The reason for these illogical markings (same distance between numerals) aperture scales on Petzvals is because:—-
- they were not used for taking photographs!
They were used on mostly horizontal enlargers where there is usually a trial and error illumination of test papers. The numbers are just a reference in getting a repeatable aperture - which gave the best result on test papers.
A lot of these Petzvals with lever iris apertures have no F indications - were also used in enlargers and the other devices around the turn of the century.
I enclose a photo of one without either makers name or iris position marks. Note the extremely long protuberence - useful in a darkroom!
The “no name” state is perhaps best explained by enlargers being well plastered with name plates , like magic lanterns.
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