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Message |
    
Morizk
Tinkerer
Username: Morizk
Post Number: 5
Registered: 11-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 08, 2012 - 02:30 pm: | 
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Hello there!
Today I had a look at how well my Canon New F-1 acutally focuses, because with my scanned negatives they often seemed a little off focus. I used a ground glass plate on the film rails and had to realise, that focusing in the viewfinder did not match the focus of the film perfectly. I then tried the same thing with my Canon AE-1, just to find the same problem. Then I tried a newly serviced Leicaflex SL-2, just to find out, that there was the same problem again.
I wonder if there is such a thing as a tolerance for focusing in cameras? From my understanding the focus should be the same in the viewfinder and on the film plane??
What do you guys think? Should i bring my cameras to a repair shop? Or are small differences in focus "normal" for mass produced cameras?!
I really wonder, and I hope someone can help me with that issue
With best regards
Moriz K. |
Ismaelg
Tinkerer
Username: Ismaelg
Post Number: 94
Registered: 11-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 08, 2012 - 08:44 pm: |
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Hi,
How are you scanning the negatives? I think your problem could be in the scanning and not in the camera. If your test gave you the same results in 3 different cameras, I don't think there is anything wrong there.
I would do the following test:
Using a known sharp fast lens (a fast 50 comes to mind) take pictures with very narrow depth of field (wide aperture) of a pattern. you can use a dozen or so AA batteries lined up perpendicular to the camera (from close to it going away from the camera). Focus in the middle one. The result should be that the one you focused on should be properly in focus while the others (farthest and closest) out of focus. In the scan, if you see that other than the one you selected is in focus, yes you have focusing issue (back focus or front focus). But if the whole image is soft, your problem may well be in the scanning process itself.
Thanks,
Ismaelg |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 1257
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 08, 2012 - 09:53 pm: |
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Your camera screens might be adjusted to match a plane of focus that's slightly behind the film rails instead of being perfectly in the plane of the rails. This is because the film has a slight curl to it, and sometimes the designers assume a small amount of curvature of the film in the focus adjustment. If you find a number of good cameras all appearing to be off in a similar way, I don't know if I'd rush them all off for repair - it could get expensive.
And to your other question, there is a tolerance for EVERYTHING in EVERYTHING. There is no such thing as +/- zero in the real world. |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 84
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 08, 2012 - 11:10 pm: |
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At very best the minimum film "pixel" is about 15 microns diameter, and the depth of focus tolerance is 2*(15 microns)*f/#, so for f/2 that's 60 microns. You'd probably never notice even twice this except maybe while taking boring photos of resolution test objects, so to really do this test well you'd have to put the ground glass within +/- 50 microns or so of the actual film plane, and then view the image from the back at very high magnification. That's tight, so I'd suspect your comparison technique with the ground glass before deciding that three different good cameras are all off. ;-) |
Morizk
Tinkerer
Username: Morizk
Post Number: 6
Registered: 11-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 03:41 am: |
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Thanks for all the replies!
The ground glass was taken from an old SLR, so it has a split image indicator on it. Thats how i could see, the focus plane was not the same. Anyways, I'm gonna shoot some test photos as Ismaelg suggested and then I'll see how that will come out.
I never thought, that the film plane was acutally behind the film rails... is that really so? shouldnt the pressure plate press the film rightly on the rails? |
Zombicams
Tinkerer
Username: Zombicams
Post Number: 2
Registered: 12-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 04:41 am: |
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The film actually travels between the rails so placing the GG on the rails will provide inaccurate results. |
Morizk
Tinkerer
Username: Morizk
Post Number: 7
Registered: 11-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 06:41 am: |
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On the Canon New F-1 there are 4 rails, 2 outer ones and 2 inner ones. The film travels on the two inner ones and that was where i put the groundglass. From that point of view, the test should have been accurate.
But theoretically it would be possible for the mirrorangle to change slightly over time, right? That was my first guess when i saw the mismatch of fucus. |
G3bill
Tinkerer
Username: G3bill
Post Number: 19
Registered: 12-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 07:34 am: |
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I'm wondering if the etched side of the glass was towards the rails? Interesting issue! |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 1258
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 09:10 am: |
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Yes, any change in the mirror stop position will affect the focus of the viewfinder. But to have it happen exactly the same on every camera you own seems like an uncommon coincidence. |
Morizk
Tinkerer
Username: Morizk
Post Number: 8
Registered: 11-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 11:01 am: |
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I had the etched side of the glass against the rails, as the film emulsion is also towards the rails.
Could anybody test their SLR? I wonder if it has to be like this in order to get the focus on the film plane...
Is there another way to test the focus of my camera? |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 85
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 11:04 am: |
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Note 100 microns is the thickness of 35 mm film, so that's the level of positioning tolerance of the lens relative to the film plane and of the focus screen relative to the lens that a camera manufacturer would want to aim for in order to maintain the ultimate sharpness and build a reputation. It would be challenging to verify this without special equipment including an autocollimator. If it seems way off, much more than this, then either something is grossly wrong or there's an error in the test - more likely the latter. ;-) |
G3bill
Tinkerer
Username: G3bill
Post Number: 20
Registered: 12-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 01:13 pm: |
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Could the thickness of the ground glass fault the test? I long ago did a test of my F-1n like Ismael's idea except used a tape measure and set a pencil at about the 18" mark for reference as my point of focus, I cant remember the result but if not perfect was close. I used a 1.8/85mm , dont think it was off but usually focused on the eyes which sometimes got me a blurry nose so focused on the bridge of the nose just in front of the eyes, think that was better, esp if someone had a huge nose |
M_currie
Tinkerer
Username: M_currie
Post Number: 292
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 09, 2012 - 10:22 pm: |
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An apparent computer glitch erased my message last night but I was responding more or less as IsmaelG did, that what I think you need to do is first to look at your negatives very closely, or shoot a roll of slides. Scanning is often not that close. If the results don't suit, then do a test of the camera.
I was going to suggest that you get a tripod, and a brick wall or something similar, and place it so the film plane is exactly 8 feet from the wall. You can then test with the lens set to the 8 foot mark, and using eye approximation, take some more pictures at 1/4 or 1/2 foot increments to see how the focus varies. But Ismael's row of batteries is more economical. I would still set the camera so that your focus point is at exactly some mark such as 8 feet, so that you can also check whether the lens and the finder agree. If the distance and lens marking are exactly 8 feet, but you must change the focus to get a clear view, something is wrong. Make sure you write down everything you do, if you take more than one picture.
The other thing not mentioned here is to make sure you are not developing a vision problem that makes focusing difficult. If the finder has any lines, circles or other marks directly on its surface, you must see those very clearly. If not, you cannot trust what the ground glass says. |
