Curved Focus?

Discussion in 'Digital SLR' started by Wilba, Mar 11, 2010.

  1. Wilba

    Wilba Guest

    What's the technical term for the kind of lens we use all the time, that
    projects a plane of focus onto an image plane?

    Is there such as thing as a lens type that has, instead of a plane, a sphere
    or ellipsoid of focus?

    Got a link to a technical glossary of lens design terms? :- )
     
    Wilba, Mar 11, 2010
    #1
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  2. Wilba

    Wilba Guest

    When I looked up rectilinear what I found only talked about straight lines
    in the plane of focus being straight on the image plane, not about curvature
    of the field of focus - it means that as well?
    That makes sense, thanks.
    He let me down. :- )
     
    Wilba, Mar 11, 2010
    #2
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  3. Wilba

    stephe_k Guest


    I thought it was "flat field"?

    Stephanie
     
    stephe_k, Mar 11, 2010
    #3
  4. Wilba

    Paul Furman Guest

    The opposite, would be "curvature of field".
     
    Paul Furman, Mar 11, 2010
    #4
  5. "Flat field" it is, and the opposite is "curved field" (of focus...).
    "Rectilinear", "spherical", etc. have to do with lens perspective
    types (see for more --
    www.donferrario.com/ruether/lens_perspective_types.htm
    www.donferrario.com/ruether/seeing_and_perspective.htm
    http://www.donferrario.com/ruether/lens-angle-of-view-and-perspective.htm
    www.donferrario.com/ruether/lens_distortion_types.htm,
    although my server is VERY slow this morning...).
    --DR
     
    David Ruether, Mar 11, 2010
    #5
  6. Yes, they can...;-)
    But, sometimes it gets more complicated, with reasonable field
    flatness being achieved only within a certain focus range unless
    the lens design includes lens physical variations that change with
    focus ("CRC", some internal focus designs, etc.), or there is a
    "happy" zoom control position that results in a compromise at
    a certain FL that results in flat field focus. Most people don't
    notice field curvature unless it is especially bad (it isn't in most
    good lenses at the most commonly used focus distances), or they
    know what to look for...;-) Also, the use of medium to small
    stops generally covers even the worst field curvature.
    --DR
     
    David Ruether, Mar 11, 2010
    #6
  7. Wilba

    Wilba Guest

    That's exactly what I wanted. Thanks a bunch.
     
    Wilba, Mar 11, 2010
    #7
  8. Wilba

    Me Guest

    It's a problem when amateur (and pro) "lens testers" shoot brick walls
    and resolution charts, particularly with wide lenses at close range.
    Some people on DPReview got all wound up about field curvature with the
    Nikkor 24-70. Perhaps there are some unique "real" circumstances where
    it matters - apart from brick walls.
    IIRC DPReview, when testing some wide lenses (at close range - so that
    the chart filled the frame) "averaged" focus setting to "allow for"
    field curvature, thus reporting relatively poor performance overall.
    Crazy stuff.
     
    Me, Mar 11, 2010
    #8
  9. And some cameras, such as the best of the tiny format Minolta 16s,
    used curved film planes in the camera and the matching enlarger.
     
    Chris Malcolm, Mar 12, 2010
    #9
  10. It's an advantage having a curved field of focus in a wide angle lens
    used for shooting interiors.
     
    Chris Malcolm, Mar 12, 2010
    #10
  11. ???? ;-)
    It may be possible to match a camera lens fairly well to non-flat film,
    but the enlarger would be an independent issue (the film image is
    sharp, or not, but no compensation for lack of film flatness in the
    camera could be made in the enlarger). It would be possible for the
    enlarger's lens to likewise fairly well match a non-flat film positioning
    in its film gate, but one wonders why one would bother, since focus
    with both the camera and the enlarger would be at best an approximating
    compromise. Price and simplicity of construction may be the answer
    (use poor lenses, then try to approximate their non-flat focus fields
    with non-flat film positioning - but the lens errors would likely be circular,
    and those of the film positioning would likely be cylindrical...).
    --DR
     
    David Ruether, Mar 12, 2010
    #11
  12. Yes, it is. ;-) In my "Subjective Lens Evaluations (Mostly Nikkors)", at
    www.donferrario.com/ruether/slemn.html, I use infinity targets to avoid
    parallelism errors, but then check lenses also near their closest focus to
    see if they change much (they often get much worse toward the corners).
    Most *good* lenses are fairly good to the corners wide open (by my
    definition for "good"...;-), but since close focus far less often requires
    high standards, unless it is poor, I don't mention it in the evaluations.
    --DR
     
    David Ruether, Mar 12, 2010
    #12
  13. Yes...8^) But it is best to know your lenses, and what they can, and
    cannot, do...
    It can be, in specific instances, but I prefer flat-field since, with few
    exceptions, interior focus can be covered easily with DOF with WAs,
    and flat-field limits me less with exteriors, landscapes, etc.
    --DR
     
    David Ruether, Mar 12, 2010
    #13
  14. Wilba

    Paul Furman Guest

    Maybe mixed up with the panorama cameras, kind of like anamorphic
    compressed wide angle movie lenses???
     
    Paul Furman, Mar 12, 2010
    #14
  15. Wilba

    Eric Stevens Guest

    The old so-called Schmidt cameras made by Fairchild used for aerial
    mapping had a high quality lens but it was still necessary to apply a
    vacuum behind the film to suck it onto a template which matched the
    actual shape of the field.

    See http://www.aerialphotolab.com/apl_museum.htm



    Eric Stevens
     
    Eric Stevens, Mar 15, 2010
    #15
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