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:

94623

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.

[muffin]

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.

Hey, you are right with your comment. I've been testing the lens on another camera today and nailed the focus 2 out of 2 times. This means the ground glass on my beloved Chamonix is somehow out of position and it wasn't really visible with my other lenses.

Hm. Not good. What can I do now?

[Mark Sawyer]

Take the back off your camera and lay a rigid straight edge across the frame. Measure the distance to the ground glass.

Next, put a film-holder in with an old sheet of film. Make the same measurement, from the straight edge to the surface of the film.

The measurements should match exactly. You probably should make the same measurements in the corners too. If the measurements are off, shim the ground glass until they match, and write a nasty letter to Chamonix.

You may also want to disassemble the Petzval and check that the elements are in the right order. Maybe a third of the old Petzvals I have came assembled wrong.

[Andrew]

question to the experts: if I focus under normal light and use IR film, I need to make a small focus correction [my old SLR lenses had a mark on the barrel to tell you how much to refocus].... IF this is collodion my understanding is that it's heavily biased towards blue and UV so is there effectively a focus shift induced by the change in colour sensitivity of the emulsion, same as going the other way towards red ??? OR is it just what you see is what you get ?

[Mark Sawyer]

question to the experts: if I focus under normal light and use IR film, I need to make a small focus correction [my old SLR lenses had a mark on the barrel to tell you how much to refocus].... IF this is collodion my understanding is that it's heavily biased towards blue and UV so is there effectively a focus shift induced by the change in colour sensitivity of the emulsion, same as going the other way towards red ??? OR is it just what you see is what you get ?

As I understand it, a great deal depends on your lens. What you're really talking about with the IR-Visible-UV spectrum is chromatic aberration, which is corrected in different ways and to different degrees in different designs. In a "perfectly" corrected lens, all colors of light from the image plane will focus on the same film plane, and none of this is an issue. But the corrections aren't perfect, and when you get out to the extreme ends, the corrections do funny things. In a totally uncorrected lens, (like a Wollaston meniscus), the UV will be slightly short and the IR slightly long. In a corrected lens, the differences in focus will be even slighter (this is most achromatic lenses), or fully corrected (what apochromatic lenses strive for), but at the extreme ends of the spectrum lenses can be over-corrected or under-corrected independently so the UV and IR shifts can actually reverse sides or both be slightly short or long.

But the differences are very small.

[alex from holland]

Ines, take of the lens form your chamonix and keep your groundglass in. Measure the distance THROUGH your camera. Front lens plate until inside ground glass. Now place your plate holder with film or glass on the camera, open it and measure the same dintance again.

[muffin]

Hello,

sorry for the late reply, I was able to measure as late as today.

BUT: There IS INDEED a difference between the depth of the ground glass and the depth of my Chamonix wet plate holder.

Measuring from the position of the front lens plate to the ground glass: 156mm.
Measuring from the position of the front lens plate to the position where the plate rests in the holder: 159,5mm.

This is a difference of almost 4 millimetres. I think THIS must be the problem.

But: What can I do now? How can I fix it?

Just for fun I measured the distance between the front lens plate and a film holder that came with the camera: perfect match, 156mm. No difference.

So, when I'm making wet plates, I need to shim the ground glass, right? How would I do that best?

And when I shoot film, I need to remove the shims.

Edit: I found a little plastic rim in my scraps that has the right dimensions and is exactly 3mm thick. Should I slip it into the holder so the plane moves forward or should I slip it into the ground glass so the plane moves backwards?

[Sevo]

This is a difference of almost 4 millimetres. I think THIS must be the problem.

But: What can I do now? How can I fix it?

Tenths of a millimetre can already be critical - 4mm is very significant. If you cannot shift the plates forward in the holder (perhaps not, if you need some depth for a drip receptacle), you will have to shim the ground glass - unless you decide to reserve the entire camera to wet plate only, I'd recommend shimming a second GG frame, as the there-and-back between shims and no shims would soon wear out the frame.

[Paul Fitzgerald]

"Edit: I found a little plastic rim in my scraps that has the right dimensions and is exactly 3mm thick. Should I slip it into the holder so the plane moves forward or should I slip it into the ground glass so the plane moves backwards? "

Neither, it would be easiest to make a 3.5mm plate to slip in like a film holder to offset the GG, then remove to insert your wet plate. No fuss, no muss, no broken GG, just store it with your wet plate stuff.

[muffin]

Great idea Paul! Thank you, this is what I'll do, sounds like the best idea.

Thank you all for your help!

[Mark Sawyer]

As you're working with a Chamonix camera and Chamonix holder, I'd consider asking Chamonix for a properly made holder. It's a standardized dimension they got significantly wrong, and at what the holders sell for, buyers should get something that works without modification.