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View Full Version : Lecture on Petzval Lenses, Tucson, Oct. 5



Mark Sawyer
2-Oct-2017, 19:54
For anyone in Tucson, AZ who's interested in historic optics, Dr. Jose Sasian of the University of Arizona's College of Optical Sciences will be presenting a lecture on "Joseph Petzval's Lens Design Approach" this Thursday, Oct. 5, 2017, at the monthly meeting of the Western Photographic Historical Society. The meeting is at 7:00 pm at the Ward Six City Council Meeting Rooms, 3202 East First Street, Tucson, AZ.

Hope to see a few of you there!

Nodda Duma
3-Oct-2017, 19:50
After many years of working as a self-taught lens designer, I went through U of A's Optical Sciences graduate program to get some formal education. Dr. Sasian was my professor for a couple of lens design courses. Very good instructor, and I was always fond of the historical perspective he provided to the design forms. He taught Petzval's design as part of his course, amongst many other well-known forms.

If you are in the area, I highly recommend attending his talk. I would love to watch a live feed if it was available.

Steven Tribe
4-Oct-2017, 02:15
If I could attend, I would ask the following questions. I have never found a satisfactory answer to them!

1. What did the redesign of the Petzval/Voigtlander Petzval - in order to bring together the visual and chemical focal lengths - consist of?

2. The Dallmeyer revision of the rear elements in order to introduce softness into the Petzval was considered as bogus by many of his competitors. They claimed that increasing the distance between the front and rear elements of the standard design acheived the same sort of "flou". Has anyone done any testing of the differences?

Mark Sawyer
4-Oct-2017, 10:31
I'll pass on your questions, Steven, but wasn't it Lerebours who did the redesign to synchronize chemical and visual focus around 1850?

Dallmeyer did several modifications to Petzval's design starting in 1860 or earlier. The 1867 design was the one that introduced softness, but it was done to spread the depth of field, not create a "flou" effect. I've noticed on other lenses that have adjustable softness that increased spherical aberration does increase depth of field, "sort of"...

Introducing enough spherical aberration to spread the Depth of field creates a very soft effect overall, so while there starts to be a sharp core behind the out-of-focus area, it's still dominated by softness, and the main focal plane has a noticeable soft overlaying image too. The overall effect is more soft than spread-depth-of-field, and if you close down the aperture to gain conventional depth of field, the spherical aberration goes away and you're back where you started.

In my limited playing with My Dallmeyer 3b and Vitaxes (same diffusion design), my observation is that the limited travel of the rear elements doesn't provide much softness, just a touch, like a still-restricted Velostigmat Series II with the diffusion ring still factory-limited, or the Cooke Portrait Lenses. Enough to soften skin texture a bit, but not a Pictorial effect, of enough to noticeably effect depth of field. For what it's worth...

goamules
4-Oct-2017, 15:46
History records Lerebours as optimizing the color focus of the Petzval design.

See you at the lecture Mark. It will be nice to see what he's been up to since our dinner that time.