Re-cementing lens elements question.

Discussion in 'Camera Building, Repairs & Modification' started by Tom1956, Jul 11, 2013.

  1. Tom1956

    Tom1956 Inactive

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    There exists threads on the topic of separating and re-cementing lens elements already. But I've seen references touching of the miking of the original cement job with a micrometer so that the repair job will end up identical. So this leads me to speculate that the thickness glue film itself comes into play. So this leads me to a question which is the point of this post. And my question is: is it really all THAT important? If it is then I believe I have devised a method that ordinary people can do with the use of a drill press. Many ordinary people have at least a tabletop drill press, or access to one.
    Once a trustworthy person answers my question I am prepared to go into my idea. I can see the point of marking the edge of the lens stack before disassembly to make sure each is turned by the clock for reassembly, but once again my question is, would it not be enough to squish out the excess glue between the fingers and take whatever finished thickness you end up with? Or are these lenses ground and engineered so that the finished thickness be exact? And to what tolerance?
    Thanks.
     
  2. vysk

    vysk Member

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    As far as I understand, lens centration is more important than end thickness, for image quality.

    If the thickness is not the same after cementing, the end effect is to change the focal distance.

    If you change it a lot, you may have to reset the lens flange focal distance, which typically involves a shim.

    For an SLR, it may not make any difference, depending on where the new focal point ends up. e.g. if it can't focus to infinity any more, you'll likely need to move the entire lens head closer to the film, which may translate to a thinner shim.

    I don't recall the tolerances, but I would look at the lens formula 1/f = 1/d1 + 1/d2 as a guide

    regards
    Vick
     
  3. Tom1956

    Tom1956 Inactive

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    For centering I intend to simply block it on 4 sides with oak letterpress furniture. The 2 projects I have presently in mind are the front sections of a 3.5 Xenotar from a Rolleiflex 3.5E, and the front pair from an older 80mm Zeiss Planar for Hasselblad.
     
  4. vysk

    vysk Member

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    I've done two Nikon 5cm f1.1 rangefinder lenses (the rear doublet) and one Nikkor 5.8cm f1.4 lens. Lots of fun.
    I've also repaired about three Leica M3 rangefinder prisms that separated.

    All successful.

    Vick
     
  5. Europan

    Europan Member

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    Why not on three points? You know, a three-legged stool never wobbles.
     
  6. johnielvis

    johnielvis Member

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    I know what oak is
    I know what a letterpress is
    I know what furniture is

    What is "oak letterpress furniture"?
     
  7. Mustafa Umut Sarac

    Mustafa Umut Sarac Member

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    it is letterpress oak furniture.
     
  8. Ian Grant

    Ian Grant Subscriber

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    You might look on the Large format website, Steven Tribe has written about recementing lenses in a few Forum threads.

    Ian
     
  9. Tom1956

    Tom1956 Inactive

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    Letterpress furniture is a set of wood blocks of various needed sizes, measured in units called points that was used in spacing of the lead type in old-style printing presses. It was used to fill out the chase so that the type could be locked in. Very accurately made and finished out of super-hard oak.
     
  10. E. von Hoegh

    E. von Hoegh Member

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    I know what letterpress furniture is. You're kidding right?

    This is what to use. You won't need the clamps though - http://www.mcmaster.com/#starrett-v-blocks/=nl80kb
     
  11. Tom1956

    Tom1956 Inactive

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    Why would I be kidding? The 2 lenses are of the same diameter. Lay the doublet down flat, presuming the back lens is either flat or concave, and then block it on 4 sides. If the back lens is convex, I have an obvious problem. I haven't disassembled the lenses yet to see. Might have to make another plan. But that plan isn't going to include spending money I don't have. I always find a way.
     
  12. E. von Hoegh

    E. von Hoegh Member

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    Tolerances in optics are not measured with a micrometer but in wavelengths of light. They're worse than watches. But - if you botch a recementing job with balsam you get all the second chances you need. You could make V-blocks on a milling machine, lapping them to the final finish - and they'd be just as good as the Starrets for recementing. For this purpose the vee need not be precisely 90 degrees, it just needs to be straight, smooth, and at 90 degrees to one end of the block.
     
  13. Tom1956

    Tom1956 Inactive

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    It sounds to me like you've got me doing the job with the lenses upright, rather than horizontal. You must be knowing the back lens is convex and it therefore won't lay flat.
     
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  15. Tom1956

    Tom1956 Inactive

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    I wonder if a dumb idea wouldn't be something as simple as a radiator hose clamp.
     
  16. E. von Hoegh

    E. von Hoegh Member

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    Not neccesarily. I'd use a surface plate for this because I have a small one. I'd place the cemented pair flat on the plate, if both surfaces were convex I'd make a ring and lap it flat/parralel. Use the V-blocks against the edges to keep everything centered, and give the balsam a few weeks at least to set. Personally I'm hesitant to use UV curing cement, it's much more difficult to decement the lenses.
    I recemented a R-R on an old Kodak folding camera using balsam from local trees. The camera was my mother's father's, I now have pictures of her taken with that same camera over a span of almost 80 years. :smile:
     
  17. E. von Hoegh

    E. von Hoegh Member

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    Now I know you're joking. Right? You're joking, right?:blink:
     
  18. Tom1956

    Tom1956 Inactive

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    I've already ordered the Canadian Balsam. If I foul up the job, I'll boil it and start over.
     
  19. Tom1956

    Tom1956 Inactive

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    Revision of Thread and question

    Upon disassembling this Rolleiflex 3.5E with Xenotar, I find the front element is a single piece of glass, not a cemented doublet. And it appears that it could possibly be etched on the front surface, or perhaps fractured coating makes it appear that way. I cannot tell because I do not have adequate magnification to see that closely. Obviously this piece of glass needs to get in the hands of an expert. It may turn out to be irreparable. This seems a dismal end to an otherwise good camera, so I float the matter out there for input. Thank you.
    [h=3][/h]
     
  20. Jon Goodman

    Jon Goodman Member

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    You could use the article I wrote: http://www.fotomozaic.ro/
    Your tongue is probably sensitive enough that you won't need to worry about clamps or jigs. At least mine has always worked just fine for work like this. Students of dentistry learn early just how tiny of a difference in height the tongue can detect. I offered to send Canada Balsam to anyone who needed it in another thread weeks ago. When that is gone I also have some very high grade rosin (some from Honduras, some from China, and more from other places). It will work just like Canada Balsam, actually.
    If the element is etched on the front surface, you will have a much more expensive and complex repair. Good luck, however.
    Jon
     
  21. Tom1956

    Tom1956 Inactive

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    Quote Received--$225.

    So I've received the quote from Focal Point on the polishing and re-coating of the front element only of the Rolleiflex Xenotar 3.5 lens element. I have the element extracted from the camera already. All it is is a single little thin piece of glass. And it's $225.00? Really? C'mon. Surely there's an alternative. $225? I wonder what other ideas there might be on this.
     
  22. E. von Hoegh

    E. von Hoegh Member

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    Do you have any clue what that entails? You can always do it yourself...:laugh:
     
  23. Tom1956

    Tom1956 Inactive

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    Yes I know what it entails. It also entails putting the lens back in the camera and sticking the camera back in the junk collection, and using $225 towards the bills, which keep coming. Or finding a more reasonable alternative. I'm somebody that $225 doesn't come easy on. Thanks.
     
  24. Jon Goodman

    Jon Goodman Member

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    I was afraid of that. The last time I checked into having one polished and re-coated it was about the same price. I couldn't justify it either...
    Jon
     
  25. John Koehrer

    John Koehrer Subscriber

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    Polish and recoat is obviously expensive. If you're going to trash it anyway, you have nothing to lose by trying plastic polish. I have used it for removing haze that wasn't touched by any solvent I had. Only Acetone, Alcohol, Ammonia and some common household stuff.
    Astronomers will grind their own lenses by hand, Their polishes can go from fine to huh? Don't worry about changing the optic unless you use power tools.

    I use two pieces of 1" angle aluminum with rubber bands to hold it in place. It holds the glass in alignment very well.
     
  26. Tom1956

    Tom1956 Inactive

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    I have a solid cake of blue buffing compound for plastic, for use with a buffing wheel. Are you talking about that, applied by hand with a cotton pad maybe?