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Thread: ground glass corners how to use please

  1. #1

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    ground glass corners how to use please

    Hi Guys

    Can somebody please tell me what to do with the corners
    I know its to do with coverage but how do I work it
    I have a Sinar f2 and a Nikkor 75/4.5

    What am I looking for

    thanks

    robin

  2. #2

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    Re: ground glass corners how to use please

    Someone may correct me here, but I think the basic operation is to peek through the corners at the aperture. If you can see all blades/edges of the aperture, you have completely covered the exposure area.

    What I don't know, is how fall-off is related to this. Maybe someone else can chime in?


    Quote Originally Posted by brighamr View Post
    Hi Guys

    Can somebody please tell me what to do with the corners
    I know its to do with coverage but how do I work it
    I have a Sinar f2 and a Nikkor 75/4.5

    What am I looking for

    thanks

    robin

  3. #3
    Whatever David A. Goldfarb's Avatar
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    Re: ground glass corners how to use please

    If the aperture looks like a circle, you shouldn't have falloff. If it looks like the two dimensional projection of an oblate spheroid (i.e., an American football), then you'll have falloff.

    If you can see the lens shade through the aperture, then that is also a source of falloff or potentially vignetting.

    You can also check that bellows sag isn't obstructing the light path from the lens.

  4. #4
    Geert's Avatar
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    Re: ground glass corners how to use please

    Robin,

    the other way'round is also possible, if your tripod is set up so you can see through the lens: with the lens stopped down at the desired aperture, look through the lens to the back of the camera to see if the corners of the ground glass are clear.

    this is particularily handy if you use a camera with viewing hood and it's the way it is teached at most LF workshops in Belgium.

    A Nikkor SW 75mm f4.5 has an image circle of 200mm at f16. That's 23mm of either rise/fall or shift on 4x5".

    G

  5. #5
    Photographer, Machinist, etc. Jeffrey Sipress's Avatar
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    Re: ground glass corners how to use please

    I thought the purpose of cut corners was to allow air to travel in and out of the interior as bellows are drawn. I've never used them for any actual photographic purpose.

  6. #6

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    Re: ground glass corners how to use please

    Oblate spheroid...that sounds like it hurts!

  7. #7
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: ground glass corners how to use please

    With a 75mm lens on 4x5" film, you will have light fall-off. If it's a modern "wasp-waist" WA lens, light fall-off will follow cos^4; if an older it will follow cos^4 (or thereabout).

    If you can see the whole aperture, you will not have more light fall-off than that. If part of the aperture is obstructed from view, either on the inside or the outside, you will have more light fall-off than that.

    It is often easier to look through the lens and see if you can see all corners of the ground glass instead.

    And yes, cut-off corners help letting air escape from the bellows.

  8. #8
    Geert's Avatar
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    Re: ground glass corners how to use please

    Quote Originally Posted by Jeffrey Sipress View Post
    I thought the purpose of cut corners was to allow air to travel in and out of the interior as bellows are drawn.
    Jeffrey,

    You're also right about that, but leather bellows are permeable up to a certain degree. Also, ther's plenty of gaps in an LF camera that allow air to pass but block the light (think about the lens board fitting).

    I already made groundglass for victorian era cameras with only 1 (one) cut corner for exactly that purpose.

    G

  9. #9
    Geert's Avatar
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    Re: ground glass corners how to use please

    Quote Originally Posted by Ole Tjugen View Post
    With a 75mm lens on 4x5" film, you will have light fall-off. If it's a modern "wasp-waist" WA lens, light fall-off will follow cos^4; if an older it will follow cos^4 (or thereabout).

    If you can see the whole aperture, you will not have more light fall-off than that. If part of the aperture is obstructed from view, either on the inside or the outside, you will have more light fall-off than that.

    It is often easier to look through the lens and see if you can see all corners of the ground glass instead.

    And yes, cut-off corners help letting air escape from the bellows.
    Hey Ole, you stole my reply!
    Go to sleep now, it's 1:13 AM morning here and also where you live! Or are you going to watch the Olympics too at 3:30?

    Seriously, I based my reply on these charts:
    modern lenses
    http://www.largeformatphotography.in...s/LF4x5in.html

    classic lenses
    http://www.largeformatphotography.info/lenseslist.html

    Everyone suffering the slightest form of GAS should consult these before purchase.

    G

  10. #10
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: ground glass corners how to use please

    Quote Originally Posted by Geert View Post
    Hey Ole, you stole my reply!
    Go to sleep now, it's 1:13 AM morning here and also where you live! Or are you going to watch the Olympics too at 3:30?...
    I'm at work, working night shift trying to get an oil well placed optimally so we can get as much oil as possible out of the ground. What's your excuse?

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