old brass barrel lens inscription a mystery

[guycam]

I recently purchased a Barrel lens and waterhouse stops to use on a speed graphic. It is about 180mm. It has the letters E.R.R. and 6 1/2 x 4 3/4 inscribed on it. I understand that the numbers may designate full frame but the E.R.R. stumps me. There is no other inscriptions anywhere. Could there be a meaning to E.R.R.? besides error? :confused:

[Paul Ewins]

Probably Extra Rapid Rectilinear. An RR is usually f8, so maybe f5.6 at a guess. 6 1/2 x 4 3/4 is half plate format, close enough to 5x7 for practical purposes.

[guycam]

Thanks, that would make perfect sense. Do you know where I could learn more about the half plate, full plate history?

[Steven Tribe]

ERR is one of standard abbreviations from the later Rapid Rectilinear era. Means usually somewhere around F6, instead of the F8. There are exceptions.

Full plate etc. and the other size systems (carte de visit etc.) are well covered in the Vade Mecum and other reference works on photographic history.