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Nomad
Tinkerer
Username: Nomad
Post Number: 2
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 30, 2009 - 05:46 pm: | 
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I have a Jupiter 3 lens that I plan to use on my Leica IIIf (which is currently away for a CLA). Mechanically, the lens seems okay - I took the helical apart and regreased it, although it wasn't too bad to begin with. While disassembled, I looked at how it might be adjusted for infinity focus on the Leica, should that be needed. I'd like to confirn that my thinking is along the right lines...
Fit the lens to the camera and set it to infinity.
Check rangefinder by sighting on a distant object.
If it's good, leave it alone, otherwise...
Take off the focus ring.
Slacken the set screws that hold the lens in its mount.
Rotate the lens in the mount until the RF is good.
Tighten the set screws.
Refit the focus ring.
Two things that spring to mind...
The focus ring has blind holes in it that its set screws go into. If the focus ring has to move, I need to drill new holes, yes?
If the focus using the rangefinder is correct at infinity, is it safe to assume that it will also be correct at other settings? In other words, is the pitch of the helical the same as on Leica lenses, such that the RF cam moves correctly to give the proper image overlap? |
David_nebenzahl
Tinkerer
Username: David_nebenzahl
Post Number: 29
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 30, 2009 - 06:33 pm: |
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I've taken apart and reassembled Jupiter-3s (for my SovCams [Zorkis & FEDs], am poor so no Leicas for me). I'm no expert on this, but it seems to me that the lens should be fine to use as-is. There's no adjustment on the lens that I know of; infinity focus is a positive stop that's set at the factory. So assuming your camera comes back with the rangefinder properly adjusted, just put the lens on and start taking pictures.
(I hope I'm right on this but am willing to be shown wrong by someone w/more knowledge than me on the subject.) |
Nomad
Tinkerer
Username: Nomad
Post Number: 3
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 30, 2009 - 06:54 pm: |
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I do plan to put it on and see how it goes - just want to have a plan in case something is out of whack.
The infinity focus is a positive stop between the outer part of the helical and the lens barrel proper, but this whole assembly goes into the collar that has the DoF markings on it, and this collar has the M39 thread that screws into the camera. The collar is fitted to the rest with a very fine pitch thread, and held in position with three set screws. Slacken the screws, and the lens/helical assembly can rotate through this fine pitch thread, meaning that its position with respect to the camera can be tuned. The downside would be that the reference mark for the aperture could move away from the centre as the adjustment is made (unless the adjustment is in whole revolutions of the collar). |
David_nebenzahl
Tinkerer
Username: David_nebenzahl
Post Number: 31
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 30, 2009 - 07:02 pm: |
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I guess you could put marks on the threaded part before disassembling it, under the assumption that the lens was assembled correctly at the factory (or the last time it was serviced). Otherwise, I guess you're going to need a collimator to set the focus. (You could do what I do--use a ground-glass screen in the back of the camera--but this is not a highly precise way to set focus.)
By the way, I ran across this page here that may be helpful: https://kyp.hauslendale.com/classics/collimator.html
[dang it--for some reason this board software doesn't make URLs clickable when you edit posts, only when you create them] |
Msiegel
Tinkerer
Username: Msiegel
Post Number: 157
Registered: 03-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, December 31, 2009 - 11:43 am: |
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Hello Nomad,
the only way, as to my understanding, to check if the lens focuses correctly on infinity would be mounting the lens, setting it to infinity and check with a groundglass on the film plane if it focuses correctly. THe rangefinder is no help here as it is in a way independent from the real focusing of the camera. So if the RF is out of focus the lens can still focus correctly on the film plane and only the RF is off. |
Nomad
Tinkerer
Username: Nomad
Post Number: 4
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, December 31, 2009 - 01:40 pm: |
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Okay, I can see now why a ground glass screen is needed - the part of the lens that pushes against the RF cam might not be consistent between lenses, so the RF itself can't be used to check. |
David_nebenzahl
Tinkerer
Username: David_nebenzahl
Post Number: 32
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, December 31, 2009 - 06:00 pm: |
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I thought that would have been obvious (that you can't use the rangefinder to check the lens' infinity focus), but maybe not.
In any case, while you can use a ground glass screen at the film plane to check infinity focus (that's what I use), it isn't the most accurate method in the world. If you really want to set the focus precisely, you need to use a collimator. Since the camera is a Leica, you might want to consider this.
Dang it, there used to be an article by Mark Overton on building a homemade autocollimator, but I can't seem to find it anywhere on the web. (This article here has a link to that article, but it's no longer valid.) I'll try to find it and post it. In the meantime you can search for info on collimators and autocollimators to find out how they work. |
Nomad
Tinkerer
Username: Nomad
Post Number: 5
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, December 31, 2009 - 06:21 pm: |
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It wasn't obvious at first because I was assuming that the ring that presses against the RF cam is positioned correctly (or within an acceptable tolerance) in every lens. In other words, position the ring correctly, and the lens is positioned correctly. Not so (and I should know better).
I'll do some searching on collimators. |
David_nebenzahl
Tinkerer
Username: David_nebenzahl
Post Number: 33
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, December 31, 2009 - 07:02 pm: |
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I think you're still confused. Setting the focus distance correctly has nothing to do with the rangefinder.
You're correct in saying that the RF cam should be at the same distance on every LTM lens (within tolerances). But this has nothing to do with focus.
Here's how it works: the first thing to do is to set the focus distance correctly (sometimes called "back focus"). This means adjusting the distance between the outer lens mounting flange and the film plane so it's precisely 28.8 mm. Since you say your camera is being CLA'd, presumably this will have been done when you get it back. (You could ask them.)
After this focus distance is set correctly, then the rangefinder can be adjusted for infinity image coincidence. Notice that this has nothing to do with lens focus. You're not altering or affecting that, only the adjustment of the rangefinder to infinity so that when the lens barrel is turned to infinity (and the RF lens cam projects the correct "infinity" amount), the two images will line up on an object at infinity.
What you're asking about here, it seems, is another aspect, which is whether the lens itself is correctly set up to focus at infinity, assuming the correct camera setup, correct? My answer is still that with the Jupiter-8, at least, this shouldn't be a concern. If it is, it's going to require at least a ground-glass checking, or better yet a collimation to check and correct it.
The ground-glass checking method is really simple, except that in your case it's not: you'd have to take the camera guts out of the body sleeve, since it's a bottom loader, and I'm guessing that's something you don't want to do.
You could get ahold of a cheap Leica copy to use as a lens test bed; you'd just need to make sure the lens-flange-to-film-plane distance is set correctly. This is actually fairly easy to do. All you need is a good caliper with accuracy to 0.001" (with a depth gauge). The distance is adjusted by inserting and removing paper shims between the body and lens flange. |
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