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  1. #1
    David Neal Minnick's Avatar
    David Neal Minnick Guest

    Default Eyepiece Focal Length?

    Can someone draw me a word-picture or direct me to a resource showing
    just how the focal length of an ep is measured? That is, if one could
    cut an ep in half lengthwise showing everything intact, from which
    part to which part is the f/l measured?

    David Neal Minnick
    Lake Elsinore, CA

    "The meek shall inherit the gulag."

  2. #2
    Edward's Avatar
    Edward Guest

    Default Eyepiece Focal Length?

    "David Neal Minnick" <> wrote in message om...

    This website shows calculations for 2-3 element eyepieces:

    Ed T.

  3. #3
    Wfoley2's Avatar
    Wfoley2 Guest

    Default Eyepiece Focal Length?

    The FL of an eyepiece is not measured from anyplace in particular, unless it is
    a single element EP. The EFL is based on the magnification with a particular
    objective, not a measurment from somewhere in it. There are formulas for the
    EFL of an eyepiece, depending on the design and glass used. The formula for a
    2-lens EP is pretty straightforward (using the FL of each single lens and the
    distance between them), but adding more elements complicates this. Generally,
    the FL printed on an EP is an approximation.
    Clear, Dark, Steady Skies!
    (And considerate neighbors!!!)

  4. #4
    William Hamblen's Avatar
    William Hamblen Guest

    Default Eyepiece Focal Length?

    On 27 Dec 2003 19:03:29 -0800, (David Neal
    Minnick) wrote:

    If you know the curvatures, thickness and type of glass for each
    element in the eyepiece you can calculate the focal length of the
    eyepiece. That's what lens designers do. If you had an optical bench
    you could measure it directly. There are natural variations in
    manufacturing tolerances so the focal length engraved on the eyepiece
    may not be exact. Or the design can change, but the markings stay the

    Lenses have imaginary planes called the principal planes where the
    rays of light that enter and exit seem to bend when you trace the
    rays. The locations for any given eyepiece depends on the design.
    The focal length is measured from those planes.

  5. #5
    matt's Avatar
    matt Guest

    Default Eyepiece Focal Length?

    "William Hamblen" <> wrote in message

    I believe the OP question was _not_ answered. The way I understood it, he
    was asking where is the _reference point_ from where the focal length is
    considered for a multi-element eyepiece . For example let's say we have a 4
    element ep , which are spread (due to their thickness and their spacing)
    over a 1" length .Let's say the eyepiece is a 7mm FL. Where is the resultant
    focal plane exactly ? Depending where is this "reference point" situated
    lengthwise , one could have the focal place very close or very far from the
    eyepiece end , or last/first element . This question is important if one
    wants to know how to connect various accessories, tube length, focuser
    length , etc . I'd like to see other answers than "just measure it" or "it's
    hard to explain" , "do a google search" etc .

    Thank you,

  6. #6
    William Hamblen's Avatar
    William Hamblen Guest

    Default Eyepiece Focal Length?

    On Sun, 28 Dec 2003 11:53:43 -0500, "matt"
    <> wrote:

    In eyepieces with an external field stop, the first principal plane is
    one focal length away from the field stop. You could locate the
    second principal plane by catching the image of the moon on a piece of
    card stock and measuring one focal length in from that point.

    To measure any better would take a well equipped optical shop. The
    eyepiece supplier would have to provide any more information.

  7. #7
    Kilolani's Avatar
    Kilolani Guest

    Default Eyepiece Focal Length?

    "Edward" <Reply@thegroup.thx> wrote in message
    Note: If you take your eyepieces apart to measure the distances between the
    elements in order to use this calculator, the chance that you will ever get
    them back together again correctly is rather small.

  8. #8
    Don's Avatar
    Don Guest

    Default Eyepiece Focal Length?

    Here is a way to find the focal length of your eyepieces by direct
    experiment that I posted a while back. It worked very well for me.

    "If you want to try measuring the focal lengths of your eyepieces here is

    a simple way that seems to give me good accuracy of about +-0.1 mm in
    the focal length even for the short focal length eyepieces. It also
    works if the eyepiece is used with a Barlow and gives the effective
    focal length of the eyepiece+Barlow combination so you can thus get the
    amplification factor.

    Its based on measuring the magnification seen when you use the eyepiece
    to project a Ronchi grating on to a distant screen (im my case a white
    wall). To use it you put a Ronchi grating behind the eye end of the
    eyepiece, with a flashlight behind the Ronchi to illuminate it. You then
    carefully adjust the separation of the eyepiece from the Ronchi so that
    a sharp (greatly enlarged) image of the grating is shown on the wall.
    You the measure the separation of the projected lines on the wall (I
    measure the span of about 5 cycles and then divide by 5). Knowing the #
    of lines/inch on the grating you divide the separation on the grating
    itself into the measured separation as projected to get the
    magnification. The only thing left is to measure the separation of the
    eyepiece from the wall. To a first approximation the eyepiece focal
    length is that distance divided by the magnification. If the distance to
    the wall is much larger than the focal length the error will be small.
    To get a better approximation you have to allow for the separation of
    the eyepiece principle planes. You can correct for that by changing the
    distance to the wall and repeating the measurement. The math for that is
    not hard but perhaps a bit long for a post here. By the way I posted on
    this in sci.astro.amateur back on about Feb 10,2000.

    If you don't have access to a Ronchi grating, a transparent
    ruler with a mm scale can also be used but the projected mm separation
    is large for short focal length eyepieces and distortion may then throw
    off the result.

    Have fun if you try it,
    Don Taylor"

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  9. #9
    David Neal Minnick's Avatar
    David Neal Minnick Guest

    Default Eyepiece Focal Length?

    Don <> wrote in message news:<>...

    Thanks, Don.

    I was more interested however, in knowing just how the designers come
    up with the measurement in a diagrammatical, drafting table
    visualization of it.


  10. #10
    Roger Hamlett's Avatar
    Roger Hamlett Guest

    Default Eyepiece Focal Length?

    "David Neal Minnick" <> wrote in message om...
    Your best bet, if you want to try 'playing' with some optics to see what
    happens, is to download OsloLT, which is a complete optical design program,
    with the 'LT' version being limited on the number of elements that can be
    used (but being free). Programs like this do all the 'hard work'. Simple
    physics gives the way that a lens element behaves, but when the different
    elements are based on different glasses, the surfaces on each side of the
    element may differ in curvature, and may use unusual curves, calculating the
    effects for different light frequencies and paths, involves a _lot_ of
    arithmetic. Design programs that do the calculations for you, have become
    the 'norm' because of this.
    Also try looking at:
    This is the 'amateur telescope makers' homepage, and there are some articles
    on there, illustrating some eyepiece designs, and the optical formulae.

    Best Wishes


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