Focusing issues with certain lens

[muffin]

Hello,

first, I want to apologize if I'm posting in the wrong sub forum. I don't really know which one is the right for my kind of "problem". And here it comes:

I have a nice 7x11 Chamonix which I use with a few different lenses. I focus with a loupe on the ground glass, and usually, the part I focus on the glass is the part that is sharp on the plate. As it should be.

Now I have this nice old petzval portrait lens, and whenever I focus correctly on something on the glass, it shows a front focus of about 3 centimetres on the plate/negative.

But only with THIS lens. Where is the problem?! I don't understand?!

If the focal plane would be wrong with my back or the holders, it would be the case with any used lens, not only this certain one?

Can anybody help me? It freaks me out. I don't know how this could happen. How can it be sharp on the ground glass but then be greatly misfocused on the final plate or negative? And it also happens with still motives that can not move, so... it is really weird.

What do you think? Your help is greatly appreciated.

Thank you!

Best regards!

[Ken Lee]

If you stop down the lens after focusing - and then the image goes out of focus - that's called focus shift. With some lenses you need to focus at the taking aperture. Perhaps that old lens exhibits focus shift.

[muffin]

Thanks for the reply Ken,

I forgot to mention that the lens does not have an aperture. I use it wide open, no Waterhouse Stops. I think the aperture is about f/4.

[Paul Fitzgerald]

"Now I have this nice old petzval portrait lens, ... it shows a front focus of about 3 centimetres on the plate/negative."

Petzval lenses have a curved field of focus both inside and outside the camera, can be a problem.
This could also be a focus shift from NOT being color corrected, which would changed off-set by focus distance.
Easiest way to check is to use panchro then non-panchro film on the same set-up.

[cowanw]

I think what Paul is referring to is chromatic aberration, which in Panchromatic film would lead to an overall defocussing of the image rather than a focus shift (as the different wavelengths of light would be focussed in a grouping around the point of intended focus.
Ortho film would show a focus shift as the focus point might be on a red yellow basis but the film would register on a green blue focus point.
At least that's my understanding, real practice may well be different.

[N Dhananjay]

I remember reading about differences between chemical focus and visual focus - something about actinic light being refracted more than the visual part of the spectrum in simple lenses. In other words, we tend to focus typically based on yellow green. However, unless the lens is corrected for lateral color, the blue and UV will focus in a slightly different region. This can be reduced by stopping down or by performing experiments and correcting (or more accurately adjusting) for them. Cheers, DJ

[muffin]

Hello all,

this sounds really interesting, esp. the part with the blue and actinic light. I was using a few black light bulbs together with my normal lights.

This is the image I'm talking about:



I did it twice because I thought I misfocused or the subject moved (he didn't!). Two times perfect focus on the eyes on the ground glass and the resulting plate has a sharp nose
I noticed this with that certain lens before but then I wasn't sure I focused right. This time I am.

[alex from holland]

Ines, you just can't test this on a living model. Try a still ife and see if you have the same problem.
As I mentioned before. Some old petzvals have this problem. I have one myself.

[muffin]

Yes, I DID test it on a still life and the problem IS there. I just had this image to show what I mean, I did not scan the still life plates because I thought I'd misfocused 4 times in a row

[Ole Tjugen]

Another possibility is that your ground glass could be very slightly out of position. That would only be visible on the final print/negative with a lens used at a large aperture, like a f:3.7 Petzval. If your pther lenses are used at f:8 or so, that small a focus error won't be visible.