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John Kasaian
16-Sep-2003, 17:30
I'm curious as to the differences between the f6.8 and f7.7 dagors. The f7.7 versions seem to be in longer focal lengths, often are said to have greater coverage, and are usually more costly---but why? Is the difference in the design of the f7.7? Is it a justifiably more desireable version of this legendary lens? I asked this question on that other forum but I thought the more savvy optical gurus over here might provide a more detailed answer. Thanks for any and all info.-------cheers!

sanking
16-Sep-2003, 18:25
There is no difference in design between the f/6.8 and f/7.7 Dagors. Basically Dagors of 12" and less are f/6.8, 14" and longer Dagors are f/7.7. The slower speed of the longer Dagors apparently had as its purpose keeping the weight down.

This answer assumes that we are comparing Dagors of the same period and manufacturer. The fact is that Dagors were made for almost 100 years and distributed under the name of many lens manufacturers (Goerz, Zeiss, Ross, Schneider, Goerz America, etc.) and there are definitely some minor design differences.

Ernest Purdum
16-Sep-2003, 19:36
Sandy King gave you an excellent answer. All I would add is that shutter size and availability is a major factor in deciding a maximum aperture. This is even more a factor now than when the Dagor's were being made.

Richard Knoppow
17-Sep-2003, 03:40
Another reason for the slower speed is the size of the chunks of glass needed to make the lens. The method of making optical glass at the time Dagors was to melt the glass in a pot and break it up after it had annealed. Pieces large enough for very large lenses were rare. Nonetheless, some f/4.5 lenses were made in quite long focal lengths. Even a 12" f/4.5 Tessar is an impressive chunk of glass and some lenses were available in considerably longer FLs