Focusing issues with certain lens

[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.