How to Figure Friggin Focal length and f Stop Thingies.

Discussion in 'Miscellaneous Equipment' started by JBrunner, Jan 16, 2008.

  1. JBrunner

    JBrunner Moderator Staff Member Moderator

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  2. Ray Heath

    Ray Heath Member

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    yeh Jason, i kinda unnerstand

    you've worked out the relative f stops

    i'm a liddle confused, is that relative to pasta divided cheetohs or is it relative to your mum's door?

    Ray
     
  3. DarkroomExperimente

    DarkroomExperimente Member

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    interesting...but the f-stop calculation doesn't seem to work on a known lens I tested -- is this only for a certain type of lens?


    I'm in Maryland...we use crab claws, not pasta

    fortunately, I prefer pasta to crabs
     
  4. JBrunner

    JBrunner Moderator Staff Member Moderator

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    It's a fundamental equation. Variables would be the actual location of the nodal point of the lens, the actual diameter used, and infinity focus. If you miss the nodal point, your focal length value will be off by that amount. If you focus on something too close, the focal length measured will be greater than the actual focal length of the lens. The longer the lens is, the more likely this is to be the case. If you measure larger than the diameter of the effective objective, like the barrel diameter, this would also skew the result.
     
  5. DarkroomExperimente

    DarkroomExperimente Member

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    "actual diameter used"....perhaps that's the issue

    I used my Konica lens...so the only measurement I didn't read off the lens was the diameter of the element
     
  6. Pinholemaster

    Pinholemaster Member

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    Did you get the coal for Christmas?

    Grin.

    I sent this link to all my photo students. Thanks.
     
  7. Kevin Caulfield

    Kevin Caulfield Subscriber

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    That is brilliant, John, I mean Jay. :wink: You really are John Belushi reincarnated, aren't you? :wink:
     
  8. copake_ham

    copake_ham Inactive

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    While there's a madness to his method - there's a method to his madness! :wink:

    Meanwhile, I'd like to figure out his fetish with cheese. Cheetoz and (presumably) cheese tortelini figure prominently in this latest installment.

    Quite odd.

    "Paging Dr. Freud, paging Dr. Freud - attend STAT - misplaced Wisconsin cheesehead is found in Utah!" :D
     
  9. Kerik

    Kerik Member

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    Nice job! You're a funny guy.
     
  10. RobC

    RobC Member

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    Its only a very approximate measure because it ignores pupil magnification. The actual aperture is the size of the entrance pupil which you can't see as it's hanging in space inside the lens. Its not the physical size of the iris.
     
    Last edited by a moderator: Jan 17, 2008
  11. JBrunner

    JBrunner Moderator Staff Member Moderator

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    What, you mean I can't use these measurements to focus a friggin laser beam for shooting down an incoming ICBM? Oh, and we haven't even begun to factor in tortellini variance......:rolleyes:
     
  12. DarkroomExperimente

    DarkroomExperimente Member

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    right...it worked fine with my simple slide projector lens, my 40/f1.8 lens...but not with the 28/f3.5

    thanks
     
  13. DarkroomExperimente

    DarkroomExperimente Member

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    tortellini variance?

    geez....I just get a handle on lorentzian linguini effects and you bring up tortellini variance?

    give a brother a break will ya?
     
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  15. RobC

    RobC Member

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    Nope you can't. But I was merely explaining why the poster didn't get the results he expected from the method as described.:tongue:
     
  16. JBrunner

    JBrunner Moderator Staff Member Moderator

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    I probably wouldn't bother to try it on a modern multi element lens. Those generally aren't a mystery. It is an effective way to get in the ballpark with old unknown barrell lenses, such as I have. In my experience it gives enough of an approximation with these old lenses to have a usable stop. Bracket exposures from the approximation to fine tune, and you are happily using your old lens.
     
  17. DarkroomExperimente

    DarkroomExperimente Member

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    K...that's pretty much what I thought....

    still confused about the tortellini variance thing
     
  18. JBrunner

    JBrunner Moderator Staff Member Moderator

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    Well, I'm still working on the math. I haven't been able to determine if tortellin variance is constant enough to be averaged. I'm afraid I ate the test sample, so until I get some more, we are all in the dark. And George, you are correct, they were parmesan...:smile:
     
  19. copake_ham

    copake_ham Inactive

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    Not to forget the critical measurements depending upon the type and "state" of the tortellini.

    Firstly, was the tortellini a "fresh" specimen or was it "frozen" or, yet, if you use the Barilla-brand variant - a "dried" one?

    Then, you have to consider - based on which version you begin with - was the tortellini then "cooked"?

    This is a critical determination as a "cooked" tortellini will have a variant shape and size from an "uncooked" one - regardless of it's initial fresh/ fresh-frozen/ dried state (see above) base constants.

    And then, one must factor into the equation the "satiation effect".

    Did JB consumed a satiating portion of tortellini before conducting his focal lenght measurements such that he was not tempted to "nibble" small portions from either the posterior or anterior ends of some or all of the tortellini used in the experimental setting?

    The quantitative and subjective factors may have considerable bearing on JB's initial findings and thus requires he continue his experiments while applying the Greek classical measuring tools of the dolmades (stuffed grape leaves) and Kalamata olives!
     
    Last edited by a moderator: Jan 17, 2008
  20. DarkroomExperimente

    DarkroomExperimente Member

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    Reimannian Tortellini have a distinctive non-Euclidean geometry which can alter linear measurements
     
  21. nick mulder

    nick mulder Subscriber

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    Theres the whole water going down the drain the other way in the southern hemisphere. This will affect the pasta making, which in turn affects the 'handedness' of the macaroni - the traditional right hand rule, becomes a left handed rule (in order to determine the flux, which you dont need anyway ... for anything) - not that hard once you learn to play the saxophone.

    Recent experiments by the folk at Cern with the LHC determined that Marco Polo was a fine chap and the 1/100000000 chance of it spontaneously developing a Death Star in orbit were unfounded.

    It been suggested its actually just the most expensive crop circle ever...

    [​IMG]
     
  22. JBrunner

    JBrunner Moderator Staff Member Moderator

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    hmmm.... looks allot like a tortellini....
     
  23. nick mulder

    nick mulder Subscriber

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    Oh crap! you used tortellini ? ! ??

    Disregard everything I said ...
     
  24. RobC

    RobC Member

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  25. Ian Grant

    Ian Grant Subscriber

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    Careful you'll have Jason of to Switzerland, he seems to be becoming obsessive about food objects.

    Link works, I remember that on TV as a kid :D

    Ian
     
  26. matti

    matti Member

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    Sweet, just what I needed! :smile:

    Too bad I'm at work so I can't turn up the volume until lunch.

    /matti