Chamonix camera 45N-1 focusing error

[Steve Hamley]

The more I read on these forums the more I see I don't know what I'm doing nor can I trust all those who post their opinions on them. Big surprise...

Mike,

Me too, but you can always trust your calipers or depth gauges. All you really want is the emulsion to be in the same place as the GG (sans fresnel lens in front of the GG), and that isn't very hard to measure.

Cheers, Steve

[Jeffrey Sipress]

Eddie & Steve, now we're getting to the truth of the matter. Plenty of chatter, but the measurements are the final word. Being a machinist and engineer (I own the place and we're ISO9000), I can measure accurately to .00005". I can use calipers, depth micrometers, or my Coordinate Measuring Machines ($60K). Fresnel, no fresnel, focusing errors, hole in the middle, whatever anyone wants to talk about, the bottom line is that the sheet of film in the holder must end up on the same plane as the frosty side of the GG. Unfortunately, not everyone has access to these measuring tools or understands their application. Bring your camera back and a holder into a nearby machine shop and have someone perform the inspection.

Game over, man.

[Steve Hamley]

Jeffrey,

Exactly - unless the GG is in the same position as the emulsion then you stick a fresnel in front of it.

I've never understood most of the thread. Chamonix's fix (the hole) is a good one if you want to use the Chamonix-supplied fresnel. If I owned one, I definitely wouldn't want the back re-manufactured so a plain GG would be incorrectly positioned by itself, although if they supplied correct shims for the GG I suppose that would work just as well and would be well within the abilities of most users.

But I'd probably buy an Ebony fresnel and cover glass and be done with it. Or have Maxwell make me one like the Ebony with the frosty side to the lens of course. As someone said earlier, you could do either and still have one heck of a bargain.

Cheers, Steve

[Mike1234]

So, from I've gathered here and from other forums...

1. Have a machine shop test to assure matched depth of frosted side of the focus panel and film (I'll add to test on all four corners as well as center).

2. Use various non-frosted fresnels of appropriate FL for the lens in use at the back (viewing side) of the focus panel and only for composing, not focusing.

3. The finest grind focus panel is best (not a surprise) but acid ethed is even better.

Or am I off base?

[GPS]

Mike1234,
despite of what Jeffrey and Steve say (and at least one of them acknowledges that he doesn't understand the thread), there has never been any problem about the plain gg film registration on this Chamonix camera. You surely can check its distance if you want but it was the handling of the Fresnel that put them in troubles - and the technically unfortunate, incorrect and dishonest explanations afterwards...

[Clement Apffel]

You make a good point on some precise measurement being kind of a thread breaker.
The distances are within tolerance? End of the story.

But…..

I just pulled out the backs of my 2 4x5” cameras.
The Chamonix GG seems to be too close to the lens by something in the 0.2 – 0.25mm range. (that would be 0.08 – 0.09”)
The Cambo GG seems to be too far to the lens by something in the same range.

I do not own any of your measurement tools though. I only was able to make a makeshift measurement.
Actually, it only points out that none of my 4x5” backs is matching GG and film plane.

My point is that I would very likely know if any cameras that all the users here own are within the 0.07” tolerances.
And actually, even a 0.05” error means a 13cm error of the point of exact focus for a 150mm shot focused at 5m.
Or a 40cm (!!!) error with a 90mm lens in those same conditions
Or a 85cm (!!!) error with a 65mm lens again in those conditions.

If you shoot any of those wide open, you’ll be off everytime. Eventhough your back is within the tolerance.
But wait a sec…
What is the depth of field at f/5.6 with those lenses we are talking about ?

Some other basic calculations:
150mm focused at 5m @ f/5.6: 70cm DOP
90mm focused at 5m @ f/5.6: 200cm DOP
65mm focused at 5m @ f/5.6: 431cm DOP
And those calculations are done with an exigent value of the confusion circle.

So my guess is that none of our cameras are at the exact 0 alignment.
Or like 1 out of 100, and by luck. And not on the entire film plane.
I didn’t even mention the film planarity that will never be perfect on all the frame.
What is that error ? 0.03” ? 0.02” ?
A 48 – 31cm error with a 65mm lens focused at 5 meters.

But we don’t care. Cause even wide open the depth of field will cover that error widely.

My Chamonix is off by 0.25mm ? the numbers says it's out of tolerance standards.
But my tests performed under my more exigent condition of uses (i.e. wide open portraits at 4 – 5m distance with a 150mm) were in my (quite demanding) tolerance of sharpness.

So end of the story for me.

Numbers and 0.00....005" precision measurements are probably a good info to have.
But at some point you have to see that this precision is not achieved on any view cameras available today. why ? because we do not need such a precision.

The magic bullet chase again....

regards,
CA

[GPS]

But DOField is different from DOFocus.

Exactly!
Now, if everyone knew that.

Just to remind you...

[Clement Apffel]

Can you explain that to me more accurately with optics formula and field examples instead of that infertile and snobbish comment?
I'm sure you can achieve that.

And please do not ignore the end of my post.

Thank you in advance.

[Clement Apffel]

I just pulled out my dusty optics lessons. Cause the OP seems to be out for a walk, weirdly enough.
So for those like me who don't remember/know what is the difference between DOFocus and DOField, here is a quick definition.
(anyone correct me if I'm wrong, please)

• Depth of field is the distance in the object space within which the object point can move and remain sharp while the film being still.
  
  
• The depth of focus on the other hand is the distance in the image space within which the film can move and stay in the sharpness tolerance while the subject point being still.
  
  
• Basically we can sum up by saying that DOFocus is the image of the DOField given by the lens.

Now, GPS, can you explain me why this difference makes my previous post irrelevant?
Or maybe I misunderstood your “just to remind you” comment? Was it just a friendly advice to make sure that I knew the words for the things I was talking about?

In that last case I thank you, because indeed I forgot those names and definition.

But my conclusion stays the same.
Thank you again for your comment.

[Steve Hamley]

Clement,

Practically speaking, the DO focus decreases with focal length and the DO field increases. With long lenses the DO focus increases and the DO field decreases. That's why the GG placement is especially important with short lenses and some specialty wide angle cameras have fixed backs.

I have a few high-end cameras that could use some adjustment, so maybe a trip to Richard Ritter this winter. I also traded for a used Ebony SV810U a year or two ago, and the previous owner had a Maxwell screen installed and sent Bill the back. Bill told him the spacing wasn't quite right and had it adjusted. So no, price or brand does not guarantee accuracy. BTW, I did measure my Wehman and it seems spot on except maybe for a burr or two on the wood which could be taken care of fairly easily - but it's very close even so and likely wouldn't be an issue with the longer lenses used for 8x10.

I believe that checking the GG depth is a skill every serious LF photographer should develop, and it's almost essential with the older, non-standard formats.

Sorry for the brief reply, I'm off to take pictures!

Cheers, Steve