Shahpness of lens at infinity - need advice

Discussion in '35mm Cameras' started by Howie Axelrod, Aug 21, 2004.

  1. When shooting subjects at (beyond) infinity, my prints are not sharp.
    Will I gat a different result depending on F-stop?

    In other words, at or beyond infinity, will I get a sharper image at
    f16 than say f 2.8 (assuming acceptable shutter speed in both cases)?

    Thanks.
     
    Howie Axelrod, Aug 21, 2004
    #1
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  2. Well there is no beyond infinity. Infinity is it.

    You may see some lenses that will focus beyond infinity due to IR light
    focusing or because the focus stop is just set that way to account for
    inaccurate lens construction.

    We need to know why your photos are not sharp before we can suggest that
    a change in aperture change will help. Of course you could just try it and
    see.

    My guess is you are having a mechanical problem with the lens.
     
    Joseph Meehan, Aug 21, 2004
    #2
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  3. Howie Axelrod

    Pål Jensen Guest

    Huh? Focusing past infinity is not for compensating for inaccurate lens
    construction but to have the ability to correct thermal
    expansion/contraction. This is common on all long telephoto lenses but also
    on some modern wider lenses. Not only does lenses contract and expand but
    also camera bodies. I've tested that there are significant diffferences
    between a camera - wide angle combination at +20 C and - 5 C when the lens
    is used wide open at infinity.
     
    Pål Jensen, Aug 21, 2004
    #3
  4. Howie Axelrod

    Pickle Guest

    Although you can't get further away from a place than an infinite
    distance, you can focus beyond infinity, which simply means bringing
    the lens closer to the film plane than for focusing at infinity. Then
    you have focussed 'beyond infinity' and, at that wavelength, nothing
    can be focused. IR 'light' focusses longer than visible light, so
    needs to be focussed shorter than infinity; however, if you wanted to
    make a picture out of ultraviolet, you would indeed need a lens that
    could go a bit more than infinity. This is academic, because the
    glass in most lenses does not transmit much UV anyway. Closing the
    aperture will improve these matters in all cases - depth of field is
    not wavelength-dependent to any large degree.

    As Paal says, the reason you see it on some lenses is for thermal
    variations. On my large mirror lens (1100mm), the actual position of
    sharp focus for objects at infinity (e.g. the sun) can change over
    quite a range.
    Cheers,
    Mark.
     
    Pickle, Aug 22, 2004
    #4
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