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Thread: Are my image circle & focal length diagrams correct ?

  1. #1

    Are my image circle & focal length diagrams correct ?

    Hi,

    I want to figure out what is the minimum image circle required to shoot an interior room with the following field of view marked in blue lines. I need to capture all the openings with some wall space on either side.

    Click image for larger version. 

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    Ideally I would like the space to be compressed as much as possible and would prefer to shoot 210mm or 150mm focal lengths.

    Iíve made the following diagrams to figure out how many exposures I would need to join to create the
    required image.

    Click image for larger version. 

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    Click image for larger version. 

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    Click image for larger version. 

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    1. Are these drawings correct? In other words is it correct to say:

    If I want the shot at 210mm I need 4 sheets of film and a lens with a image circle of 384mm or greater
    If I want the shot at 150mm I need 3 sheets of film and a lens with a image circle of 275mm or greater
    If I want the shot at 90mm I need 2 sheets of film and a lens with a image circle of 165mm or greater


    2. If I were to make all three shots would it be correct to say that the 210mm composite image would look more compressed, the background beyond the windows would appear closer than the other two configurations?

    I'm new to the large format world and trying to get my head around it, any responses would be greatly appreciated.

  2. #2
    Corran's Avatar
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    Re: Are my image circle & focal length diagrams correct ?

    If your camera is stationary on all of those diagrams you are essentially going to get the same image on all 3, with simply more film area exposed. Compression is a factor of distance from the subject + background, not focal length - a common misconception. You would need to get further away from the room to get that effect, which is obviously impossible in some situations.
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  3. #3

    Re: Are my image circle & focal length diagrams correct ?

    Thanks Corran. You've saved me a lot of $$, a lot of film and a lot of time.

  4. #4

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    Re: Are my image circle & focal length diagrams correct ?

    Of course you could slide the camera side to side and scan the result and stitch them together. Or pan the camera for that matter.

  5. #5

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    Re: Are my image circle & focal length diagrams correct ?

    You could, but corran's remark regarding the compression still holds true. You'd essentially get the same result as shooting with the same focal length on a larger sheet of film, like 8x20". That would render your 210 mildly long lens on 4x5 a wide angle lens on a larger format!

  6. #6
    Nodda Duma's Avatar
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    Re: Are my image circle & focal length diagrams correct ?

    To calculate Field of view, use this formula:

    tan(theta) = image circle diameter / focal length.
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

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