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Thread: Large format lens

  1. #1

    Large format lens

    Dear Sirs/Mdms,

    Sorry if my previous questions are confusing.

    I would like to know what is the distance from the tip of the Rodenstock Apo Sironar-N 210 f5.6 len's furthest rear surface (without the rear element) to the film plane of large format camera if i will installed the lens on the large format camera.

    The reason i need to know the above requested information is because i want to capture a 80mm (W) x 60mm (H) rectangular flat surface object onto a 15.15mm x 15.15mm cooled CCD camera sensor such that 15.15mm width of the CCD sensor completely matches the 80mm width of the object. Hence i need to demagnify by 80mm/15mm = 5.33

    I can almost achieve this 5.33 demagnification by attached the 35mm f2.8 lens onto the CCD camera and then reverse mounted the 210mm large format lens (without the rear element) on the 35mm lens. The ratio between the focal length of the large format lens and 35mm lens will provide this demagnification (i.e. 210mm / 35mm = 6, which is the closest i can get based on my existing lens).

    To prevent the captured image to have ' curvature' effects at the edges, i need to position the 210mm large format lens (without rear element) exactly as it would on a conventional large format camera (the rectangular flat surface object being in exactly the same place as the film plane would be on a large format camera).

    So i deeply appreciate if you can advise me:

    1) What is the distance from the tip of the Rodenstock Apo Sironar-N 210 f5.6 len's furthest rear surface (without rear element) to the film plane of large format camera if i will installed the lens on a conventional large format camera?

    Does this distance change accordingly to the Large format lens focal distance?

    Looking forward to your replies. Thank you very much.

    Sincerely,
    Ho Pei Jiun

  2. #2

    Join Date
    Sep 1998
    Posts
    10,339

    Large format lens

    "1) What is the distance from the tip of the Rodenstock Apo Sironar-N 210 f5.6 len's furthest rear surface (without rear element) to the film plane of large format camera if i will installed the lens on a conventional large format camera?"

    With the lens focused at what distance?

    The only specifications given by the manufacturer are at infinity. The distance will be considerably greater as you focus at closer distances.

    Since you already have the lens why not test it yourself and measure the specific distance you need?

    Simply light your target. Open the lens to 5.6. Open the shutter, hold the lens in your hand pointed at the subject or place the lens on a support like a bean bag pointed at the subject. Hold a piece of white board behind the lens and move it in and out till the subject is sharp. Measure the distace from the back of the lens to the board. If the subject is too small move the lens closer to the subject until it is the proper size on the board. Then measure the distance.

  3. #3

    Large format lens

    Have you looked through an Edmund Scientific catalog? They - and probably other vendors as well - have a pretty good selection of telecentric lenses for CCDs. They might have something you could use for your application straight out of the box and for not too much money.

    Chris

  4. #4

    Join Date
    Oct 2000
    Location
    Berkeley CA
    Posts
    153

    Large format lens

    If I understand what you are trying to do: To achieve the 5.33 reduction, use two lenses whose focal lengths are different by a factor of 5.33, and mount them front to front, so each lens is being used at infinity focus. Part of the confusion is the very different way 35 and large format cameras focus: the 35 has a fixed distance from the lens mounting surface to the film, and the focus ring on the lens moves the glass inside the lens. On large format, the focus mechanism is in the camera, it moves the camera front and the whole lens, so the distance from the front of the camera to the film is moved to about the lens focal length when focussing at infinity. Mounting two lenses front to front may cause vignetting problems: ideally, the lenses should be so close together that the aperature stops are together, obviously impossible. So you will get a small circle of image, probably too small.
    Better to use a single lens made for such a magnification: a macro lens. An enlarging lens would also work, but you would have to make some kind of mount.
    If you remove the rear element of a large format lens the result will depend on the design of that particular lens, the front element alone might be about twice the focal length of the combination and useable (convertable lenses do this), or something quite different and poor quality.

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