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  1. #1

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

    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.
    Bob

  2. #2

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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.
    Heavily sedated for your protection.

  3. #3
    lxdude's Avatar
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    Quote Originally Posted by John Koehrer View Post

    I remember being told this was normal to allow for expansion(of what?) at high temperatures.
    Expansion of the lens barrel, which causes it to lengthen.
    I do use a digital device in my photographic pursuits when necessary.
    When someone rags on me for using film, I use a middle digit, upraised.

  4. #4

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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.



 

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