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Thread: Large Format Lens

  1. #1

    Large Format Lens

    Dear Sirs/Mdms,

    I've a 150mm Apo-Sironar-N Large Format Lens. How do i adjust this 150mm Large format lens to focus at infinity?

    Please kindly advise me. Looking forward to your replies. Thanks

    Best Regards
    Ho Pei Jiun

  2. #2

    Join Date
    Jan 2004
    Location
    Halifax, Nova Scotia
    Posts
    443

    Large Format Lens

    To focus a 150mm lens at infinity the lens needs to be 150mm from the film plane. To focus closer than infinity the lens needs to be farther than 150mm from the film plane.

  3. #3

    Join Date
    Mar 1999
    Posts
    769

    Large Format Lens

    Please pardon me if this sounds like elementary, but if you are just starting with LF, it is unlike smaller formats where the lens focuses by moving internal elements, helical focusing rings etc. You have to maually focus. Place the lensboard with the lens mounted on it in the front standard. While looking at the ground glass at the back, increase and decrease the bellows length between the front and back till a focused image forms on the ground glass. Best, DJ

  4. #4

    Join Date
    Oct 2000
    Location
    Berkeley CA
    Posts
    153

    Large Format Lens

    To focus a lens at infinity put the film or CCD sensor 1 focal length from the rear principle plane of the lens. By your previous questions, you appear to be trying to set up by measurements. This fails because:
    1. The rear principle plane is not marked on the lens.
    2. The focal length is "nominal", it might be off a mm or so, by manufacturing tolerances.
    3. The CCD is covered by a glass window, of probably unknown thickness and index of refraction, so the optical CCD surface position is different from the physical position by an unknown amount (different by about 1/3 of the window thickness).
    So you do an initial setup by your best guess of the numbers, and focus by trial and error. Image some distant sharply defined object, say a tree on a distant hilltop or a star. Look at the image. Change the focus a bit, see if the image gets better. Repeat till image is sharp. This requires that the setup have provisions for fine adjustment of focus: shims, adjustment screws, focus mount, etc.
    You might calculate your focus tolerance: how much can you change the lens position at the aperature you will use before the blur circle exceeds the resolution you desire.

  5. #5

    Large Format Lens

    The design value for flange focal distance (FFD) of your lens in a Copal shutter is 142.0 mm -- this value is given in the PDF datasheet that you can download from http://www.linos.de/en/prod/apo-sironar-n.html. So to focus on infinity you should place the front of the lensboard (which is also the back of the shutter) 142 mm in front of the film or CCD. However, as Jim says, there are manufacturing tolerances and the FFD of your particular lens may be different from the intended value. So using the FFD to position the lens should get you an image, but then you need to fine tune the focusing as Jim describes.

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