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Jim23 · Posted: Thu Dec 14, 2006 3:49 am Post subject:

One of my 45 Pacemaker Crown Graphics came with a strange "inverse closeup" lens (series 6 w/39.5 adapter for the 135 Xenar) that actually throws the focus further out. What was this used for?

t.r.sanford · Posted: Thu Dec 14, 2006 4:33 pm Post subject:

Those "minus" supplementary lenses were used as a low-cost alternative to buying a longer-focus prime lens. Kodak called them "Telek" lenses, and they typically came in four focal lengths, -1, -2, -3 and -4 diopters.

The diopter measure is the inverse of the focal length in meters; opticians use it. -1 diopter=1000mm.; -2=-500mm.; -3=-333mm.;
-4=-250mm.

You calculate the effective focal length of the prime lens and the minus lens by summing the diopter focal lengths. This ignores the separation between the two components, but the result is "close enough for government work."

There has been a great deal of informative discussion about "minus" lenses on this board, and you may find a search to be very informative.