by obvious
    > > pixellation i.e. I am getting to the limit of the sensor (7MPX) which of
    > > course causes detail to go, even if focus is spot on. Its a Canon A620
    > > which
    > > gets excellent reports and has a manual focus system if required but I
    > > find
    > > it very hard to use as you tend to 'overshoot' using the focus buttons so
    > > never sure if its spot on (is it best to zoom in then focus then zoom out
    > > for the shot?).
    > >
    > >

    > Hi.
    >
    > A terrific amount of time is wasted by people worrying about the Resolution
    > of their lenses, and the accuracy of the Auto Focus system.
    >
    > If your Lens or Camera has a fault, you will soon be aware of it, unless of
    > course you can't tell good quality results from bad.
    >
    > If you really must test your AF, get yourself outside in nice bright light,
    > set a high shutter speed so that your aperture is at is maximum, and take
    > photos of a wooden fence or brick wall, from an angle. Stick something to
    > the wall or fence for you to focus on. When you examine the results the
    > wall or fence nearest to you should be unsharp, and gradually get sharper,
    > and then start to become unsharp again. The sharpest point should be at
    > your marker.
    >
    > Roy G



    Er, that's not such a good idea. The focus sensor(s) covers
    considerably more than the little square in the VF, and when presented
    with an oblique target, it cannot differentiate between the nearest and
    farthest points covered by the focus sensor area, so the focus can
    settle anywhere in the sensor area, and usually at the point of highest
    contrast, which might not be where you intended.

    Colin D.
     
    Colin D, Mar 7, 2006
    #11
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