| Author |
Message |
    
Brucegarrett
Tinkerer
Username: Brucegarrett
Post Number: 1
Registered: 12-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 17, 2010 - 05:13 pm: | 
|
I have a Canon F1n I bought second-hand a couple years ago, in excellent working condition and which looks practically brand new. But I discovered a problem I've never seen before in a 35mm SLR and I was wondering if anyone here has ever seen it and could possibly give me some insight into what might be causing it.
The problem basically is the camera does not show the correct focus in the view finder. For a while I thought it was my aging eyes and I just wasn't getting the focus right. The resulting images all had the focus just a little bit off.
One day, getting frustrated, I set the camera on a tripod and focused on a middle distance object, noted the distance indicated on the lens, then swapped that F1 body for one of my others...same lens, same object. Now the lens says the object is several feet closer.
I tried this experiment with the other F1 body, and with several different lenses and focusing screens. The result was always the same. This one camera always did not agree with the other two as to how far away the object I'm focusing on is. That would explain why the photos I take with this camera are always a bit off focus.
When I'm in focus with this camera according to the viewfinder, I'm actually focused several feet behind whatever I'm focusing on according to the resulting photos. Infinity is always very slightly off depending on the lens. It was something I hadn't notice because I normally shoot with a 24mm. But it's very noticeable with a 135mm and higher.
I can't imagine what is out of adjustment here. It isn't a bad focusing screen unless all three that I have for it have the same problem. It isn't the lenses...they all work fine on the other F1s. All I can think is it looks like there is a shim that sits under the focusing screen and maybe they come in different thicknesses and that's how the factory gives the camera a final adjustment before shipping and the wrong one is in mine. But that's just a guess.
Any ideas? Is this a problem with this model F1? My others are an original F1 and an F1n. And...pointers to someone who can repair this camera would be appreciated. It's a real nice camera, but unusable if it doesn't show correct focus. |
Chiccolini
Tinkerer
Username: Chiccolini
Post Number: 119
Registered: 06-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 17, 2010 - 06:32 pm: |
|
The mirror shifted or the mirror doesn't flip all the way down and seat properly. Repairs are best at SPTS (Strauss Photo Technical Services) in Wash., DC (check the website). |
Mareklew
Tinkerer
Username: Mareklew
Post Number: 213
Registered: 03-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 17, 2010 - 06:36 pm: |
|
Focus distance is influenced by the mirror angle too.
Since you get back focus, it means that the ground glass is closer to the lens (optically) than the film. Either the focusing screen isn't seated properly, the shim is too thick, or the mirror doesn't go all the way down.
You may calculate the amount of error by using a prime 50mm lens and the lens formula:
1/x+1/y=1/f
set the lens scale to a close distance that is clearly marked on the lens scale - e.g. 2m. Focus on the faulty camera by moving towards and away from a target (use tripod). Take measurement band and find the true distance to the object.
Now you have:
d=2m (what you set your lens to be focused)
d'= say, 1.8m (where it appears to be focused)
the 'y' in the lens formula is distance measured from the lens front nodal point to the subject. Distance on the lens scale is distance from film plane
1/x + 1/(d-x) = 1/x' + 1/(d'-x')
with sufficient accuracy as x<<d we may assume x=f in the "d-x" terms:
1/x + 1/1.95m = 1/f = 1/x' + 1/1.75m
and
1/x + 1/1.95m = 1/0.05m
so
x = 1/(1/0.05m - 1/1.95m) = 51.316mm
x' = 1/(1/0.05m - 1/1.75m) = 51.470mm
difference would be ca. 0.15mm
If you had problem following the math, just type this in google search window:
1/(1/0.05 - 1/(d-0.05))-1/(1/0.05-1/(d'-0.05))
putting in place of "d" and "d' " measured and set focus distance.
I suggest you use a 50mm lens, as these are usually neither tele, nor retrofocal design and so they fulfill the lens formula. Avoid superfast lenses (faster than f:1.8) as they may shift focus when stepped down and you won't know what does the lens distance scale is marked with.
Marek |
Gez
Tinkerer
Username: Gez
Post Number: 240
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 18, 2010 - 03:26 am: |
|
It is probably the mirror angle setting, but it is also possible that despite the camera having a pristine appearance it was worked on. Canon generally did not use shims to fine tune the mount to film plane distance, but a 'repairer' may have lost or incorrectly replaced any original shims. To accurately check this distance you will need a depth micrometer. |
Glenn
Tinkerer
Username: Glenn
Post Number: 874
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 18, 2010 - 09:11 am: |
|
One can chuck a whole reference library of theory at the problem, but I suspect it ain't going to be one jot of help to Bruce. His original appeal for help indicates that he has no experience of the mechanics of this problem - and why should he if he is a user/collector rather than a collector/repairer; however, the theory is only of any use if one uses a 'standard' calibrated lens with an accurate focus scale. It does not work where you have the possibility of two variables/unknowns in the equipment being tested - in the above case there is one unknown in the mirror angle or screen adjustment, (most likely the mirror is at fault) but when one adds the 50mm lens into the practical testing procedure you may bring in another unknown. Unless you have actually checked, there is no certainty that the values on the focus scale correspond to actual measured values- leaving one going round in circles.
My advice to Bruce is to follow Chiccolini's excellent advice - an experienced repairer will sooner be able to correct this problem. |
Stratokaster
Tinkerer
Username: Stratokaster
Post Number: 1
Registered: 12-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 18, 2010 - 04:19 pm: |
|
I have exactly the same problem with 2 of my Canon FD bodies (A-1 and AE-1 Program). They both exhibit slight back focus. The problem is especially apparent when I focus the lens to infinity - the viewfinder indicates that distant objects are not in focus, when in fact they are perfectly in focus.
I guess it may be one of common failures of Canon FD bodies. I never had such a problem with my Pentaxes... |
Glenn
Tinkerer
Username: Glenn
Post Number: 875
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, December 20, 2010 - 06:55 am: |
|
It certainly isn't a common failure in FD mount bodies - If the screen/viewfinder focus does not correlate with the image focus on the negative there is a fault with that particular body. Many of these cameras have been working away for the last 30 odd years with no thoughts of maintenance from the owners, so mirror alignment can become an issue.
Correction of this type of problem requires a methodical approach as per the manual - one should be aware that the type of screen fitted and the user's eyesight can lead to false impressions. |
Brucegarrett
Tinkerer
Username: Brucegarrett
Post Number: 2
Registered: 12-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, February 06, 2011 - 11:56 am: |
|
Just a note to thank everyone here who responded for their help. As I said in my post I just could not imagine what the problem was. The suggestions that it might be a mirror issue and take it to Struass were especially helpful. Having grown up in the Washington D.C. suburbs, Strauss was an old, familiar and very well trusted name. I was told by one of my local camera stores ages ago when I had to take a different F1 body in for repairs, that Strauss no longer took in F1 series cameras, which was why I didn't just take my camera straight to them. The guy at Strauss I talked to told me that camera store (since gone out of business) had stopped doing business with them. Live and learn.
My camera is back from Strauss now, in cherry condition and focusing correctly. Thank you all very much! |
Br1078lum
Tinkerer
Username: Br1078lum
Post Number: 50
Registered: 11-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 07, 2011 - 11:52 am: |
|
So what did the problem turn out to be?
PF |
