How to calculate distance from shutter to film ?

Discussion in 'Camera Building, Repairs & Modification' started by Falkenberg, Oct 10, 2007.

  1. Falkenberg

    Falkenberg Member

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    In my planning of making a 8x10 portrait camera I have run into some roadblocks. I have found several webpages that can calculate my front and rear focus distance at any given aperture. My question is how to calculate the distance from the lens/shutter to the film/ground glass plane. I was thinking about making a cardbox camera to have something to test the distance, but since I dont have a ground glass yet, I will not be able to see any changes.

    TIA.

    Best regards
    Mikkel
     
  2. Ty G

    Ty G Guest

    You would first need to know the focal length of the lens you are using and build the camera to that. You can figure it out without the ground glass; take a cardboard box, cut a hole in the front bigger than the lens, and cut a hole at the top so you can look inside the box. Aim to a bright object and while looking inside your box, move the lens back and forth 'til focused. Then measure that distance. You can do the same for far objects and very close objects to get the whole range.

    Ty Guillory
    www.tystintypes.com
     
  3. Falkenberg

    Falkenberg Member

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    this lens and shutter

    The Lens is a 240mm/f5.6 with a separate shutter. How would I calculate the distance with this information ?
     
  4. freygr

    freygr Member

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    Focus at infinity is 240mm from the nodal point to the film. But portrait's the person is not at infinity so the lens will be farther away from the film plain. If you have a 4x5 view camera just mount it and focus at the distance your subjects will be at and measure. Other wise a high school or first year college physics text book should have all the formulas in it.
     
  5. BradS

    BradS Subscriber

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    for lenses of conventional design, the formula is:

    1/f = 1/a + 1/b

    where:
    f is the focal length of the lens
    a is the distance from the film plane to the front surface of the lens board (roughly and subject to some assumptions about lens design) and,
    b is the distance from the front of the lens board to the plane of sharp focus (the subject).
     
  6. Falkenberg

    Falkenberg Member

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    Does this mean that with the 240 mm lens and a person at 3 meters (3000 mm) it would give me a distance 260,86mm from lens to film. Is this correct ? If this is true why are the dedicated portrait 8x10 cameras so huge ?
     
  7. Dan Fromm

    Dan Fromm Member

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    They don't use lenses that are wide angle on the format, they use lenses that are long on the format.
     
  8. BradS

    BradS Subscriber

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    Yes. Substituting the values in your example, I get...

    distance from lens to film = 1 / ((1/240) - (1/3000)) == 260.87 mm

    and as Dan has already pointed out, the 240mm focal length would not normally be considered for portraits with an 8x10 camera. It would likely be considered much too short a focal length for such a task. Also, most LF portaitist that I've encountered seem to prefer to position the camera about a meter from the subject.
     
  9. Falkenberg

    Falkenberg Member

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    wich lens

    For my project the 240mm will have to do, but wich lens should I be looking for to get the best portraits with my portrait camera ? I guess that the distance from camera to the person will be something like 1,5 to 3 meters. I want to fill the frame with the head and a little of the shoulders and neck.
     
  10. argus

    argus Member

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    To fill the frame with head and some shoulders with my appr. 440mm portrait lenses on 8x10", the lens sits less than 1 metre from the nose of my subject and have about 600mm bellows draw. (all estimations, I don't actually measure that when doing)

    I'd advise a slightly longer lens.
     
  11. larsco2002

    larsco2002 Member

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    The 240 is not a short telephoto on 8X10

    If you insist on using a short wide angle on 8X10 AND FURTHERMORE, if you plan on filling the frame with head and shoulders, you will suffer the "big nose" effect. This is considered very unflattering.

    I would consider 240 a bit more of a portrait lens on 4X5, since it is longer than the normal focal length for that form (slightly)

    Short telephotos are normally considered to render a more flattering face on all film (and digital now) formats. For instance, for 35mm, most photographers prefer for portraits, 85 to 105mm, where 50 is considered normal.

    To get back to your question, however, there was mention of using Nodal point as the measurement in the lens. I have a basic understanding of this, but I would also like to think that the aperture of any large format lens would be placed at the Nodal point (the point at which light beams cross over in the lens on the way to the ground glass). If my presumption is true, the appropriate distance for a 240mm lens would be 240mm, or 24cm from the aperture blades to the ground glass. All else would be handled by the movements of focusing. If, however, you are building or designing a camera that has a fixed distance from (no focus movement) lens to ground glass, then you are going to have to set up a measured distance for your portraits which will always be constant, and use a mock up of the lens, box, and ground glass and focus it on the chosen distance before nailing things together.
     
  12. Dan Fromm

    Dan Fromm Member

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  13. Ole

    Ole Moderator Staff Member Moderator

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    Also the reproduction ratio will be M = a/b

    So with a working distance (lens to subject) of 1.5m and a scale of 1:3 - about right for a tight head-and-shoulders portrait on 8x10", we will nees an extension of 1.5/3 = 50cm of bellows.

    Putting that into the first equation, we get f = (1/ ( 1/1500 + 1/500)) , or 375mm focal length.

    So that will be a good portrait focal length on 8x10". A 360mm or 420mm will do nicely, depending on how tight you want the portrait, how much distance you want, and how long bellows/extension your camera has.