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Monopix
Tinkerer
Username: Monopix
Post Number: 7
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 05, 2008 - 11:06 am: | 
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I thought I would put this idea up for discussion.
When aligning a rangefinder at infinity there are, it seems to me, a couple of problems. Finding a suitable subject far enough away for one and having good enough eyesight to judge when the rangefinder is aligned for another. So I was thinking about alternatives and have experimented a bit using a laser.
Basically, I shine a laser into the viewfinder, making sure it hits the rangefinder patch. The prism which usually combines the two images now acts as a beam splitter and splits the laser into two beams. One exits the front of the viewfinder, the other exits the rangefinder window. Now, if the rangefinder is accurately aligned for infinity, these two beams will be parallel and that should be easy to check for by measuring the distance between the beams close to the camera and again at a distance.
What I'm wondering is whether this would be accurate enough. With what sort of accuracy would the distance between the beams need to be measured? What sort of angular error would be acceptable?
Any thoughts??? |
Jerk151
Tinkerer
Username: Jerk151
Post Number: 21
Registered: 06-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 05, 2008 - 11:32 am: |
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Here is something I posted in another group back in August. I meant to share it here and forgot. It's me working on an Argus C4 with help of drawing from Rick Olsen. You can skip to the "Range Finder" part if you like
The Html doesn't seem to work here, so the pictures are not directly visible, but the links do work and open in a separate window.
"Well this won’t be a full work through to start off with, but a couple of things I found to be the most challenging. Oh, And this is going to be very crude because of the images you are going to be subjected to are from a P&S in a very messy work station.
<img src="http://a718.ac-images.myspacecdn.com/images01/62/l_eeeadb9f5fa8a6de52bd5698c9d06 a95.jpg">
First, there is a small set screw on the side of the lens you see in the above. That screw travels in a slot in the ring below it. When removed the lens screws off and has a Helical thread. I was very careful to mark the position of the lens. I did this because without being able to see through the lens while focusing, you won’t know where it matches up to range finder or the focal point on the film itself. You must remove the lens in order to remove the ring below it and get to the front side of the shutter. The shutter was a little sticky. I was able to clean the gunk off w/o dismantling the leaves. However, when I put it back together, no matter what I did, the lens did not go back to it’s original mark. As, I mentioned in another thread this camera had been tinkered with by someone before me, and though all the pieces are there, not too many of them were in the correct position.
<b>The Fix</b>
Ground Glass! I borrowed the small ground glass that was in my 6x9 roll back for my 4x5. I took the back off the camera and placed the ground glass on the film plain. Took it out in the sun light, set the focus dial to infinity and screwed the lens on with a distant object in view (turning it w/o the set screw in) until I got a clear, focused image on the glass. As I did, I found that the screw hole lined up perfectly with the slot in the ring. I now know the lens is in it’s correct position to focus on the film.
<b> Range Finder</b>
<img src="http://a197.ac-images.myspacecdn.com/images01/16/l_83efe12bcc7ce0afcb1ccd4b7d821 e1c.jpg">
I had never seen the inside of one before and was quite excited about it. The workings are very simple in theory, just a bit difficult to get everything just perfect in alignment.
<b>How it works</b>
The glass you look through (view finder) is the first image and straight through. In the path of this view, there is a sort of two-way mirror set on a 45 degree angle, except the reflective side of the mirror is on your eye side. So, you see through this mirror, but your sight line also makes a 90 degree reflective turn into a second mirror that lets you see through the lens in the focus dial.
<img src="http://a533.ac-images.myspacecdn.com/images01/65/l_2d013ae05d113cccb227237ad5ec3 684.jpg">
Both images are ghostly looking much like a double exposure. Out of focus, you see two images, as you focus the two images get closer together until they become one image. Then you are focused.
This camera was screwed and the images never came together, as well, one image was vertically high than the other. There are two simple screws to make these adjustments.
<img src="http://rick_oleson.tripod.com/c4rf.jpg">
Imagine is from Rick Oleson’s page
Yet, As I said, someone has already played around here so my mirrors were not on 45 degree angles anymore. At about 2:30 a.m. I woke up with a thought in my head. I went outside to the shop to try this out. I used a $7 laser pin from radio shack (I think it has something to do with Star Wars) and pointed it through the view finder. I placed the cameras film plain exactly 3ft away from a box. Set the focus dial to the 3ft mark. There were two dots being displayed on the box.
<img src="http://a470.ac-images.myspacecdn.com/images01/20/l_5671b25f3d694276e5e5f29e3a655 2b5.jpg">
<img src="http://a634.ac-images.myspacecdn.com/images01/105/l_cdac540312722c74fcbc57b59f19 d2e9.jpg">
(I told you crude pics)
I started adjusting the screws and mirrors until they lined up.
<img src="http://a6.ac-images.myspacecdn.com/images01/37/l_5eb7caa80cb32dd0c226073b440ae59 d.jpg">
<img src="http://a584.ac-images.myspacecdn.com/images01/59/l_8464ad6460fd0a391ce4413a0b3f5 3bf.jpg">
I re-assembled the camera and took it outside and focused on infinity and it seems to be right on. I have brought the camera to work with me today and hope to fire a few shots off to test the theory here.
<b>Question is</b>
When I did the laser alignment, where was the 3ft mark supposed to be? I made 3ft from the film plain to the subject. Should it have been 3 ft from the rear eye piece? Front view finder lens? or in between where the line of sight bounces from mirror to mirror? to the subject? I think I have it right, but we will find out if my test roll comes back out of focus.
Hope this wasn’t too boring.
Jody" |
Sevo
Tinkerer
Username: Sevo
Post Number: 20
Registered: 09-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 05, 2008 - 01:58 pm: |
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I doubt that laser alignment of the rangefinder itself has particular benefits - as the user will use it at plain visual precision, any better alignment there has no significant effect.
Besides, the big problems are with the lens collimation and adjustment. Most lenses are far enough off their nominal focal length that focus precision cannnot be maintained throughout their entire range without an individually matched control curve - something that only the most expensive lenses have. And the film plane is none too well defined in amateur or small workshop calibration. Even factory grade test gear will never absolutely match the back of every camera - and many repair shops and almost all amateurs don't even use a collimator, and often not even a fitting glass pressure plate to get their test targets or ground glass control screens at least reasonably close to the film plane.
Sevo |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 733
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 06, 2008 - 09:56 am: |
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http://rick_oleson.tripod.com/index-123.html
Scroll down to the bottom of the page. |
Monopix
Tinkerer
Username: Monopix
Post Number: 8
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 06, 2008 - 04:01 pm: |
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>I doubt that laser alignment of the rangefinder itself has particular benefits - as the user will use it at plain visual precision, any better alignment there has no significant effect.
I don't think that the visual limitations of the user is reason to not align the rangefinder accurately. Any errors in the alignment may add to the visual error of the user to create a greater overall error.
>Besides, the big problems are with the lens collimation and adjustment.
I don't disagree but again, it's not a reason to not align the rangefinder properly. |
Sevo
Tinkerer
Username: Sevo
Post Number: 21
Registered: 09-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 06, 2008 - 04:26 pm: |
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I actually practice and recommend the reverse, i.e. using the known good camera as the viewing tool and a piece of test film held by a glass pressure plate on the camera to be tested as the target, as that allows you to use the regular viewing instrumentation on the known good camera, halving the number of hacks needed.
In any case, even the best camera-camera collimation has a fair amount of remaining tolerances - if I'd start building my own DIY laser tools, a laser collimator would certainly be higher on my priority list than a laser RF calibrator. |
Charlie
Tinkerer
Username: Charlie
Post Number: 187
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 07, 2008 - 06:56 am: |
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I think the laser method will only work if the beam going in is absolutely perpendicular to the camera. I don't think parallelism of the two beams would be sufficient. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 734
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 07, 2008 - 08:55 pm: |
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at the risk of sounding repetitive..... |
Sevo
Tinkerer
Username: Sevo
Post Number: 22
Registered: 09-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, December 08, 2008 - 03:15 am: |
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You will only need perpendicularity for distance tests if you use a plain (dot projecting) laser. A line projecting laser (e.g. from a electronic spirit level, or as plug-on optics for better laser pointers) does away with the need to eliminate vertical errors, as it will give you a perpendicular reference in the line itself.
Going by my first attempts, laser testing might indeed come in handy for far distance non-infinity checks, where purely visual checks tend to be at the margins of the eye/finder resolution limits. For the infinity setting, the base width mark alignment test explained on Rick's page is much faster to set up and wholly adequate. |
Markus
Tinkerer
Username: Markus
Post Number: 84
Registered: 08-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 10, 2008 - 08:51 am: |
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Rick Oleson's method is fantastic! I just aligned a Kodak Medalist that way. I used to have to go outside to find an infinity target to align to, but now I can do it at night and inside.
Thanks Rick! |
Charlie
Tinkerer
Username: Charlie
Post Number: 188
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, December 10, 2008 - 03:07 pm: |
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I stil think you need to be perpindicular to be accurate. Obviously viewing the edge of the target is useless and you must rotate it in the horizontal plane until the target is exactly parallel to the rangefinder base (perpindicular to a line from the camera to the target). |
Monopix
Tinkerer
Username: Monopix
Post Number: 10
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 21, 2008 - 08:27 am: |
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Don't actually understand why perpendicularity is important. If the laser isn't perpendicular, the two beams will still exit at similar angles and so will still be parallel. If perpendicularity was important then wouldn't that mean we could only focus accurately if our eye is perpendicular to the viewfinder?
Rick's method depends on acurately measuring the distance between viewfinder and rangefinder windows and can only be as accurate as your measurement. On my ZI, the rangefinder patch isn't exactly in the middle of the viewfinder (maybe it should be but it ain't) so where do I measure from??? |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 745
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 21, 2008 - 04:42 pm: |
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If it's, say, 3mm off center (which would be pretty extreme) in your front window, that equates to 3mm off at whatever distance you choose to place the target at. If you place it at 10 to 15 feet from the camera, it's probably as close as your eye can discern. But whatever makes you feel good, go for it. |
Charlie
Tinkerer
Username: Charlie
Post Number: 190
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, December 22, 2008 - 06:36 am: |
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My point was that if the rangefinder base is say 2 inches and you make a target with the with the X's 2 inches apart and then tilt the target so the target points appear to be say 1 3/4 inches apart when viewed from the camera I don't think you will get an accurate setting. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 747
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, December 22, 2008 - 05:51 pm: |
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I have not found this to be nearly as complicated or difficult a task as this exchange makes it seem. Yes, you do need to be square to your target.
Shining a laser beam forward through the eyepiece and adjusting so that the points of light coincide at your desired plane of focus also works. They used to sell a device like that, called a "Focuspot", to clip onto the rangefinders of Speed Graphics back before most of us were born. It's a little difficult to adjust your infinity focus that way though. Measuring the 2 beams at infinity for parallelism certainly works, but it is neither easier nor more accurate than making a target with the correct distance marked and just looking at it through the finder. If your rangefinder patch is way off center in your viewfinder window, you can mark a dot on the front window with a Sharpie pen while sighting through the eyepiece to determine its exact position. Sharpie ink cleans off nicely with alcohol when you're finished. |
