Lens Focal Length & Aperture

Discussion in 'Large Format Cameras and Accessories' started by Fotoguy20d, Apr 1, 2009.

  1. Fotoguy20d

    Fotoguy20d Subscriber

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    A simple question (with, I suspect, a not so simple answer). I have a lens with no markings (or suspect markings). How do I figure out its focal length and aperature?

    In this case, it's in a brass barrell which someone seems to have modified to add an aperature in the center. The lens elements front and back do thread out if that helps in the exercise. The aperature is somewhere in the middle of its 3" or so length so it'll be difficult bu not impossible to get in there and measure the diameter of the opening.

    Right now I have the lens wedged into an extra lensboard of my 5x7 B&J - I need to check again but it doesn't seem to cover 5x7 but does cover 4x5.

    Thanks,
    Dan
     
  2. JBrunner

    JBrunner Moderator Staff Member

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    If they did it right the aperture location is the nodal point, or at least close enough for horse shoes and hand grenades. Some freak posted a video about how to calculate from there....

    Ah here it is:[YOUTUBE]http://www.youtube.com/watch?v=CIFNdfjem18&feature=channel_page[/YOUTUBE]
     
  3. jimgalli

    jimgalli Subscriber

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    Scientific exactness is not required for good round numbers and exposures. Measure from the center of the lens between the elements to the ground glass when it's focused at infinity. Hold a ruler up to the front and measure the narrowest aperture (usually a bit smaller than the front light). Divide. That's it. ie. you measure 12 inches at infinity and it has a 2 inch widest aperture, it's a 12" f6 lens. I do this all the time with these antiques and get excellent exposures. Don't do the flashlight through the lens circle on the wall bit. You're not at infinity doing that and it's meaningless unless you're an ebay seller. All of a sudden you have a 10" lens to sell instead of a 6 inch one. :rolleyes:

    Oops, sorry Jason. We were both posting at the same time.
     
  4. JBrunner

    JBrunner Moderator Staff Member

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    Don't tell my girlfriend.
     
  5. jimgalli

    jimgalli Subscriber

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    <note to self> always speak in millimeters with Jason </note>
     
  6. Christopher Walrath

    Christopher Walrath Member

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    Hope you've stocked up on tortellini.
     
  7. Fotoguy20d

    Fotoguy20d Subscriber

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    Jim/Jason,

    Thanks. I can't access youtube right now but I'll look at it later. Right now I have the lens jammed into the lensboard at its midpoint (oversize hole with some cloth wrapped around the lens to create a tight fit) so it'll be a fairly easy measurement.

    Dan
     
  8. Fotoguy20d

    Fotoguy20d Subscriber

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    I just measured the lens at around 6 1/2" (but not quite at infinity - probably around 100ft - it's pouring rain so I wasn't getting outside this morning). Just for fun, I unscrewed the front element and the length increased to around 10". Removing the rear element, I discoved that with no lenses it didn't do much - so I put the front element back in and with just that one, the length increased to a bit over 12". Out of curiosity, I reversed the lens with rear element removed and found the focal length was now out around 18" (beyond the bellows length of the camera - I might have to figre out how to mount it on my GVII).

    The aperature is almost centered, perhpas biased a 1/8" towards the back, but that might have to do with the geometry of the lens elements (front is bigger slightly) so it's pretty close to perfectly centered. Aperature at maximum is around 7/8".

    I'll try taking some photos with it tomorrow and see if it's any good. The ground glass images look pretty good with just a single element - any inferences I can draw about the construction of this lens that it seems capable of photos this way. Should I expect decent images? What might some of the shortcomings be?

    Thanks,
    Dan
     
  9. Mahler_one

    Mahler_one Member

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    Jason's videos are quite the thing....very illuminating, very clever, and worth watching.

    Ed
     
  10. Q.G.

    Q.G. Inactive

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    A traditional way is by focussing on something that makes it easy to measure scale (a ruler), measure the magnification (= size on the screen / size in real life) on the focussing screen, and measure the bellows draw.
    Then move the thing away, refocus, and measure both magnification and bellows draw again.
    After that, a bit of math will produce the focal length.

    Focal length = Difference in bellows draw / (Magnification1 - Magnification2)
     
  11. Mark Sawyer

    Mark Sawyer Member

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    Or tortellinis.

    BTW, I believe this calculation requires Fritos, not tortellinis. Everyone knows the "f" in "f/stop" stands for Fritos...