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  1. #31
    markbarendt's Avatar
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    Mark Barendt, Ignacio, CO

    "We do not see things the way they are. We see things the way we are." Anaïs Nin

  2. #32

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    As a rule of thumb, if your camera and particular lens combination will focus with the Split Image perfectly at infinity then I have always found that the remainder of the focusing range will be correct.

    I had an Olympus OM1 a good few years back and the 28mm lens would not focus at infinity and it was out of focus all the way down to the closest focus point. It was distinctly 'soft'. I had an optical engineer look at it and set it up on a collimator and it was fine from there on.

  3. #33

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    Quote Originally Posted by ic-racer View Post
    Most of the focus issues I have encountered with fixed-focus screen 35mm SLRs are from the mirror. The mirror is a likely suspect because it moves with each exposure and needs to always come back to the exact same position that it was when it left the factory. .
    Particularly problematic are elderly SLRs (as most of ours will be now) where the mirror rest is made of rubber or some other sound deadening/damping substance which degrades with time and use. The mirror stop in a Pentax MX I have is, in effect, a metal edge and a constant focussing error evident when using shallow DoF/long lenses turned out to be due to the edge having worn its way into the thick black crackle finish on the underside of the mirror, changing the mirror's position relative to the viewing screen. (Earlier Pentaxes had a flat metal pad that spread the load over a much wider area and doesn't seem to have caused a problem). A good way to worry yourself is to set up a tripod with an SLR body and lens on it and focus on an object, say, 15 feet away. Note the reading on the focussing scale precisely, then repeat using the same lens and a different body. Even allowing for the fact that the camera tripod bush location may vary relative to the film plane, the differences between indicated distance on the same lens but different bodies can be alarming!!!

    Steve

  4. #34
    Leigh B's Avatar
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    Quote Originally Posted by Mr Bill View Post
    The Gokosha autocollimators we used project a fairly narrow beam...
    That's odd. The Gokosha that I used had an objective perhaps 75mm in diameter, large enough to cover any regular lens.

    My military auto collimator has a 4" diameter objective, and will cover any lens I've encountered.

    Are you sure the tool is appropriate to your usage?

    Quote Originally Posted by Mr Bill View Post
    This would certainly be a good test application for an autocollimator with focusing lens.
    I don't understand that comment. What is an autocollimator 'with focusing lens'?
    I've never seen an adjustable lens on an autocollimator. That would defeat the purpose.

    - Leigh
    Last edited by Leigh B; 08-21-2012 at 04:59 PM. Click to view previous post history.
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  5. #35
    Leigh B's Avatar
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    Quote Originally Posted by Steve Roberts View Post
    ...set up a tripod with an SLR body and lens on it and focus on an object, say, 15 feet away. Note the reading on the focusing scale precisely, then repeat using the same lens and a different body. Even allowing for the fact that the camera tripod bush location may vary relative to the film plane, the differences between indicated distance on the same lens but different bodies can be alarming!!!
    If that's true, you have some seriously broken cameras.

    Point #1: The distance marked on the lens is from the subject to the film plane, not to the lens.
    The film plane is marked on all (decent) cameras by a small circle with a line running through it, parallel to the film.
    The line is the actual film plane location, and is used for all distance measurements.

    Point #2: The distance from the lens mounting plane to the film plane should be exactly the same in every camera using the same lens.
    For Nikon F-mount lenses this distance is 46.5mm with a very tight tolerance (IIRC ±0.05mm but don't quote me).

    Combining these two points places the lens very accurately within the optical system of the camera,
    which should result in very slight variation in reading from one body to another.

    I just tried this test using several different Nikons, from an F2AS through a D300S, and found no focus shift.
    I cannot speak to the performance of other camera/lens brands since I have no experience with them, nor examples to test.

    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  6. #36

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    Quote Originally Posted by Leigh B View Post
    That's odd. The Gokosha that I used had an objective perhaps 75mm in diameter, large enough to cover any regular lens.
    Hmmm, equally odd to me. C.R.I.S. Camera was the US distributor, but I never knew of anything there other than the types I used. They were current products up to a handful of years ago, even though they look like 1950's sci-fi ray guns.

    Quote Originally Posted by Leigh B View Post
    What is an autocollimator 'with focusing lens'?
    I've never seen an adjustable lens on an autocollimator. That would defeat the purpose.

    - Leigh
    Like this one: http://www.flickr.com/photos/2950454...n/photostream/
    It just extends the capabilities a bit.

    On this model, the focusing barrel has micrometer-like scales, with zero at the infinity-focus position.

    One use would be with respect to checking how well a zoom lens holds focus. If you refocus the Gokosha unit then read the offsets from the barrel, you can calculate the focus error of the zoom lens. Or, if the backfocus distance is in error, you could take a handful of readings, then calculate shim corrections in the lens mount.

    I don't know of any way to do these things with a pure autocollimator.

  7. #37
    Leigh B's Avatar
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    Quote Originally Posted by Mr Bill View Post
    One use would be with respect to checking how well a zoom lens holds focus. If you refocus the Gokosha unit then read the offsets from the barrel, you can calculate the focus error of the zoom lens. Or, if the backfocus distance is in error, you could take a handful of readings, then calculate shim corrections in the lens mount.
    I don't know of any way to do these things with a pure autocollimator.
    OK. The Gokosha in the Flickr photo is WAY smaller than the unit I used.
    This one seems tailored for digital sensors. Is it designed for use with camera optics or directly on the sensors?

    The one I mentioned predates digital imaging. I used it about 30 years ago.

    The military auto-collimator I have now has several internal targets, at calibrated ranges, for checking close focus.
    Nothing is adjustable by the user. The target positions are set during instrument calibration by a lab tech.

    The camera (focal plane) position is adjustable using a micrometer drive.

    This is quite a monster, with a 48" focal length. It weighs over 75 pounds and fills a six-foot-wide workbench.

    - Leigh
    Last edited by Leigh B; 08-21-2012 at 07:39 PM. Click to view previous post history.
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  8. #38

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    Sounds like a comparison between a desk-size vs a desk-top unit. The little Gokosha's footprint is roughly like a laptop computer. It's surprisingly heavy, perhaps 20 pounds, which is mostly in the base.

    My understanding is that they were intended for camera repair shops, in particular for 35mm cameras. The circular base, with 3 leveling screws, is large enough to hold a typical camera body. The normal routine would be to use a front-surface mirror at the film plane, but I've found the reflection from the surface of film to also be workable.

    They definitely predated digital. Regarding digital, I don't see how an autocollimator would even be usable. A plain collimator, as an infinity target, yes, but digital has no proper reflective surface for an autocollimator.

    I hope you got your military unit as surplus, even the little Gokoshas were priced around 5 or 6 or 7 thousand dollars in the mid-1990s.

  9. #39
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    The Gokosha looks like a nice unit. I've never seen one in that configuration, but that means nothing.

    Mine is also designed to work with a first-surface mirror at the film plane, but works fine with real film.
    You can also swing the reflex mirror assembly out of the way and use it as a regular collimator, with a ground glass at the film plane.

    My comment about using it on the sensor itself was applicable only to the fact that photographic sensors have built-in optics.
    That really doesn't sound like a productive usage; just a brain flash.

    Yes, I got these surplus. Originally they were very expensive.

    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  10. #40
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    This has been an interesting thread - I've always been curious about my lenses that are *slightly off* at infinity. The negatives come out looking fine, but the split prism is just barely off. If I recall, I've only noticed that on my old Canons and on my Pentax.

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