Chamonix camera 45N-1 focusing error

[Jeremy Moore]

WARNING: While Bill's screens are great, he will chew your ear off for hours on the phone, and it is very difficult to end the 'conversation'. Make sure you have plenty of time to kill, or fake some sort of emergency to get off the phone.

I'm planning on having it ordered for me as an XMas present. I'll give them the particulars and they can call

[lenicolas]

hi,
in your own experience, when does this focusing problem occurs?
i understand that it's only with wide lenses...
I just ordered a 135mm, should i pay extra attention?

[rugenius]

A large format short focal length lens is generally "wide" so that it can take advantage of the film real estate available. Having said that, the fresnel lens thickness and angular aperture of the taking lens are basic optical properties that determine how much shift is induced away from the film plane (properly and normally established by the GG plane as you adjust focusing and develop an image at the GG).
I don't know which 135 mm focal length lens you are referring to...
You probably have some noticeable error depending on the 135 mm lens in question. But I believe it is more dependent on the half angle from lens aperture to film/GG plane.
There are other factors to take into account such as how much of what you are shooting is intended to be in focus, Ex infinity vs macro work.
But this is all moot anyway if you just remove the fresnel, or place it between you and GG, or replace the stock fresnel with some other screen intensifier.
It's really not as big a deal as many people have made it out to be.

If your work requires the use of lenses and situations that develop problems with the stock Chamonix fresnel/ screen intensifier... then you will want to invest a bit more in accessories to accomplish your tasks.
That said,... since you don't know yet... why invent a problem...

[Bob Salomon]

"But this is all moot anyway if you just remove the fresnel, or place it between you and GG, or replace the stock fresnel with some other screen intensifier.
It's really not as big a deal as many people have made it out to be."

Yes it can be a very big deal if the camera designer designed the fresnel to be in front of the gg. If you just take the fresnel out and put it on top of the gg or just replace it with another behind the gg or just throw the fresnel away you will probably need the ground position of the gg adjusted as it expected a spacer the thickness of the original fresnel lens.

[Mike1234]

Bob, that is the problem with the Chamonix 4x5. They failed to take the thickness of the fresnel into account thereby throwing the focus position that far off straight from the factory. The end-user "solution" is to move the fresnel behind the GG... not an appropriate resolution, IMO.

[Peter K]

It's really not as big a deal as many people have made it out to be.

No it's not a big deal if one calculates the total focal-lenght of the combination of taking lens and fresnel-lens but also for every distance between the two lenses. The thickness of the fresnel-lens leave no remainder.

An auxilary lens, either a diopter-lens or a fresnel-lens, changes the total focal-lenght of the optical system, depending not only on the focal-lenghts of the two lenses - in this case the taking lens and the fresnel-lens - but also on the distance between the two lenses.

The correct formula is f_t= 1/( 1/f_m + 1/f_fl + d/f_m*f-fl), where the f_m is the focal-lengt of the main taking-lens and f_fl is the focal-lenght of the fresnel-lens, f_t is total focal length, and d is a spacing correction factor.

If a fresnel-lens is used before the ground-glass, it's influence is bigger as shorter the distance between the two lenses. So the focussing error is larger with WA-lenses focussed at infinity as with long focal-lenghts or long bellows-extentions.

To avoid all this calculations place the fresnel-lens behind the ground-glass.

Peter

[Michael Rosenberg]

"But this is all moot anyway if you just remove the fresnel, or place it between you and GG, or replace the stock fresnel with some other screen intensifier.
It's really not as big a deal as many people have made it out to be."

Yes it can be a very big deal if the camera designer designed the fresnel to be in front of the gg. If you just take the fresnel out and put it on top of the gg or just replace it with another behind the gg or just throw the fresnel away you will probably need the ground position of the gg adjusted as it expected a spacer the thickness of the original fresnel lens.

Bob,

This was already discussed in previous threads. The Chamonix fresnel lies in a recess below the gg - in other words the gg sits on ledges above the fresnel and does not rest on the fresnel. You can remove the fresnel in this camera and it does not affect the position of the gg. At All. Notta.

Mike

[rugenius]

Michael Rosenberg wrote: "You can remove the fresnel in this camera and it does not affect the position of the gg. At All. Notta..."

Thanks!!
Mike,... finally someone who gets it.

I imagine enough people that have griped about the issue hadn't realized that the GG position does not change because they may not even own one of these cameras.

No matter the current apology and explanation given by Chamonix...
My guess is,... maybe,... way back... Chamonix simply chose to protect the fresnel from scratches by placing it in front and had not considered how much of a fiasco they might have created... even if other manufacturers had done the same.

If they had stuck with the GG, and offered the fresnel purely as an accessory with a disclaimer involving fresnel thickness and focus shift with wide angle lenses of XXX mm and XX or greater angular aperture they would have saved everyone including themselves the headache of admitting to some sort of "design flaw".
And,... I'll bet people would have still bought the Chamonix fresnel/ screen intensifier because it would have been much cheaper than a new Maxwell screen.

[Larry Gebhardt]

No it's not a big deal if one calculates the total focal-lenght of the combination of taking lens and fresnel-lens but also for every distance between the two lenses. The thickness of the fresnel-lens leave no remainder.

An auxilary lens, either a diopter-lens or a fresnel-lens, changes the total focal-lenght of the optical system, depending not only on the focal-lenghts of the two lenses - in this case the taking lens and the fresnel-lens - but also on the distance between the two lenses.

The correct formula is f_t= 1/( 1/f_m + 1/f_fl + d/f_m*f-fl), where the f_m is the focal-lengt of the main taking-lens and f_fl is the focal-lenght of the fresnel-lens, f_t is total focal length, and d is a spacing correction factor.

If a fresnel-lens is used before the ground-glass, it's influence is bigger as shorter the distance between the two lenses. So the focussing error is larger with WA-lenses focussed at infinity as with long focal-lenghts or long bellows-extentions.

To avoid all this calculations place the fresnel-lens behind the ground-glass.

Peter

Peter, thanks for that explanation. I finally see why when I tested my 75 and 90mm lenses I could find no focus error. I was testing at very close distances, so the lens was extended quite a bit. Probably more than my 135mm at infinity. Must have masked the error. However I have taken over a hundred sheets with the Chamonix and have yet to see an issue in the real world, even on shots using the 75mm (always stopped down to at least f/16).

Has anyone tested moving the fresnel behind the ground glass and then putting a thin cover sheet of glass over that (a fresnel sandwich so to speak)? Seems like it would fix the problem and protect the fresnel.

[Darin Boville]

I have taken over a hundred sheets with the Chamonix and have yet to see an issue in the real world

Come to think of it, in all the threads about this issue and all the negative posts I don't think I've seen a sample image showing the problem.

--Darin