Chamonix 451N focus problem
On LF forum, there are a few threads around some issues with the way Chamonix has installed their fresnel and GG's. Apparently, there is a focus shift after focusing with the factory installed fresnel, which is installed in front of the GG, and then inserting the film holder. The focus plane shifts to make noticeable lack of sharpness on some images. Especially when shooting at larger apertures. Apparently, by moving the fresnel screen to the back, the problem should be mitigated. I am not fond of the vitriolic nature of LFF so I am asking my question here. I was wondering if the more friendly Chamonix users on APUG have seen this problem on their Chamonix cameras?
I'm not a chamonix user, nor do I use fresnels, but... I think, generally, a fresnel in front (lens side) of the GG will necessarily shift the focus... roughly in proportion to the thickness of the fresnel. It's a lens, of course it will shift the focus. That's nothing new. I guess the solution is to (1) use a thinner fresnel, or (2) figure out the offset for a particular focusing distance and correct for it, or (3) put the fresnel on the back, or (4) my favourite option, don't use a fresnel at all when focus is extremely critical. Obviously for infinity focus it's not going to be as big an issue as for those ultrashallow DOF portraits that some people do.
Maybe I'm wrong but I think any fresnel is going to shift the focus somewhat. It's all a matter of what's acceptable.
P.S. Ah, here is a useful old thread, the last comment is very good one, the best solution seems to be a partial fresnel GG. Pricey, but if you really want to do it right...
So with the sinar screen you'd do general composition via the fresnel portion and then do critical focus via the non-lensing portion.
Anywya I don't care for fresnels, they annoy my eyeballs!
There have been some backs, like Graphic backs, where the fresnel is designed to fit in front of the groundglass and is calibrated to function that way with the groundglass surface still at the film plane, but I'd be very surprised if this were true of the Chamonix. I don't recall precisely, but the Linhof Tech III may have had the fresnel on the lens side of the groundglass.
You could always measure and compare the depth of the back to the groundglass surface to the depth of a filmholder with film in it.
It does make some sense to put the fresnel in front, since it's usually plastic and easily scratched, unlike the groundglass, but it makes calibration much more complicated, and you can't easily swap out the fresnel for another one, say if you want one optimized for wide lenses, so this is rarely done on any LF camera anymore. I can't think of any contemporary LF camera with the fresnel on the inside.
SLRs, though, often have the fresnel on the inside, or have a combination screen with the fresnel portion on the bottom.
Horseman FA field camera has the fresnel on inside.
Originally Posted by David A. Goldfarb
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I'm not a Chamonix user, but I am sure you have seen the announcement on their website. The "fix" they describe seems like a stop-gap until they solve the underlying issues. The problems are serious enough that you would think a more substantial remedy should be a priority. Their apology and promise of a fix in the next model is small comfort for owners who have to make alterations to their cameras or shooting techniques in order to achieve acceptable sharpness.
Cheers... hope you are doing well. Hope to see you next June in Michigan.
Tom, on Point Pelee, Canada
Ansel Adams had the Zone System... I'm working on the points
system. First I points it here, and then I points it there...
Thanks. Forgot about that one.
Originally Posted by ic-racer
Keith, I'm not sure this is true or at least a comparable situation. I'm reasonably certain that Sinar did not put a clear spot on the fresnel because they mis-spaced the image-forming surface! I am assuming that the Sinar fresnel does not have a frosted side that faces the lens. If this assumption is true, you can easily see that the Sinar screen will also produce a mis-placed image forming surface by considering the "absurd" case of a 1-foot thick clear screen with the image forming surface behind it.
Originally Posted by keithwms
Andrew, there's good news and bad news. The bad news is there isn't going to be any "fix" for already manufactured cameras to use the existing fresnel in front of the GG. You can put it behind, and it will get scuffed up sooner or later, likely sooner. You could "calibrate" the focusing knob to re-position the film holder after critical focusing, but that's non a good solution (although workable) IMO.
The good news is that there are several perfectly workable solutions, and the Chamonix frame is apparently made to the same design specs (haven't measured one though) as an Ebony and most every other view camera for a century or more. Chamonix just put a clear fresnel in front of the frosted GG where it was not designed be according to the way the frame was actually manufactured. I'd much rather fix the fresnel or GG than the frame. I'd actually be more concerned if they "fix" the frame where a different fresnel or a standard GG would be mis-space the image forming surface rather than the current situation.
The Ebony fresnel is frosted on one side which faces the lens, and a clear cover glass goes behind it to protect the fresnel. A Maxwell can be made either way, or a Steve Hopf GG (sells on eBay as "photofixation" I believe, and has GG store) would be nice if you don't care for fresnels.
The important thing to remember is that the image-forming surface HAS to be against the GG frame in the Chamonix case, otherwise it's mis-spaced, and that mis-spacing may or may not show effects in a given situation given the many variables.
Sounds like a recall is in order but I highly doubt they'll do it. "We can't fix it", that's not a very good announcement!
Originally Posted by Toffle
I did the ruler test (photographing a ruler on a slant) with my 90mm 4.5 nikkor and my 210mm fujinon and couldn't see any issues. I also did a test with a fujinon focus chart and found that it was sharp with both lenses. Both those tests were with instant film, however. Maybe I should shoot some sheets with and w/o to be sure. My negs from normal shooting seem to be sharp when enlarged though.
Update- I just tried the fujinon back focus chart test again. I set up the chart on the wall about 4 ft from the film plane. This time just with a 8x loupe, no film. The 210mm checked out fine with and w/o the fresnel installed. The 90mm nikkor 4.5 requires an additional millemeter (aprox.) of bellows to focus w/o the factory fresnel installed.
Last edited by wildbill; 10-18-2009 at 12:03 PM. Click to view previous post history.
Reason: did some tests
I know what I want but I just don't know how to go about gettin' it.-Hendrix
Vinny, I am wondering if I just remove the fresnel, put the GG back, and cut down one of those cheap map reading fresnels from Staples to fit the correct size on the Chamonix? I do know the GG is larger than the fresnel itself so reusing the stock one is not really an option as it keeps falling off! So if I cut down the map reading one, I could make it the right size, and so it is held down by the stock mounting screws/washers.