Swapping lenses in a Petzval - Part II

OK, fans of the Petzval, if you've read the previous threads (rguinter's and mine) on moving Petzval lens elements around, you're ready for the next adventure.

I was reading my Kingslake, History of the Photographic Lens, one day and happened on this sentence on p40: "In his patent, Dallmeyer suggested that a soft-focus effect could be introduced into the image by varying the separation between the two rear elements". Just what was suggested in the threads. I couldn't resist.

The two rear elements in the Petzval used in my first thread came with a 0.243 inch spacer ring between them. I made four more rings, sizing them so that they were 0.010 inches smaller, and 0.010, 0.030 and 0.050 inches larger than the original. Then I took some 8x10's using the various rings. The elements, BTW, were in the original Petzval order, i.e., the thinner, crown element was lastmost.

While I see some difference in effect, I don't see a pattern, e.g., sharper to softer, neither in the center nor the edges.

Since it was freezing outside when I took those photos, I decided I'd better try again indoors (and at portrait distance) where I could be surer of focusing. Just for fun, I made some crops of a couple of sections.

Again, I don't see a pattern. What do you see?

Since the forum only allows four photos per post, I'll attach the original-spacing photo here, and put the -10, +10, +30 and +50 photos in the next post.

Charley

In left to right order, here're -10, +10, +30 and +50 (1,000ths):

This one was made using the original spacing:

Again, in left to right order, here're -10, +10, +30 and +50 (1,000ths):

And lastly, a few crops of the above, in this order:

-10 0 +10
+30 +50

The bright highlights of the last set of images would have told the most, but, (darn it!), the light was moving enough to be quite different from the original to the most modified spacing. I'd be tempted to make some "radical" shifts in the spacing, just to see how soft they will go, maybe quarter-inch, half-inch, whole inch (!). 50/1000" isn't much, and like the factory adjustment on the Series II Velostigmats, it may not be enough...

I've never been impressed by the "soft focus" capabilities of either the Dallmeyer or Vitax Petzvals, although each are very lovely lenses in other ways. Of course, if you discover a way to really turn the soft focus on, Petzvals could get really expensive! :eek:

Charley-- Thanks for posting your tests! It confirms what some of us have found--the "soft focus" feature of variable spacing between the two rear elements of some Petzval lenses is more marketing gimmick than useful feature. Dallmeyer didn't have anything to add to Petzval's design by reversing the rear group and adding a spacing adjustment. It was enough to get him a patent and some nice marketing points.

Mark:

The +0.050" spacer was as big as I could use - any bigger and the lens would fall out of the case :D

Barry:

I'm never sure what anything I do confirms :rolleyes:
Maybe if I reversed the order of the rear cells.....

Charley