Pentax 645 300mm ED(IF) lens, focus past infinity?

Discussion in 'Medium Format Cameras and Accessories' started by Bob-D659, Jun 7, 2012.

  1. Bob-D659

    Bob-D659 Member

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    Looking at one, but it goes past the infinity mark. This is the newer manual focus, internal focus model. Test negatives look very sharp on first inspection. Just wondering if this is a problem, or normal. From some googling, it seems some other Pentax manual focus lenses do go past infinity.
     
  2. John Koehrer

    John Koehrer Subscriber

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    I've seen a lot of longer lenses that go past the stop, not just Pentax.
    It's normally just three set screws around the barrel to loosen and set the focusing ring. If it's got a rubber grip they may be under it.
    How's it work at near focus?

    I remember being told this was normal to allow for expansion(of what?) at high temperatures.
     
  3. lxdude

    lxdude Member

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    Expansion of the lens barrel, which causes it to lengthen.
     
  4. Mackinaw

    Mackinaw Member

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    "ED" means it uses special "low-dispersion" optical glass that expands and contracts differently than ordinary optical glass. Manufacturers (many, not just Pentax) allow these type of lenses to focus beyond infinity to compensate for these different expansion/contraction rates.

    Jim B.