I recently purchased a B&L projection lens at a very low price - on examination it turns out to be a Petzval lens of 11 1/2 inch FL. I decided it was a Petzval by determining that the front element element is a cemented doublet and the rear element is an air spaced doublet - the curvatures of the glass agree exactlly with the Petzval design. It is 5 inches tall and 2.25 in diameter and casts a lovely image on my 5X7 - the glass is in very good condition.
It has an iris which I would guess was used to darken the slide image when it was used as a projection lens. The iris maximum diameter is 1.75 inches which gives an aperture of approx f6.5. If I removed the iris it would give an aperture of around f5. Should I remove the iris (which would be permanent) and use the lens only at f5 or should I keep the iris and use it at f6.5 with the option of stopping down?
If I have made any errors in calculations or assumptions please let me know - any advice gratefully received.