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

Bob-D659 · Jun 7, 2012

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.

John Koehrer · Jun 7, 2012

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.

lxdude · Jun 7, 2012

John Koehrer said:
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.

Mackinaw · Jun 9, 2012

"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.

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

Bob-D659

John Koehrer

lxdude

Mackinaw

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