Chamonix camera 45N-1 focusing error

[r_a_feldman]

I observed the issue for the first time with a 150mm f/4.5.
portraits at 4m distance (around 13 feets I believe) wide open.
resulting error is the same on everysingle portrait :
point of focus systematically 1 full meter before the subject.

did tests with the exact same setting and lens but without the fresnel : point of focus was where I set it.

the reason I didn't post image is that we speak of quite fine focus/sharpness difference and those won't appear on a attachable 700x500 dot image.
and I guess I was too lazy to do crops and stuff.
Maybe other users felt the same way.

I find it hard to believe that the fresnel would cause that much of a focus shift (25% of the camera to subject distance), especially since we are talking about what should be less than a 1mm change in the lens to focus plane distance.

Anyway, I will shut up now and await the results Jeremy's tests.

Bob

[kev curry]

Small screw driver in hand I done a bit of quick 'dry' testing today to try and understand if my Chamonix had any problems. I tested to see if the position of focus changed in relation to the camera bed or in my case the focusing scale on the focusing wheel with the gg/fresnel in three different configuation...

(Omega 3.6x loupe)

1.fresnel nearest the lens with gg on top. As sold by Cham...

2. gg only no fresnel.

3. gg nearest the lens fresnel on top.

I tested 5 lenses. SWD 75mm/5.6. SA 90mm/f8. Apo Symmar 210mm/5.6. Commercial Ektar 8/1/2''/f6.3. Nikkor M/300mm Tested at about 2m, 15m and infinity.

The 75/90/8 1/2'' showed significantly different positions of focus between configurations 1 and 2 and most pronounced at 15m. No change at configuration 3.

The 210 and 300mm showed up slight differences between configuration 1 and 2 but far less than the other 3 lenses. No change at configuration 3.


I made a very fuzzy neg a few months ago with the Ektar and couldnt understand why. I just put it down to being careless...now I know what happened.

I also discovered I could easily live without a fresnel too!

[Mike1234]

Yes, let's all just enjoy the silence until real-world testing is available. Everyone breathe now... j-u-s-t -- b-r-e-a-t-h-e.

[Clement Apffel]

I find it hard to believe that the fresnel would cause that much of a focus shift (25% of the camera to subject distance), especially since we are talking about what should be less than a 1mm change in the lens to focus plane distance.

Anyway, I will shut up now and await the results Jeremy's tests.

Bob

you make a point.
as it was on "real" shots, the focus shift distance was only appreciated on the ground before the subjects.
so I guess there is no way to actually measure the distance.
But the point is it was before the subject.

maybe not a meter. but 50cm ? 30cm ? couldn't say.

Though, the error was systematically enough to put the focusing error aside.

waiting jeremy's tests as well

[shadowleaves]

Yes, let's all just enjoy the silence until real-world testing is available. Everyone breathe now... j-u-s-t -- b-r-e-a-t-h-e.

The real-world test - I did it long time ago. Just go back this thread and find my post with an attached pic, using a fresnel with a hole drilled in the center. It was shocking to see that obvious difference in focus position with and without the fresnel, but it's good to know, after all. According to Chamonix's annoucement, the pre-drilled fresnel will be standard on every new 045n1 made by Chamonix. That's a very reasonable solution as I see it. What's more to be worried about on this matter?

[Michael Rosenberg]

I had posted this information a few months ago:

Quote:
Re: Focusing problems with the fresnel groundglass on the Chamonix 45N-1?
I was a little baffled with this thread. Two people with the same camera having identical problems with a particular focal length lens. I would have thought that the Chamonix being a new camera the position of the gg/fresnel combination would have been tested. So I thought I would test my own Chamonix, and using mm ruler verify any small differences if they existed.

For a target I used a window mat on a mat board that would provide a bright line (reflecting the overhead light), and a dark line that would be a shadow from the overhead light. Both lines would be parallel to each other, and the target would also test flatness of the gg (i.e. there should be no change in focus of the parallel lines).

I checked three lenses, my 58mm Schneider XL, 90mm Linhof Scneider Angulon f5.6, and my Apo Sirornar S 135mm. All were tested with either the fresnel in the proper position, with it on top of gg, and no fresnel - just the Chamonix gg. I used a Scneider 6X loupe to check for focus.

With the 58mm lens the focal shift +0.5 mm with the fresnel in the proper place. With the 135mm lens the focal shift was +1 mm with the fresnel in the proper place. This should not make a big difference in depth of field unless shooting wide open I would think. However, with the 90mm lens the focal shift was +3.5mm with the fresnel in the proper place. There was no focal shift when the fresnel was placed on top of the gg. I have tried 2 other gg that I have, and made the same observation.

One possible explanation is the focal length of the fresnel is affecting the focus of 90mm lenses. I do not have another fresnel to test. But I think hereafter I will keep my fresnel on top.
End Quote:

I also followed up that I had put a Maxwell screen from another camera (that I had sold) on the Chamonix and saw no focus shift. I had a thicker cover glass so I cut shims (hose washers) for the gg hold-downs.

I honestly cannot see in any of my images that there is a focusing issue, but I generally stop down f22 or greater. I have had my camera almost two years. I think the only way this issue affects me is maybe where dof was critical and objects closest to me weren't as sharp as I may have thought - but that maybe operator error.

Mike

[Clement Apffel]

I find it hard to believe that the fresnel would cause that much of a focus shift (25% of the camera to subject distance), especially since we are talking about what should be less than a 1mm change in the lens to focus plane distance.

Anyway, I will shut up now and await the results Jeremy's tests.

Bob

I just did some basic optical calculations and here are the numbers :

with a 150mm lens. when focused on a 5m away subject, a 1mm focus shift towards the user means a 86cm focus shift in the "object space"
and a 0.5 focus shift is a 47cm shift.

4m away subject, 1mm focus shift is a 56cm shift
and 0.5mm focus shift is a 30cm shift.

[Mike1234]

That's certainly enough to make a difference....

[mortensen]

After reading this thread and many others it seems that many believe fresnel lenses are actually unnecessary elements that are counter-productive to accurate focusing and often inhibit more than they help. Or am I missing something? So why are they so common if they suck? After reading many posts I get the strong feeling that the fresnel SHOULD be mounted to the outside (viewer side) of the GG and that we should keep an array of different FL fresnels to accomodate specific FL ranges of lenses and the appropriate fresnel should only TEMPORARILY be placed on the GG as needed to compose/focus. Does this sound right to those in the know?

I second that question!!! It would be really nice to hear some opinions from the experienced folk around here. Or is it a topic for a new thread, Mike?

I've taken my fresnel out on the 'right' side of the GG of my 45n-1, but still haven't found any mechanism that gives me the ability to put it on, when demanded only (I'm not going for the paperclip trick, though it probably works fine). Haven't tried shooting without the fresnel yet - we only have around six hours of daylight in DK currently... and those are working hours.

[Sal Santamaura]

...It would be really nice to hear some opinions from the experienced folk around here. Or is it a topic for a new thread...?...

No, it's a topic for a multitude of old threads.

I can only speak for myself, but suspect that other experienced folk around here also tire of the forum being used as a chat room rather than the rich resource it is. Perform a Google advanced search under the domain largeformatphotography.info and you'll find a wealth of answers to these repeated questions.