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Thread: Math: Calculating distance scale based on focal length, FFD, helical travel

  1. #21
    Corran's Avatar
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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Thanks Jim, for the past hour I've been looking around trying to figure out what those variables meant on the optical data sheet, and just came back to see you've answered my question!

    So if I am reading this correctly, HH' is the "internodal point," yes?

    I plan on making an Excel spreadsheet with all of this data for as many lenses as I can find and will share it here.
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  2. #22
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Quote Originally Posted by Corran View Post
    So if I am reading this correctly, HH' is the "internodal point," yes?
    H is the front nodal point. H' is the rear nodal point.

    So HH' is the distance between the two, the internodal distance.

    Please note that H and H' define their functions in the lens design.
    Their physical location can be anywhere, and in any order.

    H' is the point from which all light headed toward the film emanates, the apex of the cone.
    Front nodal point H has a similar function for inbound light, though not as intuitively obvious.

    - Leigh
    Last edited by Leigh; 9-Sep-2017 at 20:13. Reason: re-ordered the text
    If you believe you can, or you believe you can't... you're right.

  3. #23
    Corran's Avatar
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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Right, I said that wrong, I did mean the distance inbetween. So that makes sense.

    I found this interesting site when searching more info on internodal distance:

    https://people.rit.edu/andpph/text-g...al-length.html
    Bryan | Blog | YouTube | Instagram | Portfolio
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  4. #24
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Quote Originally Posted by JimL View Post
    Does anyone know if similar data is available anywhere for Rodenstock or Nikon lenses?
    The Fujinon brochures give all the data necessary for their lenses.
    Full datasheets are available for all modern lenses of all major lens brands.

    My main lens kit has all of the Rodenstock APO-Sironar-S lenses.
    I had no trouble finding their engineering data online.

    The same is true for Nikon and Fujinon lenses. I have and use lenses from both makers.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  5. #25
    jim landecker JimL's Avatar
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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Quote Originally Posted by Leigh View Post
    Full datasheets are available for all modern lenses of all major lens brands.

    My main lens kit has all of the Rodenstock APO-Sironar-S lenses.
    I had no trouble finding their engineering data online.

    The same is true for Nikon and Fujinon lenses. I have and use lenses from both makers.

    - Leigh
    Leigh, if you could share any links to Rodenstock info it would be greatly appreciated! None of the brochures I've found have anything on principal points, etc.

  6. #26
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Quote Originally Posted by JimL View Post
    Leigh, if you could share any links to Rodenstock info it would be greatly appreciated!
    None of the brochures I've found have anything on principal points, etc.
    Hi Jim,

    I'm sorry but I no longer have that info

    My research on these was many years ago.
    The results have not survived multiple computer updates.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  7. #27
    Corran's Avatar
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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Using the data from Schneider's website, I made an Excel spreadsheet to calculate common focus distances, up to 3.75mm which is one full revolution of the Mercury UWA focus mount.

    Anyone else can adapt this data to their lenses and focus mounts. If you know the focus mount's extension amount in a 360 degree turn, you can calculate where to place your markings by dividing the extension amount given for a focus distance by the extension at 360 degrees, and multiply that by 360 to get the degrees to turn the helical to where your marking will be placed.



    If anyone sees an issue with these calculations, let me know!! I can send this sheet to anyone who wants it and you can input your own data.
    Bryan | Blog | YouTube | Instagram | Portfolio
    All comments and thoughtful critique welcome

  8. #28

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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Bryan, I'm sure your calculations are fine, but what if the lens in hand's focal length doesn't match the number engraved on it?

  9. #29

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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    Quote Originally Posted by Dan Fromm View Post
    Bryan, I'm sure your calculations are fine, but what if the lens in hand's focal length doesn't match the number engraved on it?
    Indeed, if you don't know the EFL of your lens and you don't know how accurate the infinity position of your lens is (down to 0.01mm both values) it is entirely futile to produce spreadsheets with values down to 0.01mm precision.

  10. #30

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    Re: Math: Calculating distance scale based on focal length, FFD, helical travel

    On the other hand Corran, photographers must be in awe to see you indicate them the distance to the subject right down to 1 micron! Um, just don't smoke that stuff every day, for your own good.

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