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Thread: finding out the focal length when focussed closer than infinity...

  1. #1

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    Question finding out the focal length when focussed closer than infinity...

    Hi - I have just purchased a casket set of four lenses. Unfortunately the focal lengths are not marked.

    Obviously I can find out the focal length by focussing on infinity.

    However in my small garden I cannot focus on anything that is far enough to use for infinity.

    If I focus on an object say 10 meters away (I will measure it) is there a formula that I can use to find out the focal length at infinity?

    Thank you in advance.

    John Wills

    www.londontramways.net
    www.southernrailway.net

  2. #2
    Cogito, ergo sum
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    Re: finding out the focal length when focussed closer than infinity...

    Here you go:
    http://en.wikipedia.org/wiki/Focal_l...In_photography

    If you want to check focal length at infinity, just put it under the sun light. Move the lens until the sun light focus to smallest point. The distance from that point to the lens is approx FL.

    BTW it's better if you use google or forum search engine before asking .
    Life = Love + Passion + Responsibility

  3. #3

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    Re: finding out the focal length when focussed closer than infinity...

    There is the formula

    f = u v /(u + v)

    where u is the distance to the subject and v is the distance to the image. Unfortunately, you have to measure the distance u to the subject from the front principal point and the distance v to image along the rail from the rear principal point. You probably won't be able to figure out where the pricipal points are. But if you measure both distances from the front of the lensboard, that should be good enough for most normal lenses. It could yield a significant error, however, for a telphoto lens.

    I'm not sure why you have to do this in your garden. Just go out on the street and use a building or some other object that is not too close. That may be sufficiently far away. Also, if you use two such objects and get essentially the same distance of the image along the rail, that is a check on whether you are close enough to infinity.

  4. #4
    Jim Graves Jim Graves's Avatar
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    Re: finding out the focal length when focussed closer than infinity...

    Good question ... glad you posted it here ... I like jumanji's solution of using the sun ... that has never occurred to me even though it seems an obvious solution.

  5. #5
    ghostcount's Avatar
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    Re: finding out the focal length when focussed closer than infinity...

    Quote Originally Posted by jumanji View Post
    ... If you want to check focal length at infinity, just put it under the sun light. Move the lens until the sun light focus to smallest point. The distance from that point to the lens is approx FL.
    Ahhh... the ant burning method.
    "Sex is like maths, add the bed, subtract the clothes, divide the whoo hoo and hope you don't multiply." - Leather jacket guy

  6. #6

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    Re: finding out the focal length when focussed closer than infinity...

    I always use the comparison method.
    Hold two lenses up and focus the window frame on a white wall. One of these must have a "known" focal length. Swap the known lens until you get a match - or a very close match. If you have a measuring scale on this wall, you can "measure" the size of the window frame image with a "known" lens and unknow lens. Then do a simple division calculation. This method also gives an indication of the F value of the unknown cell. You can see the difference between and F5 and F6 (as an example) very easily.

  7. #7
    Cogito, ergo sum
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    Re: finding out the focal length when focussed closer than infinity...

    Quote Originally Posted by ghostcount View Post
    Ahhh... the ant burning method.
    Yep that's it! A funny game when I were a child. Summers came, I took a magnifying glass, went out and burned ant and waste paper .
    Life = Love + Passion + Responsibility

  8. #8
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: finding out the focal length when focussed closer than infinity...

    Quote Originally Posted by jumanji View Post
    If you want to check focal length at infinity, just put it under the sun light.
    The OP is in London.

    The sun may not be available.

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

  9. #9

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    Re: finding out the focal length when focussed closer than infinity...

    Have a look at this link it will help.

    http://www.giangrandi.ch/optics/lenses/focalcalc.html

  10. #10

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    Re: finding out the focal length when focussed closer than infinity...

    John,

    The focal lengths are marked on the edge of each element - 15, 25, 35cm, etc. On the sheet I had posted with that set, it gives the focal lengths for the various combinations, but each element alone is already marked. If you need a copy of the sheet, I can email it to you.

    Cheers,Richard

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