Petzval cell replacement

[cyberjunkie]

I have a nice early Petzval. It's a projection lens, with no waterhouse slot, flat focusing knob, and a short barrel. Unfortunately the front element is missing, but the two air-spaced glasses at the back are in perfect shape.
When i say that the barrel is short, i mean that the back cell is not fitted in a conventional way, with one or two retaining rings and possibly a spacer, but is mounted in a tube, which in turn is fitted to the barrel. The tube length is more than 3cm. I guess that the front cell had, more or less, the same shape and dimensions, because the shape of the lens shade allows to mount a similar cell at the front of the lens.
The length of the proper barrel is reduced because about 6.5cm of the total length were made by the two tubes of front and back cells.
The threads at the front and at the back are of 54mm, the only difference between the cells should be the glass diameter: screwing the back cell at the front there is no room to mount the shade, because the cell is a little too large. The front cell should have 54mm thread as well, but should also sit flush to the barrel diameter. That is not strange, as i have seen some Petzvals where the front glass is a little smaller than the lens at the back.

I hope i have described the situation with sufficient detail.
I would like to fix the lens in some way, but i have not experimented with Petzval lens swapping before...
I even tought about fitting a +2 close-up lens with a 52mm thread in place of the front cell (i guess that a 54mm thread should be difficult to find). What makes me dubious is that the separation between the cells would be totally off!
I am not sure about the focal either, a 500mm cemented achromat (like a +2 close-up lens of decent quality) was my first choice, just because a 2 1/2+ would be very difficult to find, and a 3+ could be a little too short :confused:
Unfortunately i don't have a close-up lens of suitable size at home, and i'm not willing to buy anything before asking on this forum. I am sure that many of you made this kind of experiments.
I am open to any advice. What i know for sure is that i'd hate to give it up. The two back elements alone are worthless, but the only front achromat i have at home is much smaller: no use for this Petzval, but i'd like to try to make a landscape lens fitting the cell at the back of an empty small barrel, with rotating diaphragm, i found time ago on the bay.
I end my post with a small appeal: if you have in some drawer an unused Petzval front cell with 54mm thread, and you have no use for it, i would be more than happy to pay for it, if the price is reasonable

have fun


CJ

[walter23]

Maybe find an inexpensive tessar of about the same total focal length as the Petzval was supposed to be, and use the rear group (which looks similar in design to a petzval group)? If it sucks, you'll still have the tessar to play with, so it's win-win in that respect.

Or you could try one of the groups from a rapid rectilinear (my initial suggestion but then I looked up the lens diagrams).

Of course it's probably not going to be exactly correct, but it might be interesting.

[c.d.ewen]

CJ:

Who needs the front cell in a Petzval? Look at Post #3 of this thread. (http://www.largeformatphotography.info/forum/showthread.php?t=66676)

Charley

[cyberjunkie]

Thanks Charley, interesting stuff.
The solution is not easily applicable to my lens: the few other Petzvals i own are made the same way, the back elements can be freely rearranged. This Petzval has a true cell, like modern lenses (BTW, the elements are not easily removed, the rings have no indents for the tools, you should get a very thin rubber cylinder of the right size... IF the threads are not stuck!).
Nevertheless, while i've read with interest the mentioned thread, i don't think that the modifications would allow to get something unavailable with other (and simpler) optics. I have seen the results obtained with simple plano-convex and bi-convex lenses, and they don't differ that much :D
I will try soon a +2 close-up lens (i think that even the cheap ones sold online are achromatized these days): i think it will easily cover 8x10", with soft focus effect wide open, and with sharper than expected results with a stop in front of the lens.
As i already stated, i have a smaller loose cell from some kind of Petzval. I got it with other stuff i purchased. It's an achromatic doublet, and should almost fit an empty barrel (probably from a WA Rectilinear) with rotary diaphragms. I would be delighted to do it on the cheap, but i am afraid that an adapter ring should be fabricated. IF only i could find a way to assemble a quick and dirty fixture to attach the cell to the back of the barrel, i bet the result would be pleasing, both aesthetically and functionally.
The only problem would be with the stops. The existing ones were made for a shorter focal and for another use: it would be better to drill bigger holes on the stop disk, so to get a soft focus effect at least with the two larger ones. It would be a cheap imitation of some of the best known diffused focus portrait lenses.

Back to the Petzval, i'd like to understand if reducing the effective lenght of the barrel, and modifying the focal of the front cemented achromat, would make a GREAT difference, or just a slight one.
For sure there would be a change in coverage, but i have no idea at all if the performance, and the typical signature of the lens, would change in a very visible way.

have fun


CJ

[Carsten Wolff]

...not to hijack this thread unduly, but has anyone ever come across a Petzval where the two cemented surfaces in the cemented doublet in front had slightly different radii?
I've got a disassembled lens where all 8 surfaces have different curvatures....