View Full Version : A couple of lens technical questions

4-May-2007, 07:32
Question #1)
I have an 12" rapid rectilinear with waterhouse stops that sit in the center of the lens between the two equal elements. I recently bought a front-mounted shutter. I have roughly calculated the fstops for the waterhouse stops. However, if I instead use the iris on the front mounted shutter do I need to calculate the fstops differently with the iris forward of the front element than when it was between the two?

Question #2)
I have a convertible f5 petzval. When the rear element is reversed and placed on the front of the shutter alone, it becomes roughly an f8 lens (and surprisingly sharp at that). However, with the rear element mounted alone, is the depth-of-field now; a) identical to the original pair at f5, b) equivalent to the original pair at f8?, or c) something else?

Ernest Purdum
4-May-2007, 08:45
#1. Using the diaphragm of the front-mounted shutter is not a good idea. It will cause vignetting.

#2. Something else. The DOF will now be governed by the focal length and aperture of the rear element alone.