# How to Figure Friggin Focal length and f Stop Thingies.

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

1. ### JBrunnerModeratorStaff MemberModerator

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2. ### Ray HeathMember

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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. ### DarkroomExperimenteMember

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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. ### JBrunnerModeratorStaff MemberModerator

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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. ### DarkroomExperimenteMember

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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. ### PinholemasterMember

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Did you get the coal for Christmas?

Grin.

I sent this link to all my photo students. Thanks.

7. ### Kevin CaulfieldSubscriber

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

8. ### copake_hamInactive

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

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!"

9. ### KerikMember

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Nice job! You're a funny guy.

10. ### RobCMember

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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.

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11. ### JBrunnerModeratorStaff MemberModerator

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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. ### DarkroomExperimenteMember

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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. ### DarkroomExperimenteMember

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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?

15. ### RobCMember

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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.

16. ### JBrunnerModeratorStaff MemberModerator

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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. ### DarkroomExperimenteMember

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

still confused about the tortellini variance thing

18. ### JBrunnerModeratorStaff MemberModerator

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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...

19. ### copake_hamInactive

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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!

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20. ### DarkroomExperimenteMember

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Reimannian Tortellini have a distinctive non-Euclidean geometry which can alter linear measurements

21. ### nick mulderMember

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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...

22. ### JBrunnerModeratorStaff MemberModerator

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hmmm.... looks allot like a tortellini....

23. ### nick mulderMember

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Oh crap! you used tortellini ? ! ??

Disregard everything I said ...

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25. ### Ian GrantSubscriber

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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

Ian

26. ### mattiMember

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Sweet, just what I needed!

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

/matti