View Full Version : Rear focus with Petzval lens

4-Jan-2014, 13:45

first happy new year to everybody!!!

I bought an unknown Petzval lens last year. I tried it with film and i noticed it had a kind of glow and was a bit off focus.

Today, i tried it again more seriously with a LensAlign target, it is really hard to focus. It is glowing a lot even if i reversed the rear group like a Dallmeyer.
The lens is rear focusing by around an 1 1/2". All the others lens i tried today are in focus...

What could be wrong with it?

4-Jan-2014, 14:54
? is the rear group assembled correctly? sellers frequently want to clean the lenses for sale and put them back wrong.... if you've just reversed the whole group it won't fix one element in the group facing the wrong way.

4-Jan-2014, 14:55
Make sure the most curved surfaces of the two rear lenses are nested, facing each other inside. Does the cemented pair in the front look intact and not fogged?

alex from holland
4-Jan-2014, 16:18
This is the way how the lens parts should be mounted.

4-Jan-2014, 17:27
Glass is clean but have a slight separation on front element (Blue color).
This is the way i reinstalled it.
If i compare the ring between the lens in the rear element, it is thinner (half) than the one on a Derogy of the same focal length.

5-Jan-2014, 08:04
After reading about spherical aberration, i realized this lens wasn't corrected. It is normal that my focus point is behind my target.
What will be the best way to focus it so i can sharper image at full aperture? It will still be soft but more on my target.
Try to get the best image i can, close the iris then reopen it after or focus in front of the target...

I have others Petzval lens but all are sharper than this one.

Steven Tribe
5-Jan-2014, 09:26
The good thing about Petzval's rear cell is that it can be cleaned easily. The bad thing is that the elements can be installed incorrectly. Even worse, is the fact that there are some standard diameters which means there is never any certainly that the mounted rear pair (or even the front achromat) are the original ones. I have a Pertzval with different French/German makers written on the achromat and the wider of the rear pair.

What you describe suggests that these lenses are not the original set making up the Petzval.

5-Jan-2014, 14:08
i Guess this is the problem because it is really off compare to 3 others lens...
I can't look at the achromat because i can't removed it but it seems to be installed properly.
I'll try again to reverse both elements just to see if the focus will move forward instead of be behind the target.

Here is some scans:

5-Jan-2014, 15:14
It could be just a poorly made lens. You say it's unmarked, and you're comparing it to Derogy, who only made photographic lenses, and good ones, to my knowledge. Your unknown lens could be a bad one, or even had the wrong glass swapped into it at some point.

Steven Tribe
5-Jan-2014, 16:08
Glass is clean but have a slight separation on front element (Blue color).

This doesn't sound like balsam separation - so the problem could be here.