Check your Chamonix

[Jeffrey Sipress]

A recent long and winding thread about focusing issues with the Chamonix 4x5 got me to take a closer look at mine. I'm fortunate to own a precision machine shop, and so I have the inspection equipment to measure these things accurately. When I went to remove the glass and fresnel, I noticed that the ground glass was resting on the fresnel, and not actually resting on the narrow steps in the wooden back made to accept the glass. This is because the plastic fresnel is thicker than the recess in the wood that is to contain it. Although the angle of view in the pic here does not show this discrepancy, it does show the corners in question. The GG (top layer) is resting on the plastic lens under it, and not contacting the ledge in the wood it is supposed be against. I measured two film holders (with film in them), a Toyo and a Fidelity, and they were within .003" of each other and the specification, which I believe is 5mm (.197"). On the camera back, the distance to the wood step for the glass measured the same. The camera was made well! My fresnel is lifting the glass about .010" above it's proper location. I need to find a thinner fresnel, or carefully mill or file the edges thinner to allow the glass to seat. If you have a gauge you can measure the step from the fresnel to the outer wood ledge. The wood should be even or higher than the plastic. This post and my suggestion is not intended for the non-technical oriented camera owner.

[Steve Hamley]

Jeffrey,

Not necessarily so. The image forming GG surface should be in the same place as the film WITHOUT the fresnel - the fresnel shifts the focus by about 1/3 the thickness of the fresnel. There should never be a lens (even a zero power lens) in the focusing path that isn't there when the film is exposed IF the image forming GG surface is at the same depth as the film. That's why you always focus with rear-mounted filters in place, not focus then add a rear-mounted filter, because otherwise, the zero power filter would shift the focus by virtue of not being in the image path when the focusing was done.

Please don't get everyone started again.

Cheers, Steve

[BarryS]

I think my head may explode if any more of these threads pop up and I already hear the troll scrabbling out of his dank cave, but Jeffrey is bringing up a valid point. The ground glass surface should be in the same plane as the ledge on the holder--which is the T-distance for a standard 4x5 film holder. If anything is displacing the physical ground glass position (a mouse, chewing gum, swizzle stick, or an overly thick fresnel screen), the focus won't be accurate. This has nothing to do with the optical displacement of the plane of focus due to the fresnel screen.

[Jeffrey Sipress]

Thanks, Barry. The concept is simple: plane of frosty GG equals plane of film. I'm sorry, but I don't know the stuff that steve is talking about.

[rugenius]

Jeezzz... DejaVu...
You guys are saying just about the same thing in different verbage.
The GG = the film plane 4-ever and a day when you view the image with only the GG.
Using a screen intensifier such as Fresnel lens distorts the film plane focus by some degree.
In some cases this is significant...
In other cases the shift in focus is insignificant...

With so many use cases involved it's difficult to define all parameters for each use case and lens.
To generalize, a wide angle lens shorter than 150mm focal length might lend to less than optimal focusing in the situation described by numerous LF forum participants.
Furthermore,... many camera makers use the same setup with GG and Fresnel lens and the issue has been greatly exaggerated with the Chamonix 045N.
That's my opinion not fact.

Bill

[GPS]

A recent long and winding thread about focusing issues with the Chamonix 4x5 got me to take a closer look at mine.
...

My fresnel is lifting the glass about .010" above it's proper location. I need to find a thinner fresnel, or carefully mill or file the edges thinner to allow the glass to seat.
...

You're surely right, Jeffrey, in you assumption about how the camera back should be made. The fact that it isn't is the sad fact, already known (as said in the long and winding thread... ) - i.e. Chamonix "corrected" one mistake with yet another one...
That there are cameras with even worse manufacturing quality and a constructional concept is hardly any consolation, is it?

[vinny]

Ladies and gentlemen, GPS!

[Marko]

Ladies and gentlemen, GPS!

Indeed!

[Jeffrey Sipress]

reminds me when I was shopping for a monorail, and started comparing the distance between the front and rear standards by measuring at all 4 corners. I found almost all cameras were off by .5 to 1mm.

I understand that you are using this comparison simply as a reference, but your comparison is a waste of time. Who cares if the standards are ever exactly parallel? We always move them to where they need to be in the course of using the camera to make an exposure. It matters not where they start.

DISCLAIMER: I began this thread to approach the subject in a strictly mechanical and dimensional sense. If the community here thinks I am a troll by furthering this subject, I will voluntarily cut my hair, take a bath, and buy a new suit.

[GPS]

I understand that you are using this comparison simply as a reference, but your comparison is a waste of time. Who cares if the standards are ever exactly parallel? We always move them to where they need to be in the course of using the camera to make an exposure. It matters not where they start.

DISCLAIMER: I began this thread to approach the subject in a strictly mechanical and dimensional sense. If the community here thinks I am a troll by furthering this subject, I will voluntarily cut my hair, take a bath, and buy a new suit.

You see Jeffrey, I take it in the sense you wrote it too. 0.25mm gg displacement is not something one wouldn't need to care about- last time I was focusing my new camera (135mm Sironar-S with a helicoid) at infinity I could see that 0.15mm displacement was already clearly less sharp on the gg...
But the people with the Chamonix religion are so easy to wind up - you cannot keep the thread be approached in " a strictly mechanical and dimensional sense"...