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Thread: How to calculate ground glass distance?

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    Senior Member RicanJoe's Avatar
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    Default How to calculate ground glass distance?

    I'm completely done with my 35mm adapter but cant seem to find the proper distance to place my Ground glass at. If I place it too close, I cant get long/far shots cause it is out of focusing range, but if its too far I cant get proper dialog shots. In commercial adapters, are/is the glass mounted on a sliding system? How do I calculate the proper distance I should place my ground glass in front of my camcorder lens in order to obtain the proper focusing range my 35mm vivitar lens is capable of? thanks in advance

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    Legend
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    The ground glass must be placed at the focal point of the lens. Quoting from Daniel's pdf tutorial (from www.jetsetmodels.info):
    "For Nikon lenses the focal [length] is 46.5 mm. But by using a Canon Ee-S / Ee-A focusing screen you have to shorten the focal flange to 45mm because
    the matte side is facing the camcorder lens."


    Hope that helps.
    Sharp Shooter

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    Legend tcindie's Avatar
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    The GG would never be on a sliding system. If you think about how a camera works that question should answer itself. The film moves along a fixed plane, the lens is in a fixed position, but when you zoom or focus the elements move and refocus the light onto that fixed film plane.

    In a 35mm adapter, the ground is acting as the film plane. This works exactly the same as it does when the ground glass is being used in the viewfinder of an SLR camera, only in a 35mm adapter it's the camera that looks at the ground glass, not your eye.

    As for proper alignment, Worley's comment is right, but additionally setting the focal ring to infinity and taking the whole assembly outside on a sunny day to check proper alignment can be beneficial too.

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    Senior Member RicanJoe's Avatar
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    Quote Originally Posted by Worley View Post
    The ground glass must be placed at the focal point of the lens. Quoting from Daniel's pdf tutorial (from www.jetsetmodels.info):
    "For Nikon lenses the focal [length] is 46.5 mm. But by using a Canon Ee-S / Ee-A focusing screen you have to shorten the focal flange to 45mm because
    the matte side is facing the camcorder lens."


    Hope that helps.
    Is my lens bad for what I'm trying to do?(I cant even place a lens hood on it cause the whole front area spins around when I turn the focusing ring, I thought It was broke at first). Do you recommend I get a nikon lens? Also how would I find the focan length on my lensCould it be on the lens itself (Im not a photographer)?
    thanks

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    Legend tcindie's Avatar
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    That's a nice lens, but it is a bit on the slow side, and generally speaking zooms aren't the best choice for use with a 35mm adapter, though they CAN work.

    I'm not familiar with Yashica or Contax cameras, but I'd guess the distance from the back of the lens to the film plane is similar to a nikon or canon.

    As for the lens hood issue, it should be said that you don't NEED a lens hood, strictly speaking.. but you could mount a matte box on a set of rails, it would actually offer more shade from off axis lights, and be a better protection from lens flares than a small lens hood -- with the added bonus of making your rig look more like a professional movie camera.

    The easiest way to find focal length is to set the lens to infinite focus, and the largest f stop setting (so the iris is at its smallest diameter), take it outside on a bright day and move the lens until its projected image on a piece of paper is in focus, then measure that distance. That measurement would be the focal distance, and should be the space between the back of the lens and the actual focal plane (in the case of a nikon ground glass, it would be the side that faces the lens, or the side furthest from the lens for a canon ground glass).

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    Senior Member RicanJoe's Avatar
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    Quote Originally Posted by tcindie View Post
    That's a nice lens, but it is a bit on the slow side, and generally speaking zooms aren't the best choice for use with a 35mm adapter, though they CAN work.

    I'm not familiar with Yashica or Contax cameras, but I'd guess the distance from the back of the lens to the film plane is similar to a nikon or canon.

    As for the lens hood issue, it should be said that you don't NEED a lens hood, strictly speaking.. but you could mount a matte box on a set of rails, it would actually offer more shade from off axis lights, and be a better protection from lens flares than a small lens hood -- with the added bonus of making your rig look more like a professional movie camera.

    The easiest way to find focal length is to set the lens to infinite focus, and the largest f stop setting (so the iris is at its smallest diameter), take it outside on a bright day and move the lens until its projected image on a piece of paper is in focus, then measure that distance. That measurement would be the focal distance, and should be the space between the back of the lens and the actual focal plane (in the case of a nikon ground glass, it would be the side that faces the lens, or the side furthest from the lens for a canon ground glass).
    Thanks alot for the help! Ill do that tomorrow morning

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    Senior Member RicanJoe's Avatar
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    Will a nikon or canon lens give me less light loss? Or does light loss have to do with the ground glass/projection material in my diy 35mm adapter?

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    Legend tcindie's Avatar
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    the total amount of light loss is based on your lens (its settings and speed) and the amount of other elements (GG, achromat, etc) that the light has to pass through.

    The main thing to look at on the lens is the f-stop range. nikon has a 50mm prime lens that can be found on ebay for cheap, that goes from f1.4 to f16
    The lower the number the brighter the light coming through and consequently the shallower the depth of field.

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    But you can loose some sharpness in some of the faster lenses so some people shoot with AF 50mm f/1.8 for performance/pricepoint balance.

  10. #10
    kyle.presley
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    When you go to set the proper distance from the rear element of the lens to the focal plane (this is the flange focus), open the iris all the way up, don't stop it down. Stopping it down deepens the depth of field (as opposed to shallow), thus making it harder to tell when the background is truly in focus for the full exposure range. Do go out on a brightish day, but open the iris so the image will pop into focus easier, and you won't be second guessing it.

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    Legend tcindie's Avatar
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    Kyle,

    That is true.. but to first find the focal distance it would be better to start with settings that will put the full image into focus before playing around with shallow focus, or you'll end up moving the lens and focus ring for a long time.. The only other way to do it would be setting up objects to focus on at known distances.

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    Senior Member RicanJoe's Avatar
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    thanks for the replies everyone. Im am now upgrading my 35mm adapter a smoother grounded glass(actually its a compact disk) and a new lens, but I need to know which is the most beneficial(works best) and most commonly used lens in the 35mm adapter world. I take it that 50mm f1.8 and below are best, is that correct?
    thanks again for the great support!

    EDIT: WOW I just found this
    http://cgi.ebay.com/Leica-Leitz-Noct...QQcmdZViewItem
    Im damn sure not gonna buy it, but is that good(is it the king lens) for 35mm adapters?
    Last edited by RicanJoe; 2007 November 25th at 19:44.

  13. #13
    kyle.presley
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    there is no need to play with the focus ring at all. You set the focus to infinite, so if it's set at infinite and the iris is full wide, it will pop in focus easier when adjusting the GG. If the iris is closed it won't "pop" as easily and makes it more likely that the flange focus will be off.

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    Quote Originally Posted by RicanJoe View Post
    thanks for the replies everyone. Im am now upgrading my 35mm adapter a smoother grounded glass(actually its a compact disk) and a new lens, but I need to know which is the most beneficial(works best) and most commonly used lens in the 35mm adapter world. I take it that 50mm f1.8 and below are best, is that correct?
    thanks again for the great support!

    EDIT: WOW I just found this
    http://cgi.ebay.com/Leica-Leitz-Noct...QQcmdZViewItem
    Im damn sure not gonna buy it, but is that good(is it the king lens) for 35mm adapters?


    Just get a new one!

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    Legend tcindie's Avatar
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    Quote Originally Posted by kyle.presley View Post
    there is no need to play with the focus ring at all. You set the focus to infinite, so if it's set at infinite and the iris is full wide, it will pop in focus easier when adjusting the GG. If the iris is closed it won't "pop" as easily and makes it more likely that the flange focus will be off.
    Hmmm.. good point. Hehe..

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    Legend tcindie's Avatar
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    Quote Originally Posted by ESTEBEVERDE View Post
    Just get a new one!
    Hell.. in that price range you'd be better off going with something like an arri mount and using high end motion picture primes.

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    Quote Originally Posted by tcindie View Post
    Hell.. in that price range you'd be better off going with something like an arri mount and using high end motion picture primes.

    Exactly!

    Give me the $100 Nikon 50mm anyday of the week!

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    Quote Originally Posted by RicanJoe View Post
    ...

    EDIT: WOW I just found this
    http://cgi.ebay.com/Leica-Leitz-Noct...QQcmdZViewItem
    Im damn sure not gonna buy it, but is that good(is it the king lens) for 35mm adapters?
    One of these just sold at auction in Europe for $35,000.00!


    OUCH!


    CUT!

  19. #19
    Legend tcindie's Avatar
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    That's ridiculous. 35k for a metal tube and a couple chunks of glass.. erf!

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    Quote Originally Posted by tcindie View Post
    That's ridiculous. 35k for a metal tube and a couple chunks of glass.. erf!

    Yap! Waaaaayyyy to much.

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    Senior Member RicanJoe's Avatar
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    Quote Originally Posted by ESTEBEVERDE View Post
    But you can loose some sharpness in some of the faster lenses so some people shoot with AF 50mm f/1.8 for performance/pricepoint balance.
    AutoFocus? I dont think an auto focus will work with a 35mm adapter. Did you mean AI?

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    How do you determine the proper distance between the focusing screen and the HV20? I'd like to make my adapter as compact as possible.

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