I have decided to start a new thread about the well known focusing problem with Chamonix 45N-1 camera so as not to discuss it in threads not fully related to it.
Here is the announcement of the problem together with its solutions (taken from Chamonix cameras website):
Announcement from Chamonix view camera regarding Fresnel lens problem of 45N-1
After repeated testing, it has been confirmed that the Fresnel lens of our 45n-1 cameras will cause focus shift of various degrees when used with certain lenses. The main cause of this problem is that the Fresnel lens is a convex lens with rounded bands; it has its own unique focusing and other characteristics. When you place a Fresnel lens between a ground glass and a lens, it may cause some kind of distortion of the light coming from the lens.
This is a very old problem. Past and current manufacturers of view cameras all have to deal with this problem. Different manufacturers have chosen different solutions to this problem:
1. Some manufacturers like Sinar use Fresnel lenses that can be taken off. This is due to the fact that a very large part of Sinar view cameras are used for commercial photography where large apertures are frequently used. 2. Some manufacturers put the Fresnel lens behind the ground glass. This solution avoids the problem of focus shift, but causes another problem: damages and scratches of the Fresnel lens as it is made of soft materials prone to scratches. 3. Some manufacturers use a Fresnel lens that has a small area in the middle that is transparent and non- magnifying to avoid the problem. 4. Majority manufacturers will tolerate this focus drift by putting the Fresnel lens in front of the ground glass. 5. Some manufacturers like Ebony use a special piece of glass which combines the Fresnel lens and ground glass into one.
In the design stage of 45N-1 camera, we did not plan to put a Fresnel lens on it. When the prototype of 45n-1 camera came to light, so many potential users asked that we put a Fresnel lens on the camera as a standard feature. So we decided to add a Fresnel lens to the 45N-1 camera. Before the production, I made a trip to Japan just to purchase the special ground glass with build-in Fresnel lens like the one Ebony uses. But manufacturers of these special screens in Japan refused to supply us for reasons unknown, maybe for fear of competition. So we had no other options but to choose to tolerate the problem of possible focus shift with certain lenses.
In my experience and from what I have learned from test results, it seems that lenses of different structure and lenses of different focal lengths will have different focus shift when used with Fresnel lenses. Certain lenses, for example, will produce slight focus shift when stopping down due to their unique structure of the lens. Based on this, I do not recommend that users of 45N-1 camera to change the position of the Fresnel lens as all cameras are tested for the accuracy of the Fresnel position before leaving our factory. By changing the position of Fresnel lens, one can’t solve the focus shift problem for all the lenses, only some of the lenses.
My recommendation for those of you who have this focus shift problem is the following:
1. Take off the Fresnel lens and use your camera as usual. 2. Return your focus screen to your dealer and our factory will modify it by drilling a hole of 16mm diameter in the middle of the Fresnel lens. This small hole can prevent or fix focus shift that may or has occurred. We have done repeated tests with this method and the result is very satisfactory. The small hole will not interfere with your composition on the ground glass and will easily prevent possible focus shift.
For all 45N-1 camera users, I sincerely apologize for this design error caused by my improper judgment. This warns us to be more careful in future and we will fix this error. For future 45n models, this 16mm hole on the Fresnel lens will be standard.
What is interesting on this announcement is the way how Chamonix tries at the same time to announce “we made a constructional mistake on our camera” and to induce in the reader a thought that the same mistake is found on other cameras too. Also, the whole text is ridden with technically incorrect statements.
Take the first paragraph – Chamonix doesn't say truth about their technical mistake. The cause of the focusing error on their 45N-1 camera is the fact that, originally, the camera was made with a ground glass focusing screen only. When Fresnel lens was added (causing a focus shift) the position of this sandwich in relation to the film plane was not changed thus adding a focusing error to the camera. The error is therefore made by leaving the gg-film plane distance unchanged.
But in their explanation Chamonix puts 2 different causes on the same level – the Fresnel lens focus shift in itself and the omission of Chamonix constructor to take it into account. Chamonix wants us to believe that their original mistake is due to the Fresnel lens characteristic and not their own omission. The two things are in fact different.
The different focus shift in Fresnel lens (due to the different focal lengths of taking lenses) is not of the same nature (and it is much smaller) as the original incorrect focusing distance introduced to their camera by not taking Fresnel in account at all.
Then the incorrect logic continues. n.1) Sinar has the removable Fresnel screen on the external side, not the internal one as the Chamonix camera in question. Therefore this screen is not removable in order to avoid a focusing error like the one present in Chamonix camera, contrarily to Chamonix instigation!
n.4) The majority of manufacturers tolerate this focus drift – or so Chamonix says. Wrong – the majority (except Chamonix) puts the Fresnel + gg sandwich to the correct focusing distance from the film plane in the first place. What is then tolerated then is the much smaller (in many cases not detectable) focus shift due to the different focal lengths of taking lenses. And even this shift is sometimes taken care of using different Fresnel screens for different focal lengths…
Then Chamonix steers us to believe that they became a victim of a Japanese manufacture who didn’t want to sell them their Fresnel + gg sandwich – “So we had no other options but to choose to tolerate the problem of possible focus shift with certain lenses”. What a pathetic lie! As if Chamonix couldn't focus their Fresnel + gg on their cameras correctly because of that!
And the incorrect mixing of two different focus shift causes goes on in Chamonix announcement – pathetically, Chamonix doesn’t recommend removing the incorrectly focused Fresnel lens because different lenses cause a different focus shift anyway… But what about the focus shift caused by incorrect Fresnel + gg positioning on this camera in the first place? Not a one word about it yet! So much to the honesty of Chamonix manufacturer… ”By changing the position of Fresnel lens, one can’t solve the focus shift problem for all the lenses, only some of the lenses” says Chamonix. Surely not, but you can put the sandwich in question to the right distance in the first place, letting then different focal lengths have their own (smaller) focus shift.
Only at the end, the writer – after leading us to believe that all sandwiched combinations have this error – suddenly apologizes for “this design error caused by my improper judgment”. Well, a design error? Surely. But what about the erroneous explanations afterward?