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02-22-2011, 01:58 PM   #1
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Spotmatic focuses past infinity?

Hello, I have just remarked that my Spotmatic focuses past infinity with any lens I use (Takumar, Helios, Jupiter). Is this a problem - will it lead to focus inaccuracy - or is it just some feature that noobs like me don't understand?

02-22-2011, 02:07 PM   #2
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That depends. Usually the camera mount section was fixed on the chassis with 4 screws, and copper washers were sandwiched between them. Though appear identical, those washers have different thickness determined in the factory for precise film plane distance. Often those washers were mixed up or missing after serviced. If they were left out, the film plane distance will decrease and any lens on this camera will focus past infinity (assuming the lens was accurate to begin with). This might cause the focus of the film plane not matching the viewfinder. You can tape a piece of tracing paper on the film plane and inspect the focus with a loupe and see if it matches the viewfinder.
02-22-2011, 02:14 PM   #3
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Alternate cause: the focus screen is out of kilter. Or, there's a diopter in the eyepiece (just to be complete with this analysis)
02-22-2011, 02:33 PM   #4
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Hi
It depends on your idea of infinity, a few blocks away is not infinity. The distant
mountains are.
Also the older Metal lenses especially the Russian ones are adjusted very slightly past
otherwise on a very hot day or in the sun it would not reach infinity due to expansion.
I set mine up just past. After all they are manual focus and you must check focus anyways in the viewfinder.

On the other hand if it is Waaayy past infinity I would sell it to Stephen Hawkings he
could probably use it

02-22-2011, 02:35 PM   #5
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I'm reading your replies with the dictionary in front of me
If I understand well, if this is the case with the missing washers, the focus should remain correct on the final images. If the viewfinder or focus screen is faulty, the images should be out of focus. I guess I'll have to shoot a test roll of film to find out.
Is it easy to remedy these problems?
02-22-2011, 02:45 PM   #6
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QuoteOriginally posted by kcobain1992 Quote
If I understand well, if this is the case with the missing washers, the focus should remain correct on the final images.
Not necessarily. I once had a Super Program with this exact same problem and the viewfinder didn't agree with the film plane. Since the washers were gone, the tech recalibrated the focus screen matching the film plane. The con is that the lenses will always focus past infinity but at least you can trust the viewfinder again.

QuoteOriginally posted by kcobain1992 Quote
If the viewfinder or focus screen is faulty, the images should be out of focus. I guess I'll have to shoot a test roll of film to find out.
Is it easy to remedy these problems?
Recalibrating the focus screen precisely will require a special instrument (forgot what it's called) which produce a virtual infinity target so that the focus screen can be calibrated to be precise at all 4 corners (not just the centre).
02-22-2011, 03:07 PM   #7
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QuoteOriginally posted by bobpur Quote
Hi
It depends on your idea of infinity, a few blocks away is not infinity. The distant
mountains are.
Also the older Metal lenses especially the Russian ones are adjusted very slightly past
otherwise on a very hot day or in the sun it would not reach infinity due to expansion.
I set mine up just past. After all they are manual focus and you must check focus anyways in the viewfinder.

On the other hand if it is Waaayy past infinity I would sell it to Stephen Hawkings he
could probably use it
Now that's a good one indeed. The problem of focusing past infinity is pretty philosophyical indeed. But the lenses are not calibrated to focus past infinity. Mounted on my other M42 body, a Zenit, they focus at infinity just fine. By infinity I mean anything past 10 meters away. I'm not sure if the lens actually makes any difference between stars and a few blocks away. On the Spotmatic, when I focus on a distant object, it really looks in focus at the 10m mark, which is just a step back from infinity. Focused all the way to infinity, nothing looks that sharp anymore.
02-22-2011, 03:07 PM   #8
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Another thought, use a tape measure, and see if the viewfinder agrees with the measured distance. Alternately, lay the tape measure out, focus on some spot, and see what the lens says vs what the tape says. This will tell you a lot about the geometry of the focusing system: if these are badly off then there's something wrong with the system. If these are close, then something else is going on at infinity.

What in the view finder do you look at to determine you're out of focus?

02-22-2011, 03:14 PM   #9
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The trees in front of my house
I'd like to measure the distance indicated by the lens, but to be honest I don't know "from where to where" should that distance be.
I have no idea if the camera has ever been serviced, as I'm not its original owner (I'm just 18) but bought it recently from a German.

Last edited by kcobain1992; 02-22-2011 at 03:21 PM.
02-22-2011, 09:01 PM   #10
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The trees in your front yard are nowhere near infinity. I would suggest something about 150-200 meters distant at a minimum and 1 km being preferable. Think a large distant building.

If it still focuses past a truly distant object, you can do an actual check at the focal plane with a microscope slide and frosted tape for mending paper. Use the tape surface as a type of ground glass. Even better would be to use abrasive paste to make your own piece of ground glass. Put the camera on a tripod, open the back, open the shutter on "B", affix the glass to the film plane, focus with the viewfinder and check the focus on the ground glass with a loupe.

You might also be able to get a local camera repair person to check it out for free.

Steve
02-23-2011, 04:55 PM   #11
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Mirror position can also affect focus. Check the mirror stops.

Chris
02-25-2011, 05:13 AM   #12
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I can't check focus at the moment, because I have film in the camera. For what it's worth, it focuses even past the horizon. I don't know how to check the mirror stops, the focusing screen also looks ok to me, everything looks fixed in place so I don't know what could have moved.
02-25-2011, 06:29 AM   #13
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Spend a couple of frames shooting at a target at a measured distance (use a tape measure or a known accurate camera): one at the distance engraved on the lens = your measured distance, and the second focusing via the view finder. Try to shoot as wide open as you can, and if you can work in some surface that carries from near to far, even better! Note which is which (stick a note in the scene?) and examine the results.

Do the same shooting at something very far away - one with the lens at infinity stop, the other at what the view finder shows.
02-25-2011, 06:27 PM   #14
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I don't know why you'd worry about focusing past infinity. It would only matter if you shoot starts, can't accuratly focus in the dark and just twist it all the way expecting to be at infinity.
02-26-2011, 04:00 AM   #15
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I am worried because it can't be just the infinity mark that's out of focus. I suspect that the viewfinder gives inaccurate focus, because, as I wrote above, the same lenses on another body focus at infinity twisted all the way with no problem.
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