Home invention Hasselblad Mirror Checking?

Discussion in 'Camera Building, Repairs & Modification' started by Tom1956, Mar 24, 2014.

  1. Tom1956

    Tom1956 Inactive

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    Anybody who has kept up with my camera repair escapades poorman-style could have expected me to cook up another scheme. Working on Hasselblads seems to have risen to the top of these endeavors. By "risen to the top", I hope it doesn't end up as merely more internet pond scum.
    Working on Hasselblads require factory made and approved jigs, fixtures, and tools that few people can own. And
    usually these same people have factory training. Far and above us mere mortals :smile:
    But careful attentive work CAN be done, perhaps as within 5 total percent error of factory work. Certainly 8% at the outside.
    Now that this verbal windage has made my self-expectations a standard for home-made excellence, I have an idea. On the gliding mirror models, What if a home made jig made of perhaps PVC pipe fittings or other common materials could be fabricated or fashioned to shoot a cheap laser toy dead center of the bayonet flange, at a near-perfect perpendicular angle right down the camera's throat? Should not that beam end up near dead center of the ground glass? Obviously if the mirror stops were off, the beam would end up higher or lower.
    Al comers welcome to shoot down my boob plan. Or approve it. Gratitude, HTF
     
  2. Tom1956

    Tom1956 Inactive

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    Make that 2% error and 5% max. So far, I'm within .04mm on a 71.40 measurement in setting up the chassis.
     
  3. lxdude

    lxdude Member

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    The screen height adjustment at all four corners would have to be dead nuts and you would have to know that it is.
     
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  4. Tom1956

    Tom1956 Inactive

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    I had already considered that. At this stage in calibration, all 4 screws would be bottomed out, for the better or worse of the plan. First a 71.40mm chassis length was done, within the tolerances of my more crude dial caliper (instead of the Hass fixture). I was happy there. Right now I'm working on fashioning a ground glass to check lens infinity stop. But moving on, it seems to me mirror angle comes next. And the only way to zero in on that would be by screen squareness to the chassis assembly. Necessarily all one CAN do is bottom out the screen pegs and hoping for the best on that count, so you can shoot your mirror angle. Right? Then adjust your screen pegs last.
     
  5. lxdude

    lxdude Member

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  6. Tom1956

    Tom1956 Inactive

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    Thanks, it was the 503 repair manual. I've read all those available on the free internet already. They call for the Hass fixtures, tools, and jigs. And I understand their use.

    BTW--at this time I'm dealing with a 553 ELX.
     
  7. ic-racer

    ic-racer Member

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    On my 35mm cameras with easily adjustable mirrors, I spend some good time observing the horizon at the top, and bottom and middle of the viewfinder, then adjust as needed.
     
  8. Tom1956

    Tom1956 Inactive

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    What do you mean observing the horizon?
     
  9. ic-racer

    ic-racer Member

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    Horizon being a surrogate for infinity. If the lens is collimated to the film plane, then focus errors observed with the focus screen signify that the screen and/ or mirror would be off. To set the mirror angle I observe the horizon at the top of the screen and at the bottom. Should be just as clear and sharp in both cases. If not, tweak with a screwdriver (in those cameras with the capacity).
     
  10. Tom1956

    Tom1956 Inactive

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    Understood, ic-racer. But I have a couple problems with that. First, I have to get in my truck with tripod, camera, and tools and drive a long way before there is any horizon. Further, I've been doing these sorts of home-baked ideas for too long, and am moving on in my repair studies to wanting to devise bench-testing variations of expensive and unobtainable factory fixtures and jigs. And do better work than these slow cumbersome and imprecise tripod tests on a pretty day.
    I wish I could better understand the construction of the official Hasselblad mirror jig, but have never seen one. But I can surmise how it must work and come up with a variation. Reverse engineering, I suppose.
     
  11. lxdude

    lxdude Member

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    Tom--

    If you cannot establish high precision on your bench equipment and know that you have, you will not have precise results, and you are only going to chase your tail. The horizon method is precise in function, as it uses an established infinity point, not an artificial one, and you can see when adjustment is out. You do not have to use the horizon; it can be a tree on a hilltop several miles away, for example. I prefer to use the moon at night, as it is a high contrast true infinity target. Using the focusing screen image to set mirror angle might seem crude, but it's not. It's a simple and real-world method that works very well, because it relies on a non-variable target that requires no calibration. Once precise mount-to-film plane distance is set, a ground glass at the film plane establishes the point of precise focus, then the mirror angle and screen height are matched to it.
    Not to denigrate the bench method, which is much more practical in a commercial situation. But to build your own precision optical test setup requires a precise means of fabrication. How are you going to precisely determine center of the lens mount? How are you going to know that the laser beam is precisely perpendicular to the film plane? So using a real-world infinity target and calibrating by observing focus on the screen is faster, easier, more practical, and more trustworthy.

    Some points:

    > Achieving some percentage of factory precision really isn't good enough. It's either in tolerance or it's out. I wouldn't want any repair person deciding that out of tolerance by 8% or 5% or even 2% is good enough. Don't worry about setting up an optical bench when you can use other means to set mirror angle- instead get some better precision tools, like a depth micrometer that reads to .01mm.

    > You should bring that .04mm down to .02 or better, to give some room for error. Using the caliper is iffy, and the closer it reads to exact, the more likely you will be in tolerance. If your present measurement is precise, then you are out of tolerance, because the factory tolerance is .03mm. It makes no sense to start out not knowing where you are, because you will never know where you will end up.

    > Make sure the focusing screen is flat, as stated in the manual. This is essential.

    > With the screen height screws all the way down, the shortened optical distance from the lens mount flange means that the screen image should be out of focus 'beyond' infinity. By turning the lens focusing ring off its infinity stop you will increase that distance until infinity focus is reached somewhere on the screen, and you can adjust the mirror angle until all parts of the screen image are in focus. That's your final mirror angle setting.

    > Turn all four screen height screws the exact same amount to raise the screen to its true setting for infinity, using the ground glass at the film plane to establish true infinity focus and adjusting the focusing screen to that.

    > If your lens mount to film plane distance is off, properly adjusted lenses will not focus to exact infinity at their infinity stop and any lenses you adjust to the body you are working on will not be right on other bodies. If you have access to a properly adjusted lens, you can use it to check the lens mount to film plane distance. Use it with it set to its infinity stop in conjunction with the ground glass at the film plane to check infinity focus-if it's not right the body needs adjustment. You can also use a known good body to calibrate your lens' infinity stop and then use that lens to check the body you're working on.
     
  12. Tom1956

    Tom1956 Inactive

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    lxdude, I really appreciate your attention to my thread, and anybody else who might answer. I'd like to throw a thought out there for anybody to shoot holes in my logic. So...
    If the mirror is off it's 45 degrees, and the front (low end) is drooping too low, then does that not mean that the image will be moved UP on the screen? I have constructed a screen from a Rolleiflex that stands exactly where the film would be ±the thickness of cheap grade scotch tape, which is about .015mm. Pretty precise, for home-made boob work. So far infinity on the camera screen and the back Rollei screen matches up very well, telling me this 553 ELX is already OK.
    But this is a learning experiment. So back to the mirror angle question: If a mirror that droops in the front, moves the image UP on the Hasselblad screen, then the Moon placed dead center on the back Rollei screen, is off-center high on the Hasselblad screen, then is that not the proof that mirror angle is off?
     
  13. lxdude

    lxdude Member

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    Two questions:

    Why the -+ the scotch tape thickness? Isn't the screen in contact with the film rails?

    What do you mean by the mirror drooping? Do you just mean the mirror front is adjusted low, or is it something with the gliding mechanism?


    Also, it comes to mind that if there is a small machine shop in one of the cities you drive through, you could talk to the guy there and ask if they could check your caliper for accuracy. They have gage blocks which are highly precise, and they could use them to check at the exact Hasselblad flange distance along with the entire length of the caliper. Offer a little quid pro quo and I expect they'd do it for free.
     
  14. Tom1956

    Tom1956 Inactive

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    Yeah, bottom edge of mirror too low, for instance (stops too low). My caliper is accurate, based on measuring stuff like guitar wire, and other odd things of known factory accuracy. And on my home fabricated GG back, I used 3/16 hobby plywood from my RC plane building, and enough tape of various kinds to stand the GG off from the back panel of the camera where the film would be. The film is behind the gate opening by some distance. You can't just slap your GG on the camera back.
    I was happy with my caliper accuracy when I measured the camera about 80 times and kept coming up with 71.40mm ± .02-.03mm most of the time.
     
  15. lxdude

    lxdude Member

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    Ok, I get it with the back. The insert side-loads, right? I was thinking of my Bronicas, where the film rails are accessible with the insert out.

    Regarding the image centering, I would say it is not absolute proof that the mirror angle is off. Maybe the focusing screen and the film gate are not centered to each other. I don't know what percentage of coverage of the image a Hasselblad screen gives, but lots of cameras are around 90- 92%, giving adjustment room.
     
  16. Tom1956

    Tom1956 Inactive

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    Yeah, that's the question I had too--whether the viewing screen is a dead-center arrangement with respect to the bayonet mount of the lens and the film gate. They could be all over the place for all I know. For instance on a Honeywell Pentax Spotmatic, the screen is 92% FOV and off-center in both directions. Dismal.
     
  17. lxdude

    lxdude Member

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    I remember from old camera magazine tests that it was very common for cameras to have viewfinder images that were off and even tilted. I used to wonder why they couldn't have made a fixture that would insert in the film gate and the screen opening and align them before the mirror box was tightened.
     
  18. shutterfinger

    shutterfinger Subscriber

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    From my limited work on vintage LF SLR's a improperly positioned view screen/view ground glass will make a square or rectangle appear as a trapezoid if the alignment is incorrect.

    Any focused distance can be used for alignment as the optical distance to the image surface of the view screen/ view gg must be equal to the optical distance to the film plane. As I stated in a response to one of your other threads focus at the film plane then adjust the focus screen/view opening ground glass to match the film plane.

    A ISO 1223 test target, http://www.graphics.cornell.edu/~westin/misc/res-chart.html , or a USAF 1951 lens test target, http://en.wikipedia.org/wiki/1951_USAF_resolution_test_chart can be used for alignment purposes. Print out a high resolution copy on paper or transparency film on you photo printer.
     
  19. lxdude

    lxdude Member

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    This is true.
    However, I suggest an infinity target because first, it does not require establishing parallelism between the target and the film plane, and it allows the camera to be swivelled to put the single target anywhere on the ground glass and focusing screen with no practical error; second is that because lenses are corrected for infinity, error induced by curvature of field is minimized.
    Personally, I also find it to be less hassle, but others might not.
     
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  20. Tom1956

    Tom1956 Inactive

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    mirror position.jpg Once again I thank all you guys for your input on my thread here. Attached is the page out of the 555/553 manual on mirror positioning. It just seems such an easy task if one could procure or fabricate a jig of like kind. Surely it can be done. I'd give my eye teeth to actually see one of these. Wouldn't be the first time I reverse-engineered a gadget. How difficult could it really be? For instance even common things like PVC pipe fittings these days have been so automated in their manufacture as to be within billionths of a germ's hair in tolerances.
    I've thought about it today, and though I can't measure or prove it, but I'm sure for instance that an imaginary straight line drawn dead center of the film gate opening through the lens bayonet would pass dead-center there also. This IS a Hasselblad, after all. Now to think of a way to determine if the focusing screen is on-center with front bayonet, with whatever crop being equal on all 4 sides. Given these variables, then fabrication of a test instrument similar to this should be pretty easy. We're not computing a course to the moon, where the last digit in an irrational number is necessary to keep from flying a thousand miles past it.
     
  21. Tom1956

    Tom1956 Inactive

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    Well that's the point--to do a bunch of cameras.
     
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