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life_in_sepia
9-Oct-2010, 19:35
As a matter of curiosity I was wondering: does the addition of an internal iris diaphragm reduce the maximum aperture on the old petzval lenses? For example, if in 1895 someone bought a Dallmeyer 3B with an internal iris, would he or she have been sacrificing a little speed for the sake of gaining some ease of aperture adjustment, ie, as opposed to the waterhouse-stopped version?

I have a 2b (1930?) with internal iris, marked at f3 wide open. And I have a 3b (1910?) with internal iris, marked at 3.5 wide open. So the markings themselves may not be totally accurate, and in any event don't seem to answer this question.

The advertisements from the day would seem to indicate that there is no difference. From Dallmeyer's 1901 catalog:

Patent Portrait Lenses. These were then in 3 speeds as:
f3.16 Quick Acting Portrait Lenses Type B
No 1 6in focus 2in dia CdeVisite 7.00 Wh or 7.75 with iris.
No2 8.25in 2.75in dia also Cde Visite 12.75 Wh or 14 with iris.
No3 11.5in 3.5in dia Cabinet Portraits 19.00 Wh or 20.5 with iris

Anyway, I thought someone here my know the answer. Thanks for your thoughts. Ed