is there something wrong with my lens???

Discussion in 'Digital Cameras' started by Mark N, Sep 1, 2003.

  1. Mark N

    Mark N Guest

    Hi, i just got a Sigma 20mm f1.8 lens for my 10D and am very happy with it
    so far (about 100 shots)

    The only thing is that on some pictures there is a very noticeable blur on
    the right hand side of the images produced

    I took a test picture of my computer screen

    http://www.marknowak.co.uk/images/3806.JPG (caution 2.9Mb unedited straight
    from the camera)
    or
    http://www.marknowak.co.uk/images/3806-smaller.jpg (420Kb 20% jpeg image of
    above)

    In this picture the left hand side is in focus as is the center. However,
    the right hand side is very out of focus in the last quarter of the image.

    Is there a problem with this lens or are sigma wide angle lenses a 'bit
    dodgy'.

    I have noticed this on both closeups and long distance shots


    Thanks for any assistance, Mark
     
    Mark N, Sep 1, 2003
    #1
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  2. Mark> Hi, i just got a Sigma 20mm f1.8 lens for my 10D and am very
    Mark> happy with it so far (about 100 shots)

    Mark> The only thing is that on some pictures there is a very
    Mark> noticeable blur on the right hand side of the images produced

    Mark> I took a test picture of my computer screen

    Mark> http://www.marknowak.co.uk/images/3806.JPG (caution 2.9Mb
    Mark> unedited straight from the camera) or
    Mark> http://www.marknowak.co.uk/images/3806-smaller.jpg (420Kb 20%
    Mark> jpeg image of above)

    Mark> In this picture the left hand side is in focus as is the
    Mark> center. However, the right hand side is very out of focus in the
    Mark> last quarter of the image.

    It should certainly be possible to get pictures that are sharply in focus
    across the frame. However, I suspect that a computer screen is not the
    best possible test target.

    Let me suggest the following:

    1) Put the camera on a tripod.

    2) Set it to aperture-priority mode.

    3) Select f/8.

    4) Manually focus the lens on infinity.

    5) Go outdoors.

    6) Point the camera at something far away with lots of fine
    detail. A stand of trees would work just fine.

    7) Take some pictures.

    8) Examine them closely.

    If you cannot get sharp pictures of distant objects at f/8 with the
    camera on a tripod and the lens focused on infinity, something is
    broken. If you can, your next task is to try changing things until
    the pictures are no longer uniformly sharp. At that point, you will
    know a lot more about the problem. If you come back and post new
    images, along with test conditions, I'm sure someone will be able to
    help you figure out your problem.
     
    Andrew Koenig, Sep 1, 2003
    #2
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  3. Mark N

    Stuart Guest

    Firstly - on Sigma lenses - I have a 15-30mm EX wide angle zoom as well as
    a 30-70 EX zoom and a 70 - 200 zoom, all of which I think are more than
    satisfactory. (Hope this thread doesnt turn into a long diatribe about Sigma
    lenses - there are a few usual suspects out there that will try to make it
    thus)

    Second - the 20mm lens behaves like a 32mm lens in 35mm SLR terms because of
    the sensor is smaller than 35mm film.

    Now - your picture - you dont say what your aperture setting is and what
    distance the subject is from the focal plane - the RHS is definitely out of
    focus but this could be just down to the depth of field for that distance
    and angle - particularly if shot at f1.8. Try a little bit of geometry -
    pythagoras and all that - to validate !

    If I assume that distance from the page photographed is say 10" and the page
    width is 8" and the clearest focus is 1/3 from the LH edge then the distance
    to the RH edge is about 11.5". Is this within the depth of field at f/1.8 ??

    Stuart
     
    Stuart, Sep 1, 2003
    #3
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