Chamonix 451N focus problem

On a Linhof Technika, the fresnel is to be placed on the rear after the ground glass, so lens focus directly on GG. Grooved side of fresnel towards GG. The fresel is retained by help of a spring loaded clip in each end of GG length. Fresnel is convenient for eye focusing and needed when making light-metering direct on GG (according to owners manual). For this measurement, set meter f to 1.0 and set lens f to recording setting. Make measurement for exposuretime.

I suggest you check depth of film holder to distance of GG from film holder face with a caliper, and then try to rearange fresnel, f.x. turn it so opposite face faces GG or, if possible set it on outside GG.

Regards, Morten

The more I have thought about this, the more naïve I feel about understanding the optics of Fresnels. I likely was incorrect in my prior post. I assume that if the Fresnel is on the inside of the ground glass as with the Chamonix, that the plane of focus on the Fresnel/ground glass combination is different than the plane of focus on the film. If so, is the difference between the two identical to the thickness of the Fresnel? If this is the case, it should be easy to measure the thickness of the Fresnel and then calculate the effect for different focal length lenses focused at different distances and at different apertures. If the focusing problem only relates to the thickness of the Fresnel, then one solution would be to adjust the distance between the lens and the film plane by that amount.

On the other hand, I am not sure that the problem is that simple. Does the image, in fact, focus precisely on the front surface of the Fresnel or at some place within the grooves or perhaps on the back surface of the Fresnel, which would be the same as the surface of the ground glass? Does it matter if the grooves are facing away from the glass or pressed against the glass? Does the fact that the Fresnel, itself, acts like a lens effect the position of the plane of focus by creating an aerial image and if so where is that located and how is it different than the plane of focus of the film?

To summarize, am I correct in assuming that the image focused with the Fresnel is in front of the film plane by an amount identical to the thickness of the Fresnel or is there more at work here?

Its not about the thickness of the fresnel but more about the light transmission/lens effect of the fresnel. The focus/film plane is checked at the factory with the GG installed. Then the fresnel is installed in front of the GG, shifting the focus by a small amount. The amount of focus shift varies based upon the focal length of lens being used and is not a constant amount regardless of focal lengths used. The constant is the GG internal surface which translates to the same as the film plane.

Stick the fresnel behind the GG, which is what I did with my Crown Graphic. Get an acetate cover sheet (used for reports etc) from Staples & cut to size if you want to protect the fresnel.

Ian

I hope this doesn't offend anyone who has one of these but I expect used cost of this model Chamonix to be affected. I'll just buy a used one at a good price and replace the defective GG/fresnel.

Mike, I suspect it will hurt resale values. So you should be able to get a good deal. I have tested mine and the the fresnel makes no noticeable difference, even with the 90mm lens. Shots with the 75mm also come out sharp, but I haven't tried it wide open. Worst case you end up with a camera without a fresnel - not really a big deal.

L Gebhardt said:

Mike, I suspect it will hurt resale values. So you should be able to get a good deal. I have tested mine and the the fresnel makes no noticeable difference, even with the 90mm lens. Shots with the 75mm also come out sharp, but I haven't tried it wide open. Worst case you end up with a camera without a fresnel - not really a big deal.

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Thanks for sharing your experience. However, I'm assembling a dedicate 6x12cm kit based on a lightweight 4x5 body and the widest lens is a 38mm SA XL so I'll need a good WA focus panel.

Mike1234 said:

Thanks for sharing your experience. However, I'm assembling a dedicate 6x12cm kit based on a lightweight 4x5 body and the widest lens is a 38mm SA XL so I'll need a good WA focus panel.

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You would definitely want to replace the stock fresnel then. It doesn't work very well with the 75mm lens (double images and seeming blurring in the corners). Getting a fresnel designed for such a wide angle lens would make sense. Placing it behind the glass would also be more convenient if you want to change fresnels with the lenses as some suggest you should.

L Gebhardt said:

Placing it behind the glass would also be more convenient if you want to change fresnels with the lenses as some suggest you should.

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Hmm... I hadn't thought of that and missed those posts. I figured I'd just buy a second back if I find that need to switch focus panels but switching fresnels is certainly more economical. However, as others mentioned damage is much more easily done to an exposed plastic lens. Has anyone mentioned whether or not the fresnels work just as well mounted to the exterior of the GG?

Earlier this years I put a fresnel on my Crown Graphic behind the screen, I can use my 75mm f8 SA easily now, although it's going back on my 6x17 again next month, where it's the only lens I use.

Ian

There has been a rash of people (well, specifically one person) who do not own a Chamonix haranguing over on the LFForum about this issue. Because of this I am conducting a test this evening on film to see what part the fresnel plays in the final image. I don't want to deal with posting this and keeping up with discussions in multiple locations so I'm just going to provide a link from this thread to the one over at the LFForum detailing my testing procedure, asking for any suggestions you may have, and will serve as the repository for the resultant photographs.

http://www.largeformatphotography.info/forum/showthread.php?t=56511

Shoot on rock-solid ground, not on the 2'nd floor, or someone will cry foul. Ehh... they probably will anyway.

Mike1234 said:

Shoot on rock-solid ground, not on the 2'nd floor, or someone will cry foul. Ehh... they probably will anyway.

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Not a problem. I'll be making the test in the basement of a building where I regularly do large scale art reproduction.

Jeremy said:

There has been a rash of people (well, specifically one person) who do not own a Chamonix haranguing over on the LFForum about this issue. Because of this I am conducting a test this evening on film to see what part the fresnel plays in the final image. I don't want to deal with posting this and keeping up with discussions in multiple locations so I'm just going to provide a link from this thread to the one over at the LFForum detailing my testing procedure, asking for any suggestions you may have, and will serve as the repository for the resultant photographs.

http://www.largeformatphotography.info/forum/showthread.php?t=56511

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Thanks, Jeremy.

I've been following the rants over there for some time. (I've been a lurker there for a couple of years, but every time I start thinking about joining a discussion, a flame war breaks out, and things get nasty... besides, I'm afraid that on my first post, people are going to think that I'm the newest alias for a certain W.L. touting the wonders of a certain camera modification. )

In terms of the Chamonix discussion on this board, even though I do not use that camera, I have had some things to say. (mainly in terms of customer service) I don't like to see things get warped by personality issues, though.

I'll be watching to see how your testing is received over "there". I hope you'll have some pictures to support your findings to finally give creedence to one side or the other.

Cheers,