Divergent lensspeed engraving from specification, how common ?

**Richard Årlin** · 27-Sep-2004, 06:28

I have not been able to resist buying some of the tiny little gems named Cooke wide angle anastigmats series VIIb, a 82mm in brl, a 133mm in brl. and a 158mm in a syncro compur. They are all specified as f6,5. Strangely the 133mm # 579604 is engraved f6,3. It is confirmed by Cooke that this is the case and the lens is entered in the books as a f6,5. There are at least two different possibilities. Either it was wrongly engraved or it was measured to be faster and the worker took the trouble to change the tools he used without bothering to write it down in the book. Now if either has happened once it may happen again and there could be other lenses out there with a faulty engraving. Just curious,

Richard

**Ernest Purdum** · 27-Sep-2004, 07:01

I haven't noticed any f stop engraving errors, but I used to have a "Wolensak" lens and still do have an "Apoc hromatic" specimen.

**Arne Croell** · 27-Sep-2004, 13:56

I've seen it once on a Docter Tessar sold on ebay (no typo of the seller - it was clearly visible in the photo) that was marked 4.8 instead of the standard 4.5. I am sure there is some spec somewhere what error is allowed for the difference between real value and "standard" engraved value. maybe 1/6th of a stop, since the usual scale goes in 1/3rd increments? Btw, the difference between 6.3 and 6.5 is somewhere around 1/11th of a stop....