Petzvals: To extend. Or not to extend. That is the Question.

[rguinter]

With Petzval lenses in magic lantern mounts, the rack and pinion mechanism obviously was used originally to focus the image on the projection screen.

But I'm wondering what basic effect (if any) is obtained when the same lens is now used on a view camera. One can focus easily on the ground glass regardless of whether the magic lantern mount is racked forward or backward or somewhere in-between.

I believe the Petzval becomes more of a "soft focus" lens when the spacing between the front and back lens-elements is maximized.

Just wondering if this is this correct or perhaps I am mistaken. I seem to recall reading this somewhere but I have little experience with Petzvals and don't know for sure.

Thanks to those who can explain. Bob G.

[BarryS]

The rack and pinion moves the lens whole lens barrel back and forth--there are no changes in cell spacing. The rack and pinion was originally added to lenses to allow a fine focusing control before cameras had geared standards.

[goamules]

You are on the right track Bob. But it's the two air spaced rear lenses that are sometimes adjusted for a soft focus effect. See the recent post here about the different configurations.

[rguinter]

The rack and pinion moves the lens whole lens barrel back and forth--there are no changes in cell spacing. The rack and pinion was originally added to lenses to allow a fine focusing control before cameras had geared standards.

Ah yes. Thanks Barry.

Now I'm thinking my question was (perhaps) a dumb one since I didn't notice the whole assembly was fixed and simply moved back and forth. I only have the one lens to work with but it moves as you say.

So unless there is some way to disassemble and adjust the spacing between cells, the racking does essentially nothing.

Thanks for the explanation.

Bob

[BarryS]

Bob-- Not a dumb question at all; I wondered what the rack and pinion did with my first Petzvals. After I bought a reproduction daguerreotype box camera, I got a real feel for how useful it is to have the lens rack and pinion. As Garrett mentioned, there are a lot of Petzvals that have a pseudo soft focus effect by varying the distance between the elements of the rear group.

[c.d.ewen]

Garrett was talking about this thread , where I shuffled lenses in a Petzval. I, too, once read something about extending the length of a Petzval's barrel, and made a 1/2" extention to screw into this Petzval's barrel. You be the judge of the extension's effects: the first photo below is taken with all lenses in the original Petzval design, separated at the original barrel's length; the second is taken with the same lens arrangement, but the separation was increased by 1/2".

Maybe sometime when I'm not busy, I'll try making an extension to separate the rear cell elements.

Charley

[goamules]

I'd missed this factor you experimented with Charley. At least one early lens, the Jamin Cone Centralisateur, had a provision to change the distance between the front and the back elements (as apposed to the distance between the two rear lenses). I haven't played with it on my Cones, but need to see what the effect is.

If I recall it was intended to reduce aberrations at different focal distances. Perhaps it would move the "swirl focal distance" (spherical aberrations) so they wouldn't be as apparent at different focus spots.