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  1. #1

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    Distance between lens and film?

    Quick question I've wondered about for when building a camera...

    How do you know what distance the lens should be from the film to be able to focus it, and how far the bellows should move in and out?

    Tom

  2. #2
    Steve Smith's Avatar
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    Most lens manufacturers publish a figure known as the flange distance for their lenses. This is the distance from the front (emulsion) surface of the film to the front of the lens board or rear surface of the lens/shutter assembly. In theory when set at this distance the camera will focus distant (infinity) objects. For closer objects the lens needs to be further away from the film. This can be worked out mathemtically or by using a ground glass to focus with.

    Here is an example from Schneider's data pages: http://www.schneideroptics.com/info/...6,8-120mm.html


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  3. #3
    wiltw's Avatar
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    Very tricky question! Lenses can be designed to have different distances which might be equal to the FL or less than the FL (so-called 'telephoto' design), or more than the FL ('retrofocus' design). Unless you have a spec sheet to refer to, the only dependable way to figure it out is to use the lens to focus an image of the sun onto a brick and measure that distance (don't focus on a piece of paper unless you intend on starting a fire!)

  4. #4
    Ian Grant's Avatar
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    LF cameras used to be called Double & Triple extension, that's the amount of bellows extension compared to a standard lens. At double extension a lens will focus 1:1, but this was more important for using longer focus lenses (non telephoto).

    If building a 5x4 camera then decide what's the longest lens you want to use, typically most will just accept a 300mm focussing to about a metre. The bellows need to close a close as possible if you want to use a wide angle lens.

    Ian

  5. #5

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    Thanks for the replies. I've just looked up a lens I would probably like to use, the Helios 44M-4. I found this page, with a diagram. http://www.zenit-camera.com/lens-helios-44.htm

    It says the distance from rear element to image plane is 38mm. I'm assuming the image plane is 35mm film. Say I wanted to use medium format film, is there an easy way to work out that distance? I'm assuming the lens would be further from the film.

    Thanks again,
    Tom

  6. #6
    Ian Grant's Avatar
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    The focus, so film distance, is the same regardless of the format.

    You need to find a lens with it's own shutter ideally, this Angulon is an example.

    What sort of camera are you planning on ? Have a look at Steve's 6x12

    Ian

  7. #7

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    Calculating Bellows Requirements

    There's a simple calculation that lets you do this fairly easily based on the lensmaker’s equation

    1/f = 1/i + 1/p

    where f = focal length of the lens, i = image-to-diaphragm distance, and p = subject-to-diaphragm distance.

    Knowing any two of these three variables allows calculating the third. f is known and we choose p, so all we need is

    i = pf/(p-f)



    At infinity focus the image-to-diaphragm distance is i = f.

    You have to decide how close the camera must focus.

    Example: You want to design a camera to use a 300mm lens from 1 meter to infinity. We’ll suppose that you’ve measured the distance from the mounting flange to the diaphragm at 15mm.

    The film to diaphragm distance at infinity is i = f = 300mm.

    At p = 1 meter = 1000mm

    i = 1000*300/(1000-300) = 428.6mm

    Taking the 15mm flange-to-diaphragm distance into account your camera must have a film plane to front of lens board range of at least 315mm to 443mm.

    You can repeat the calculation for the shortest and longest lenses you think you might use on the camera to determine the range of bellows extensions necessary. Don’t be surprised to find that the shortest lenses might require a dedicated short bellows or a recessed lens board. That’s because the longer bellows required for the other lenses might be fully compressed and the short lens is still too far from the film to focus at infinity.

    That’s why camera makers supply short or “bag” bellows and recessed lens boards for very short lenses.

  8. #8

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    With regards to the distance I was thinking if that distance was set for 35mm, would the image from the lens then cover a full 6x6 format right to the edges or just the center portion?

    The shutter was another thing I was wondering about. I have a Kodak Junior 620 medium format bellows camera which has it's own shutter in the lens, but then I would be worried about the quality of the glass. If I used the Helios, I was thinking I would more or less chop up a Praktica SLR I have no use for so I also have the ready made M42 mount attached with shutter, and could use that as the front part of the camera that moves when focussing. All just for experimentation purposes - whether it would work or be worth it is another matter.

    The type of camera I would like to make is something like that link to Steve's 6x12 (I have 2x tinny old 6x9's) but I would love bellows, but just from an aesthetic point of view. I'm thinking now I might just build a new body for the Kodak as a cheap experiment and probably convert it to 6x6 as I love square images.

  9. #9
    ic-racer's Avatar
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    Quote Originally Posted by TomMessenger View Post
    With regards to the distance I was thinking if that distance was set for 35mm, would the image from the lens then cover a full 6x6 format right to the edges or just the center portion?
    You will likely get a circular image with that lens on a 6x6 segment of film. It might be interesting if that is what you want.

  10. #10
    Steve Smith's Avatar
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    Quote Originally Posted by TomMessenger View Post
    With regards to the distance I was thinking if that distance was set for 35mm, would the image from the lens then cover a full 6x6 format right to the edges...?
    Probably not.

    Quote Originally Posted by TomMessenger View Post
    I have a Kodak Junior 620 medium format bellows camera which has it's own shutter in the lens, but then I would be worried about the quality of the glass.
    You could certainly use that lens but all it will gain for you is the knowledge and experience of building your own camera. This is obviously worthwhile but it may make more sense to get this camera working if possible and get hold of something beyond repair to use the lens from.


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

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