Can a Petzval Design Be Used Backwards?

[Bob Hubert]

Hi All:

Recently acquired (on this forum) an early Hermagis lens of a Petzval design. Some previous owner reversed the lens in its focusing sleeve to hide the threads that used to hold the lens hood.

I'm going to test the lens both backwards and forwards, but I am curious if anyone has run into this before? Will an asymmetrical lens produce a different image if used in reverse?

Thanks in advance for any information.

Bob

[Jim Noel]

"I'm going to test the lens both backwards and forwards, but I am curious if anyone has run into this before? Will an asymmetrical lens produce a different image if used in reverse?"
If you are going to test it, why ask the question? You will get your answer when you test.

[goamules]

I am surprised that this question hasn't come up before. I first noticed Petzvals being used backwards then I bought the brass tube ones for the old multi-cameras. I mistakenly thought the threaded end was the "back", and most of them had the elements in the wrong way for that (I since learned the threaded end of the brass tubes are the front).

But - they seemed to work both ways. I didn't notice a difference, and still don't. But I've not done a careful, scientific test. Just held them up the the ground glass, mostly. Who is the optician that can tell us why or why not?

[Mark Sawyer]

A positive lens system is a positive lens system in both directions; it still throws an image, and the differences are usually slight. You can mix the elements all around in spacing and order a fair amount and still get an image, (Dallmeyer did this with their Portrait Petzval, reversing the crown and flint glasses at the rear, and changing the rear spacing to induce a little softness.) It might induce some aberrations, curve the focus plane, etc, but it's surprising how much you can rearrange lenses and still get an image...

The instance where reversing lenses are most discussed is in reversing modern lenses for macro use, since they are optimized with the long distance in front and the short distance behind, but again, the difference is slight. Petzvals pre-dated such concerns.

[Sirius Glass]

IIRC Petzvals are symmetric lenses so turning them around will show little or no difference.

If a lens is not symmetrical, which is typical of many modern lenses, the the reversed lens can be used as a "macro" lens. YMMV

Steve

[Fotoguy20d]

I mounted a projection petzval in my speed graphic today and got some interesting results. First, the focal length seemed to be about an inch or two shorter (around 4" at infinity instead of the 5-6" I had expected). Second, it didn't swirl as much as I expected, and, really odd, it got very swirly when focused beyond infinity. But, instead of swirling, when focused fairly close, light coming through some distant trees took on that petzval blobbiness. Here's where it gets interesting. I checked to see if the glass was reversed bu the cemented pair was where it belonged and properly assembled. Not so, as it turned out, the back pair. They were completely reversed inside the sleeve - borrowing Dan's construction diagram, the flat surface on the flint was on the right, and the flat of the crown on the left. Once I reversed them, the focal length went back to where it should have been, the swirliness came back in true crazy petzval fashion, and the blobiness disappeared. So, something to play with - try reversing those rear glasses to alter the swirl/blob behaviorof a petzval. Going to try it on some other lenses and see what happens.

Dan

[c.d.ewen]

Bob:

You might take a look at this thread. (http://www.largeformatphotography.info/forum/showthread.php?t=66676)

Charley