Koni Rapid Omega 180mm f/4.5 lens focusing problem

Discussion in 'Medium Format Cameras and Accessories' started by altair, Oct 3, 2012.

  1. altair

    altair Member

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    Hey all.

    I recently purchased a 180mm lens along with a Rapid Omega 100 body and 2 other lenses (I now have 2 Rapid Omega 100 bodies). I seem to be having a couple of problems with the 180mm lens:

    a) The focusing is way off. I've taped scotch tape to the film plane of the camera body (after taking off the film back & the camera's top plate) to see if what the rangefinder says is focused is really in focus. Turns out, it's not. The rangefinder focusing is way out of alignment. Subjects that are 12 ft away will only be in focus in the film plane when I use the distance scale on the focusing knob (i.e I set the knob at 12ft for the 180mm, and checking the scotch tape GG, I see that the subject is in focus). Using the rangefinder patch in the viewfinder, the same subject is definitely not in focus on the film plane when the patch says it is. But, the funny thing is that the rangefinder focusing with my other lenses (58mm and 90mm) are spot on on the same body, and my checking with the scotch tape on the film plane confirms this. I've also tried using the 180mm on my other Rapid Omega body but the results are the same.

    b) The gridlines in the viewfinder for the 180mm is visible when the lens is focused at infinity, but when I focus at closer stuff the gridlines for the 180mm lens just disappears from view in the viewfinder. Also, when I look through my 'scotch tape ground glass', the view that I get with the 180mm lens doesn't change at all when I focus the lens at closer subjects. Isn't the view supposed to change/shift to compensate for parallax?

    Help!

    -Dani
     
  2. altair

    altair Member

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  3. rjbuzzclick

    rjbuzzclick Member

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    The 180mm should have two pins that go into the camera body that are mounted to the back of the nameplate above the lens. I believe they adjust the rangefinder and frame lines parallax for the lens. It sounds like your lens might be missing one or both of these pins. The 90mm has one short pin, and the 60mm has no pins. If the rangefinder is correct with your other lenses, then it's probably your 180mm lens that has issues and not you camera bodies.

    The 180 grid lines disappear at around 12 feet because that's the close focus limit with the 180mm lens and the rangefinder.

    You won't see parallax correction on the ground glass, you'll see it in the viewfinder. The frame lines will shift down to the lower right of the viewfinder as your focus distance gets closer. The parallax correction happens in the viewfinder to compensate for its offset from the lens.
     
  4. altair

    altair Member

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    Reid, my 180mm lens has both of those pins, mounted just like you said, behind the nameplate and slightly to the right. Their length (that protrudes) doesn't differ by much. The longer one is 8mm while the shorter is 7mm.

    Re: parallax correction, yes..but shouldn't that compensation also be visible in the ground glass across the film plane? When I test the focusing accuracy of my 90mm lens, and when I gradually shift focus from infinity to the nearest distance (3.5ft), I can see the view on the ground glass gradually constricting and shifting a bit to the right, as per the shifting framelines in the viewfinder.

    Now, with the 180mm lens, this doesn't happen at all when I rack the focus from infinity to 12ft. Strange, because I was expecting a telephoto lens like the 180mm to have an even more pronounced parallax compensation that will be visible in the viewfinder.
     
  5. rjbuzzclick

    rjbuzzclick Member

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    I'll measure my 180mm when I get home tonight and see how long they are. They should both butt up against something inside the camera, make sure that they're not getting mis-aligned when you mount the lens, or that they're not bent.

    The parallax error is in the viewfinder, not the lens. You will see the image get slightly wider or narrower on the ground glass with focus distance, but that's just the slight field of view shift that all lenses have when you move them closer or father away from the film plane. Large format photographers deal with this all the time. The film plane image won't (or shouldn't) shift side to side or at an angle during focusing since the lens only moves perpendicular to the film plane, not at an angle to it. Think about what you see when you focus through an SLR, there is no side to side image shift, only possibly slight expansion and contraction of the whole image.

    The angle that the rangefinder viewfinder is from the spot your are focused will change from what the lens sees depending on how far you are from your subject. If you've ever cut off part of your subject with a non-parallax corrected RF viewfinder because you forgot to tip up a bit during a close-up, that's parallax error. You can get the same issue with TLR's at close distances. Close up lens sets for TLR's will have a prism for the viewing lens to correct for this.

    The only side to side or angle movement you should see that has to do with parallax correction is the frame lines in the viewfinder. The frame line borders will also constrict or expand slightly with focusing distance as well.
     
  6. MattKing

    MattKing Subscriber

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    If the image on the ground glass thrown by the 90mm lens shifts to the right as you focus, then the focussing mechanism requires repair.

    The shifting framelines are there to help match the view through the viewfinder to the view seen by the lens, which is of course not in the same location as the viewfinder.
     
  7. rjbuzzclick

    rjbuzzclick Member

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    Well, I think we might have a lead on your problem. The pins on my 180mm lens are 18mm long for the outside one, and 15mm long for the inside one. They are spring loaded however, and when I push them into the lens, they both stop at about 6mm. From your description of your lens, it sounds like the spring for those pins maybe be broken or not connected on your 180mm lens, and therefore not providing the right amount of pressure on the rangefinder mechanism. If your handy with tools, it looks like that cover comes off with just four screws.