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David_nebenzahl
Tinkerer
Username: David_nebenzahl
Post Number: 114
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 01, 2010 - 05:24 pm: | 
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I'm thinking of building myself a collimator. I'm getting tired of stepping outside during the day with a camera with a piece of ground glass held on the back by rubber bands, adjusting the focus at infinity. I'd basically like to "bring infinity indoors", as they say.
So I looked at the article on building a collimator on our own Monopix's (Peter Robinson) site (it's here).
Looks simple enough.
My question is, how does this work? I know the reason for using a collimator--providing an image at virtual infinity to focus a test camera on. What I don't understand is how Peter's rig, basically a target inside the guts of an old camera with a diffuse light source behind it--can provide such parallel beams of light. Wouldn't one get the same effect by just focusing the test camera on anything? Obviously not, in practice. But I don't understand how Peter's target ends up located a virtual infinity. I'd like to understand how the process works, then build my own collimator, probably out of what's lying around in my parts boxes.
So I looked around on the web for an explanation of collimation. There weren't very many Google hits, and most of them were astronomy-related. So if anyone knows of a good explanation out there of how this works, it would be appreciated.
One of the pages on this site raises the issue of collimation, but provides no details. It does refer to an article by Mark Overton, who used to have an excellent page up explaining how to build a collimator (it was quite a bit more complex than Monopix's rig), but since Geocities shut down I can't find his stuff anywhere. |
Mndean
Tinkerer
Username: Mndean
Post Number: 184
Registered: 08-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 01, 2010 - 10:14 pm: |
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Isn't Peter's rig similar to Rick Oleson's? I thought to use the collimator, you would need a known-good lens on the collimator body for it to work correctly. I didn't see that detail in Peter's article.
As for the whys, I read a mindbendingly detailed article on that once years back in a cinematographer's reference guide and I've forgotten most of it, but they're very fussy about focus accuracy of their lenses (especially zooms, IIRC). If you're looking for details on lens collimation, that may be a possible source. Of course, those were done off-camera so the details of how it was done differed. |
David_nebenzahl
Tinkerer
Username: David_nebenzahl
Post Number: 116
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 01, 2010 - 10:30 pm: |
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D'oh! It's the lens, of course, dummy (talking to myself here).
Now it all makes sense; the collimator's lens is focused at infinity, which instead of capturing an image of an object at infinity projects an image (whatever is in the film gate of the collimator) at "virtual infinity". So that's where those parallel beams come from; the collimator's lens.
So all one needs is a known good camera focused at infinity. In my case, trying to make the rig out of the crap I have lying around here, this raises a chicken-and-egg question: how do I get a "known good" camera focused at infinity if I don't have a collimator?
But at least now I think I more or less grok it.
I think I'll go scour Rick O's site to see what he has to say on the subject. |
Prasanna
Tinkerer
Username: Prasanna
Post Number: 45
Registered: 10-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 10, 2010 - 05:08 am: |
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Just use any camera with which you have made good pictures by focusing it at infinity. Note the setting on the distance scale in the lens. [Most likely it would read infinity; some times it could be off the mark, as the scales are not that accurately marked; there could also be a shift due to the mount on the camera body.]Of course make sure to focus with the aperture fully open. Then set it on the bench [a make-do optical bench] with the lens at the same infinity setting and project the light from behind. Then adjust your test candidate camera. Regards, sp |
Monopix
Tinkerer
Username: Monopix
Post Number: 106
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 10, 2010 - 08:47 am: |
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Yes, you do need a known good lens to start with and I didn't make that clear. I use my Zeiss 50mm f/1.4 which has never been messed around with so I'm assuming it's set correctly. It certainly appears to be when I use it on a camera and point it at a distant subject. But if you don't have a known good lens, just check one out by pointing it at the moon one night. Once you have one you would be happy to say is focussing at infinity correctly, then you can use it to set up others. (NOte, strictly speaking, when you check out the lens you should do so on the body you are going to make the collimator out of and do it by checking the image at the film plane, not by looking through the viewfinder).
I think the (maybe) unique feature of my setup is I built the thing vertically so you can just stack everything up and be sure of it being aligned. If you try and build it horizontally, you have the problem of getting everything level and on the same axis. Also, it's an assembly of parts which I can put together when I need it and, other times, I can use the parts for other things. So I'm not tying up my 50mm. |
David_nebenzahl
Tinkerer
Username: David_nebenzahl
Post Number: 137
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 10, 2010 - 12:41 pm: |
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OK, that makes sense. Just to confirm and clarify: you seem to be saying that it's OK to adjust the reference lens in the collimator by eye on a sharp subject at infinity; is that correct? (Otherwise, one would need a correctly-set-up collimator to adjust the collimator's lens, raising an obvious chicken-and-egg problem.)
That is how I now adjust infinity focus on my lenses, by stepping outside and using the telephone poles and whatnot that I can see far away down the street as subjects. So far as I can tell this is adequate, judging by the results I get on film. (I use a piece of finely-ground glass and a loupe to check for sharpness.)
It would just be nice to bring all of this indoors ... |
Monopix
Tinkerer
Username: Monopix
Post Number: 107
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 10, 2010 - 02:33 pm: |
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Yes, I'm basically saying that. For me, the benefit of a collimator is being able to do everything on the bench. I'm not looking for any more accuracy than I can get by focusing on the moon (or whatever). If someone is looking for perfection, then I guess they need something better. This works for me. |
David_nebenzahl
Tinkerer
Username: David_nebenzahl
Post Number: 138
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 10, 2010 - 02:39 pm: |
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Thank you for that confirmation (and for your guide on the web). I am now officially looking for a donor camera body and lens to make my own rig. I'm thinking of making some adapters out of plastic plumbing pieces to accommodate different cameras without marring them. |
Tamasflex
Tinkerer
Username: Tamasflex
Post Number: 1
Registered: 09-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, November 08, 2010 - 12:09 am: |
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Bringing infinity for cameras:
http://www.AceLayouts.com/uploads/images/2010-11-08/ngGAaPc6vO.jpg |
Scott
Tinkerer
Username: Scott
Post Number: 117
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, November 08, 2010 - 07:05 am: |
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"Infinity" (or far away objects) is one reference when collimating a lens.
For cameras with which I anticipate using for portraits, or at distances between 2 and 5 meters, I usually decide on an medium distance of 3 meters or so, and then measure the exact distance with a tape measure from the film plane to a particular luminous object (like a TV), check the image at the film plane with a loupe, and then set the focus at 3 meters.
Everybody tells me that approach this is just all kinds of wrong, and that I obviously don't understand the theory behind collimation-- that the focus must be set at infinity.
My opinion is that it probably dosen't make much difference either way, and that it you mainly want landscapes, then go ahead and set focus at infinity.
With some cameras, I would rather have precise focus at 3 meters, and rely on DOF and margin-of-error at longer distances- rather than vice-versa. |
Monopix
Tinkerer
Username: Monopix
Post Number: 167
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, November 09, 2010 - 06:16 am: |
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That would make sense if you wanted to accurately focus at 3m (or whatever) using only the scale. But other than that, I don't see the point. |
Der_uli
Tinkerer
Username: Der_uli
Post Number: 18
Registered: 05-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 12, 2010 - 09:39 am: |
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Tamasflex' link above is a very good technical drawing of the method I use too:
Using a telescope as a collimator makes inifity adjustment much more precise. This is due to the bigger enlargement, compared to a camera/lens combination. Every fault in the camera under test is exaggerated. On the other hand, precise infinity focusing of the telescope is less critical.
You may read my loong story about focus adjustment at this website.
-uli |
Paul_ron
Tinkerer
Username: Paul_ron
Post Number: 228
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 13, 2010 - 08:37 am: |
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The longer the lens, the more critical the adjsutment. Cameras don't need such critical adjsutments sine you have DOF on your side. So how accurate the adjsutmment will you need?
BTW the egg came first since chickens weren't arond while lizards were laying em eons prior. |
Der_uli
Tinkerer
Username: Der_uli
Post Number: 19
Registered: 05-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 14, 2010 - 04:44 am: |
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I agree with Paul_rons argument on DOF only when shooting at f/4 or more.
My assumption for needed accuracy: The enlargement for testing should be a few times higher than the biggest enlargement ever made from the negative. For color film, 12-15x enlargement is a limit (30x40cm prints). Extremely fine-grained black&white films allow a little bit more, say 20x enlargements (50x75cm prints). Therefore I prefer total magnifications of 30-120 for critical testing.
In the collimation test setup, we have a multi-stage enlargement. The lens on the camera under test projects the film plane to infinity. Its enlargement may be calculated like a magnifying glass: standard viewing distance of 250mm divided by focal length. So, with a 50mm standard lens, you'll get 5x magnification. _Shorter_ lenses give more magnifcation here.
But since we do not look with naked eyes at this, we have to multiply the number with the enlargement of the telescope. This is simply its focal length divided by that of the eyepiece. Binoculars have printed this number on them, e.g. "10x50" where 10 is the enlargement and 50 the front lens diameter.
BTW, it should be larger than the diameter of the lens under test. Else you will miss the light from the outer parts - an effect like stopping down the lens. Mirror telescopes do not work here for a similar reason: their center is blocked.
If we use a second camera instead of the telescope, we need the enlargement factor of its lens/viewfinder combination. Usually, this is close to 1 for standard 50mm lenses. (With digital cameras, you may have to take a crap factor into account.) Here, _longer_ lenses give more magnification: Simply divide the focal length by that of the "standard" lens.
An example: use an 28 mm wideangle lens on the camera under test. Magnification is 250/28 = 9. For observation, a 200 mm tele on a full-frame digital camera (standard lens 50mm) is used, with a magnification of 200/50 = 4. Total enlargement of the observed film plane is 9x4 = 36.
This number is ok if you only want to check infinity adjustment. But you may even explore the picture quality of different lenses with this collimation method! That will be a totally different topic...
-uli |
Cyberjunkie
Tinkerer
Username: Cyberjunkie
Post Number: 2
Registered: 07-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 11, 2010 - 12:16 pm: |
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I am trying to guess how a makeshift collimator could be used for checking the correct performance of a certain lens.
Expecially with large format cameras/lenses, shooting pictures to check for image quality can be expensive (vey expensive if you shoot 8x10").
Using a home-made collimator to visually check for aberrations would be easier and quicker. Sometimes very old lenses were improperly reassembled, or a DIY re-cementing project can produce a misaligned group. Finding a solution would be a little easier with an optical test bench available at home!
My best guess would be to use a ground glass, with five crosshairs, etched in the center and close to the four corners, to check for the consistency of the plane of focus. If there is something else that could be done, please explain. I am sure that it would be of some interest for many forum readers.
have fun
CJ |
Der_uli
Tinkerer
Username: Der_uli
Post Number: 21
Registered: 05-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 17, 2010 - 02:56 am: |
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You are right, the collimation method works the same way with large format cameras too. I once replaced the 35mm camera in the picture above with an old 5x7" Mentor Panorama. The ground glass was rough enough to get a usable picture. To test the alignment of camera and lens, no additional markings were necessary. If it were, I'd apply a water-soluble ink on the matte side.
Another question was if the ground glass would be in the same plane as a film in its holder. For that, I used a dud sheet with white ink spots in the middle and the corners. But the film holder was not inserted completely. Instead I left some space to run a cable inside the camera's bellows. A 12 V Halogen bulb was attached to it to light up the film. Now the ink spots on the film surface can be observed much enlarged through the telescope!
However, you are not asking for camera alignment, but for lens testing. I did much of that with the collimation method, too, but had no time yet to put this online.
Some hints relevant to this are in my focus adjustment link above. The method in short: Black ink marks on the ground glass had minute holes in them. Lit up from behind, they serve as bright point sources which are imaged to infinity by the camera's lens. Playing with the lens' focus reveals many aberrations: spherical, color, coma, astigmatism, field curvature, but not distortion. You need 30-100x total magnification for this, and of course some knowledge: read Harold Suiter's book "Star testing astronomical telescopes", which is the same method but with the bright points on the other side. Maybe you can google for "star testing" in the amateur telescope makers domain.
-uli |
Cyberjunkie
Tinkerer
Username: Cyberjunkie
Post Number: 3
Registered: 07-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, December 27, 2010 - 03:48 pm: |
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Thanks for the advice.
What i had in mind was lens elements alignment.
Controlling proper lens assembly, and checking a lens after re-cementing, would be the two most useful uses that come to my mind.
Any more infos (or Internet links) about home made optical test bech would be very welcomed.
have fun
CJ |
Der_uli
Tinkerer
Username: Der_uli
Post Number: 23
Registered: 05-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 06, 2012 - 04:44 am: |
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Update to "my loong story about focus adjustment at this website":
The website has been renamed and is now available at http://olympus.dementix.org/Hardware/tutorials/FocusAdjustment/omadjust.html |
Hanskerensky
Tinkerer
Username: Hanskerensky
Post Number: 175
Registered: 05-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 06, 2012 - 02:57 pm: |
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Hi Uli, an interesting story. Indeed long but worth reading ! Thanks for sharing. |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 60
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, March 10, 2012 - 09:51 am: |
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Seems to me that there are two problems with setting the infinity focus - knowing that the object is really at infinity, and knowing that the film will really go in the plane where you focus. With some vintage cameras the latter issue can be significant and hard to correct for, and it seems to me there's no better way to address the former issue than looking at something that is really far away. Any benchtop system using a real object and a second lens will be subject to errors in that setup, so now you have to verify two systems instead of one. It's simple enough to just open your front door and look out at distant trees or houses with a ground glass and a magnifying glass.
Going beyond that without making things less accurate, not more, is hard - what I personally would do if I really wanted to nail it would be to bounce a HeNe laser back and forth across a big room until the beam filled the lens, and check the focus of that beam at the back of the camera. You'll see a diffraction pattern, and changing the lens focus in and out makes best-focus pretty obvious to within the limitations of the lens. |
T6nn
Tinkerer
Username: T6nn
Post Number: 26
Registered: 06-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, March 10, 2012 - 05:05 pm: |
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Jeffik I'm a bit skeptical on your hypothetical laser setup: First, how will you verify that all the mirrors (and there has to be a lot of them?) are perfectly flat? If only one of them isn't, the infinity will be off. Second, the monochromatic light of a laser would focus at the same distance as the white light only in case of a truly apochromatic lens, in any other case there could be a difference. I would prefer a light source as close as possible to the light the lens gets used in.
If one needs to check whether the film actually is where the lens is collimated to, I believe the best tool is an autocollimator. They are often used with a mirror in the film plane, but albeit with a reduced image brithness, they work with a piece of actual film too. Even off the optical axis. |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 61
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, March 10, 2012 - 07:23 pm: |
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Flatness of the mirrors isn't critical here, they just need to be flatter than the divergence of the beam over the small area of the beam, so that the beam expands. Just bouncing one off the length of a typical house a few times off 2-3 mirrors is enough, a laser is spatially coherent but the divergence isn't zero. And if the lens is focused for red or green, it will be focused within the chromatic aberation range for other colors too - better than this isn't meaningful (or defined, really) for white light through a real lens.
But agreed, if you want to build one then an autocollimator would be the best way to go, reflecting light off a piece of actual film placed inside the camera. |
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