Which Petzvals have the Dallmeyer modification?

[Steven Tribe]

Well, I have some news about this. In my newly acquired Le reve data booklet, I can see that from about 1875, Gasc & Charconnet, made a version of their Quick Acting Petzval, called B in their catalogues, which is described as " imite ceux de Dallmeyer". Now I happen to have a quick acting G&C Petzval sold by Bryant in the USA and I noted that the rear cell had the 2 lenses mounted separately - without giving a thought to what this might mean. It does, indeed, have the Dallmeyer rear layout. Adjustment can only be made by taking off the rear cell and screwing the inner cell.

These were made in 3 sizes:

13cm efl, lens diameter 54mm
16cm efl, lens diameter 70mm
25cm efl, lens diameter 81mm

It is last one I have.
It might be a good idea to check your Bryant/G & C lens!

[Steven Tribe]

And another one!

Again, I overlooked the logical meaning of a rear cell, where the two lenses are joined by a screw thread.
This is a large Derogy, with export engravings "London and Paris". Exactly the same system as with the above G & C and with no graduation marks inside. I image there are details in Le Reve booklet no. 16 which includes Derogy.

I now have this booklet which includes Derogy. There is no mention of "Ross" and "Dallmeyer" versions in the text.

[goamules]

Well good work detective! I'll have to check my G&C Quick Acting and see if it's that layout too.

[Mark Sawyer]

Just as a note, I'm cleaning up a couple of 14-inch Vesta lenses today and re-ordering them to the Dallmeyer layout. I've seen several notes in Wollensak catalogues that they are "built on a modified Petzval formula", built on a modification of the Petzval formula", and "an improvement of the Petzval formula". Wollensak used similar language in describing the Vitax, ("a modification of the Petzval", etc.), so I'm guessing both lenses use the Dallmeyer layout.

If anyone has information contradicting or confirming this, I'd love to hear it...

[Steven Tribe]

The modification may be the general redesign, initiated by Hermagis and Voigtlander, in the 1850's which moved the visual and chemical focus to the same plane.

[Mark Sawyer]

The modification may be the general redesign, initiated by Hermagis and Voigtlander, in the 1850's which moved the visual and chemical focus to the same plane.

The only re-design by Voigtlander that I know of is the 1878 modification wherein Voigtlander cemented the two rear elements, resulting in a less-sharp lens. (Obviously, that modification is pretty recognizable when you disassemble the rear, so it's not on any Petzvals I've seen.) Can you point me to any information on the 1850's redesign?

[Steven Tribe]

Here are two sources. Sorry about the quality.

The Voigtlander extract includes the number of turns necessary on the lens focussing knob. What a problem they must have had in the 50's!

[Mark Sawyer]

Interesting! I've had surprisingly little issue with chemical vs. optical focus with Petzvals on wet plate. But as far as I know, all my Petzvals are 1860 or later, so perhaps the modification was common to all Petzvals by then. I wonder what Hermagis' "modification" was, re-ordering the rear elements, different spacing, different curves to the lenses?

Perhaps I'll just have to shoot a Vitax and Vesta in both configurations and see how they compare. But that still may not tell me how the were assembled by Wollensak...

For those who don't read French, here's a translation of the first text from someone who reads it poorly:

PORTRAIT LENSES

LENSES OF ORDINARY MOUNTS

Petzval lenses (4 lenses in 3 groups) having blur due to residual chromatic aberration (anachromatic objectives), presence of a chemical focus different from the optical focus (image modification of the order of 2%, thus requiring focus correction. Hermagis was one of the first to greatly reduce these disadvantages.

(Meaning a focus correction of 2% was necessary before Hermagis' modification.)

[thafred]

Can someone explain to me why "chemical focus" was different to optical focus? Intriguing..
Is it because the lens doesn't focus all the colors on the same plane but the orthochromatic emulsion records only blue light? Since the human eye is least receptive of blue light we focus the other wavelengths more naturally so what looks sharp on the ground glass is blurry on emulsion?
That would mean that to get the sharpest focus out of my petzval I would have to use a color filter on panchromatic film?

Thanks for clarifying!

Back on topic. I bought a nice big (nondescript) petzval some time ago (somewhere there is a thread here) that had terrible picture quality until I reversed the configuration of the rear group, making it into a dallmayer effectively. I don't have a clue who made this lens but it was obvious that the previous owner had cleaned it and put it together the wrong way around.

[Mark Sawyer]

"Chemical focus" (aka "actinic focus") is only an issue with non-panchromatic processes. The old orthochromatic films and plates were sensitive to UV, violet, blue, and green light, which focuses at a closer distance from the lens than the visible spectrum, especially so with the older non-apo-chromatic lenses. So after focusing on the visible light, photographers would shorten the focus (usually by 1% to 2%) to get the UV-blue wavelengths that would actually be making the image in focus. Modern panchromatic emulsions match the human visible spectrum very closely, so no correction is necessary. A very strong filter at either end of the spectrum can affect the chemical focus; that's why there's a focus correction for infrared photography.

Hope that helps...

And yes, I've run into many Petzvals that were re-assembled in seemingly random order. I've always re-assembled them in the classic order, but with some (a few? most?) later Petzvals, they may have been designed for the Dallmeyer layout. I'm not sure which, hence this thread. I believe the Vitax and Vesta were Dallmeyer-style.