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Greg
19-Feb-2019, 17:45
How does one determine if a lens is a Dagor formula optic? Have searched this out and have found many mixed results. A simple clarification most grateful to have to use.

Drew Wiley
19-Feb-2019, 17:53
A dagor is a symmetrical pair of cemented triplets. With only four air-glass interfaces (one one each side of each respective triplet), internal reflections can sometimes be hard to detect. But a tessar has six interfaces instead, and plasmats, many. But with Dagors, the two symmetrical sides often screw apart from each side of the shutter, so it's quite obvious they're matched, unless you have something old in barrel only. The elements also tend to be thick, so a lens can be a bit heavy even relative to its size (but so can tessars).

Dan Fromm
19-Feb-2019, 18:26
Short answer, as Drew hinted but didn't spell out, a single dagor type cell will have two strong reflections from the two air-glass interfaces and two weak reflections from the glass-cement-glass interfaces. As he said, the weak reflections can be hard to see.

Are you asking a general question or about a particular lens?

Mark Sawyer
19-Feb-2019, 23:52
"Re: How to recognize a Dagor lens by the reflections"

If you hold it up to a mirror and the rim says "ROGAD" with the letters backwards, it's a Dagor...