Petzval question, barrel extension ring

[Andrew]

Hi all !

I've got a petzval lens with what seems to be an odd feature: the rear end of the barrel has a removable "extension ring" that adds about 1.5cm to the seperation of front and rear elements

the lens focuses infinity at 9" with the extension in place and 8 1/2" without it [those distances taken from the slot for waterhose stops]. It's a slower design and I'm guessing about F5.6 Coverage is just about 5x7 at infinity and certainly covers that format at closer range. There's just a little swirliness in the out of focus background.

I'm guessing that the slot in the barrel makes it a portrait lens rather than a projection lens. There's no makers marks

My questions for the experts are:

what's the extension ring for ?
and
I've read that the slower petzvals have less swirl, is that correct ?

cheers

[Steven Tribe]

The only thing I can think of is this might be an adaptor to allow the front lens to be installed at the rear as a meniscus landscape lens, whilst the double lensed rear cell is carefully stored somewhere. This combination petzval/meniscus lens is known in the Darlot/Jamin double thread ended paysage/portrait creation. The purpose of this was to alter the distance from the stops in relation to the rear mounted lens (that is, nearer). If I am guess correctly, the focal length of the pretzval is 9" (with the extension). When the extension is removed, and the front lens mounted in the rear - it becomes a landscape meniscus of xx" focal length. Does this make sense to anyone else?

[Steven Tribe]

I think I am right. If you look at the Darlot/Jamin paysages/portraits on e**y 360215161098 - you will see that the last photos show the moveable extended part of the barrel.

[Andrew]

Steven, thanks for taking the time to think about this and reply...

The front cell will screw directly into the rear of the barrel just as well as it will also screw into the extension ring as all three threads are the same. This begs the question of why have an extension in there when using a single element? Surely the front cell will behave the same used by itself whether it's on the extension or directly on the barrel? Seems to me that the extra piece is redundant unless it's doing something to the image by changing the spacing when two calls are in use? Oh, unless there's something critical about the placement of the single cell relative to the aperture stops!!! Could that be it???

I now have question that will show my ignorance... does the front group need to be reversed to use it as a meniscus lens? I don't have the lens on a camera at this time but just projecting an image onto a white card it seems to form an image either way.

[Steven Tribe]

The distance from the aperture to the menicus lens, when mounted in the back, is critical otherwise there be so much of the image size cut off by the aperture being so far away. That 1.5 cm makes a big difference. I have never seen this simple meniscus / petzval combination before and I think it is a genius idea some manfacturer had and it is not just a copy of the original Darlot idea. It must from after around 1860 as waterhouse stops first were common after that time. The meniscus will work fine just screwed in in the reverse direct to the light it was when mounted in the front. Take care of this objective - it may be a solitary survivor - or even a one off job:

[Andrew]

Thanks again for the input... to get this sorted out in my own head, I'll obviously need to get this lens on a camera and see what it's doing by shooting some film in the different conformations!

[Steven Tribe]

But first you must make some temporary waterhouse stops! The fact that there have been no other contributions like "I've got one of these too" supports my postulate, I think.

[c.d.ewen]

But first you must make some temporary waterhouse stops! The fact that there have been no other contributions like "I've got one of these too" supports my postulate, I think.

Well, not a 'me, too' post, but a 'so that's why" one.

Attached are photos of an ordinary enough Petzval, except that it splits in the middle. If you take the rear cell off and replace it with the front cell, you can unscrew the front half of the barrel.

Exactly how critical is the placement of an aperture in front of a meniscus?

Charley

[Steven Tribe]

Thanks for the interesting (exciting) imput Charley! I would have thought distance was pretty important as the Petzval design optics demands a long barrel in relation to the lens diameter. Seems like a good idea to make a petzval that can be adapted by shortening the barrel, using the existing aperture/waterhouse stops and fitting the front balsamed lens at the rear. Does yours have stops? - if not, it would make an interesting soft/plastic lens!

[Steven Tribe]

Oh, there is a difference, though. In you case Charley, the rear piece would be used to mount the front lens at the back for a landscape lens as this "half" has the mounting thread for the flange!