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11-08-2011, 10:10 PM   #1
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extension tubes, min focus distance & infinity

If extension tubes decrease the minimum focus distance of a lens do the tubes prevent it from focusing to infinity?

11-08-2011, 10:18 PM   #2
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Yes.

The only way to increase magnification without losing infinity focus is to use a teleconverter.
11-08-2011, 11:30 PM   #3
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If you put a standard camera lens on extension (tubes and/or bellows) then yes, you lose infinity focus. Camera lenses are built to reach infinity focus at their minimum distance from the frame (film or sensor) where the lens' optical center is at the focal length distance. Pushing that optical center further away reduces the focus distance. No standard camera lens on extension can reach infinity focus.

The story is a little different with enlarger lense (ELs) and projector-copy-process-etc lenses, which lack a focusing mechanism. Those can be mounted on tubes or bellows or focusing helicoids selected for whatever focus range is desired. I generally find that such lenses longer than 80mm can reach infinity focus on my PK or M42 bellows. (I have a couple odd 60's that work too.) I need to put about 60mm of tubes behind a 162mm EL before it reaches infinity focus on bellows, which extended can focus very close.

A note on focus distance: A non-reversed lens cannot focus closer than its focal length, which is also where it reaches maximum magnification. No matter how much extension you put behind a 50mm lens, it can't focus closer than 50mm from its optical center. Reversed lenses are another matter, but we won't worry about that now.
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