Focus beyond infinity and other things.

Discussion in 'Digital SLR' started by bob, Jan 26, 2008.

  1. bob

    bob Guest

    One thing I've noticed, is that I can focus some lenses beyond infinity, in
    other words, after focusing on some distant mountains, and even at the infinity
    marking on the lens, I can still turn the focus ring farther, blurring the
    image. Even the moon would be blurred...

    Is there a reason for this? I seem to remember my old cameras from the 70s
    stopped at infinity as if there was a calibrated stop, you couldn't go farther.
    That was handy because you just needed to twist the barrel to the end to shoot
    far away... no need to look for focus.


    Another thing, I used to zoom into a subject, focus, and then zoom back, knowing
    that my focus was dead on now... but these days, the lens goes out of focus when
    I zoom it back - it requires a new adjustment... Is this something new or is it
    just my lenses?

    thanks!
     
    bob, Jan 26, 2008
    #1
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  2. bob

    Charles Guest


    The story I remember about my Vivitar 600 for this aspect was the
    allowance was to compensate for temperature variations. I think it
    made it easier to manufacture, didn't need to hew so closely to the
    specs, looser tolerances.

    Now for a story about why my eyes are that way.
     
    Charles, Jan 26, 2008
    #2
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  3. bob

    RichA Guest

    To allow for heat-produced thermal expansion of the bodies, which are
    primarily aluminum and plastic.
     
    RichA, Jan 26, 2008
    #3
  4. bob

    BC Guest

    Not true. It is most often related to the change of refractive index
    with temperature (dn/dt) of certain glass elements within the lens.
    ED glass and fluorite are particularly prone to this. A little known
    fact is that you often want the lens mount to be made with high
    expansion materials to counteract the dn/dt effects of ED glass and
    fluorite. Plastics are sometimes used in the mount to achieve this
    compensation (aka passive athermalizartion) on some extremely high end
    optical systems. In less critical systems like photo lenses the usual
    solution is to provide some overtravel on the focus mechanism.
     
    BC, Jan 26, 2008
    #4
  5. bob

    jean Guest

    How else could they take pictures of Buzz Lightyear?
     
    jean, Jan 26, 2008
    #5
  6. bob

    Jeff Guest

    wrote in
    It's not something new. In ye olden times these were called varifocal
    lenses (could change focal length but not retain focus) as compared to true
    zoom lenses which maintain focus when zooming. The manufacturers expect
    the majority of users to let the autofocus adjust the lens before each shot
    whether or not the focal length changed so they don't build all lenses to
    maintain focus when zoomed. This also explains the extra focus ring
    movement because the ring position that focusses infinity at one focal
    length would be different than another focal length.
     
    Jeff, Jan 26, 2008
    #6
  7. It's varifocal rather than true zoom. Having auto-focus decreases the
    importance of maintaining focus, and varifocal is easier to design and
    probably cheaper to manufacture.

    David
     
    David J Taylor, Jan 26, 2008
    #7
  8. bob

    ben brugman Guest

    There are different reasons for this :
    1. Thermal (and moisture) expansion of the materials (or even the lenses).
    2. Af systems now can overshoot the focus and adjust for that.
    3. Tolerances do not have to be as strict if exact infinity focussing was
    done.

    Is this new ?
    1. Not totaly older manual lenses containing a fluorite glass also did not
    have a 'fixed' infinity.
    (Fluorite glass was supceptable to moisture differences).

    Other lenses normaly had a fixed infinity, with manual focusing this was
    convenient, you could set your lens to infinity without looking. So normaly
    the lens was did not go beyond infinity, but how exact this was I can not
    tell.

    ben
     
    ben brugman, Jan 26, 2008
    #8
  9. bob

    Paul Furman Guest

    It would be hard to deal with at wide angles but is manageable for long
    lenses. My old manual focus Tokina 300/2.8 over-focuses infinity but
    it's easy to see where you are focusing. It is tedious getting the moon
    in focus though!
     
    Paul Furman, Jan 26, 2008
    #9
  10. bob

    bob Guest

    On Fri, 25 Jan 2008 22:00:49 -0500, wrote:

    Thanks again for clearing that up!!
     
    bob, Jan 27, 2008
    #10
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