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