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Chenard
Tinkerer
Username: Chenard
Post Number: 10
Registered: 11-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, April 25, 2007 - 02:03 pm: | 
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Hi Everybody,
I have an Agfa Isolette III, with the 85mm f4.5 Apotar lens and the Prontor-SVS shutter. I've CLA'd the shutter and rangefinder, unstuck the focusing helix, and everything seems to be working as it should. But I'm having trouble getting the focusing scale calibrated.
I initially set the scale at infinity by carefully focusing on a distant object with a ground glass. But, then when I use the ground glass to check the focus on an object exactly 4 feet from the film plane, the distance scale reads 3.5 feet. That's quite a bit of rotation from 4 feet--about 20 degrees.
On the other hand, if I calibrate the distance scale while focusing on an object 4 feet away, then it becomes impossible to focus on anything farther away than 30 feet or so.
If I check the close focus with the lens stopped down a little bit, the scale error increases instead of decreasing.
Keeping in mind that I've had the whole thing completely disassembled, I'm trying to think of assembly mistakes I might have made that would cause the linearity of the focusing scale to be wrong. Perhaps something is wrong with the geometry of my reassembled camera?
The camera is producing reasonably good images, btw. Ground glass focusing with this camera has kind of a weird quality though. Small details don't tend to pop in and out of focus as I've seen with other lenses. They sort of fade in and out over a larger rotation of the helix.
Any ideas? |
Charlie
Tinkerer
Username: Charlie
Post Number: 61
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, April 25, 2007 - 05:40 pm: |
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Are you performing these checks with the lens wide open? What happens when you set the lens at infinity while focussing on an object at infinity (say at least 100' away)? |
Chenard
Tinkerer
Username: Chenard
Post Number: 11
Registered: 11-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, April 25, 2007 - 05:54 pm: |
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Yep, I'm performing the checks with the lens wide open. This is all happening with the lens set as you suggest--scale set at infinity with the lens focused on an object at infinity.
I did try checking the close up setting with the lens stopped down a bit, because I'd heard that the focus might shift with aperature at close distances with the apotar. I found that it did shift, but in the wrong direction! |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 251
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, April 25, 2007 - 09:07 pm: |
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You didn't remove any of the individual elements from their mounts, did you? It sounds like you may have an element in backwards. The only explanation for the focus scale error is that the focal length of the lens is different from what it was originally, and the focus shift on stopping down would suggest severe spherical aberration - both of which might be caused by a reversed element. |
Chenard
Tinkerer
Username: Chenard
Post Number: 12
Registered: 11-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, April 25, 2007 - 10:11 pm: |
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Actually, both the rear and center elements have been out of their mounts.
I installed the center element with the more concave side towards the back of the camera, and flatter side towards the front. The machining of the lens sleeve and the bevel on the concave side of the lens seem to suggest this is the way to do it.
I installed the rear element with the flatter side towards the front, and the more convex side towards the back. There's not a lot of curvature to this lens and it is very thin.
The center element is thicker, and the rear element is thinner.
Does this sound right?
Additional info that might support your spherical abberation theory: I mentioned that objects don't pop in and out of focus, but rather tend to fade in and out. When they do this, I think I can see them changing shape as well, from being stretched along one axis to the other.
If the elements are in fact installed the right way around, can you think of any assembly errors I may have made that would change the focal length?
THanks! |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 252
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 26, 2007 - 04:50 am: |
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The rear element sounds correct, not sure about the center - these are often so small and so steeply ground that it's hard to tell which way is which. I'd try flipping that one and see if it helps.
The main other possibility is the spacing being wrong, if one element is not fully seated in the mount... but a reversed element seems more likely from the sound of it. |
Alex
Tinkerer
Username: Alex
Post Number: 31
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 26, 2007 - 06:05 am: |
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This is fairly similar to the problem I encountered after taking apart an Isolette with Solinar to clean it. I inadvertantly put the centre lens element (the one behind the focussing element) back in the wrong way round, and found I couldn't calibrate the focussing. Turning the middle element round the other way fixed it. |
Chenard
Tinkerer
Username: Chenard
Post Number: 13
Registered: 11-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 26, 2007 - 01:57 pm: |
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Okay, I've tried flipping the rear and center elements and combining them in all of their combinations. The combination I described above is the only one that works well at all.
Here's a question: Is the distance measurement supposed to be taken from the film plane, or from the lens? At distances less than 4 feet, the 4" or so displacement between the lens and film plane become significant in terms of how much the helix has to be rotated. Plus, the rangefinder and lens scale readings are different, but both are correct for their respective distances from the subject.
Could it simply be that I'm measuring from the wrong place? |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 254
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 26, 2007 - 05:16 pm: |
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Standard practice is to measure from the film plane, but I don't know if this was followed in all cameras. |
Chenard
Tinkerer
Username: Chenard
Post Number: 14
Registered: 11-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, April 30, 2007 - 10:39 am: |
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Thanks for all of the thoughtful replies.
After reading Roland and Caroline's website more carefully, I see that they are quite specifically saying to measure from the lens:
<http://www.rolandandcaroline.co.uk/focadj.html>
Given this, when I focus the lens wide open, with a ground glass, on objects 4feet from the lens, the scale reading comes out fairly close to but not exactly 4 feet. There is some wiggle room here because the image doesn't pop in and out of focuse suddenly, but tends to come in and out gradually as I mentioned before. Not too far off, though.
The interesting thing is that when I stop the lens down to f8-f11, the plane of focus shifts considerably, so that the sharpest image results in a focus scale setting of about 3.5 feet, or about a 20 degree rotation of the helix from 4 feet.
I repeated this experiment with another camera fitted with a f3.5 75mm Solinar lens, and I noticed a similar thing. Close up and wide open, the image is in best focus fairly close to the scale setting, but stopped down, the lens must be focused a significant rotation closer to get the best image.
In both cases, the sharper image obtained by shifting the focus setting was significantly better, as one would expect when stopping down.
Is this a property of front focusing lenses? It would certainly explain the comments I have heard/read about these lenses not being so good as distances become closer than 8 feet--depth of field is shrinking, and the plane of focus isn't where we expect it!
Any thoughts? Anybody feel like slapping a ground glass onto their Isolette to see if this is normal or not? |
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