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View Full Version : Calculation of a Petzval lens f-stop



Jay Decker
3-Oct-2009, 17:11
Have an old Petzval with 10-1/2 inch focal length. The lens has slot for waterhouse stops. When calculating the f-stop do you use 1) front element diameter, or 2) the diameter of the hole thought the waterhouse stop?

Mark Sawyer
3-Oct-2009, 17:18
The diameter of the hole in the Waterhouse stop, as measured through the front lens, which will magnify somewhat.

Jay Decker
3-Oct-2009, 18:06
The diameter of the hole in the Waterhouse stop, as measured through the front lens, which will magnify somewhat.

Thank you!