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.
> 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.
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?
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.
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.
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.
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.
Originally Posted by lxdude
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.
Originally Posted by Tom1956
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/~wes...res-chart.html , or a USAF 1951 lens test target, http://en.wikipedia.org/wiki/1951_US...ion_test_chart can be used for alignment purposes. Print out a high resolution copy on paper or transparency film on you photo printer.
This is true.
Originally Posted by shutterfinger
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.
Attachment 85181Once 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.