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Thread: exit pupil

  1. #1
    Tom Rauschenbach's Avatar
    Tom Rauschenbach Guest

    Default exit pupil

    Sitting here with a pair of 25X100 binos on a trpiod and a pair of 11x70
    binos on a tripod, and looking at them from a couple of feet away (call it
    70 cm) I see that the 25x100s have an eyepiece lens 26mm in diameter and
    the smaller pair has an eyepiece lens 22.5 mm in diameter. From this
    distance I see an illuminated circle (both are focused on the same object)
    about 4mm in the larger pair and about 8mm in diameter in the smaller
    pair. Is this the same phenomenon I've heard called "exit pupil" ? Are
    these numbers about what one might expect ?

    The larger instrument's aperture/magnification ratio is 4 while the
    smaller instrument's ratio is 6.3. I would have expected a noticable
    difference in image brightness, with the smaller instrument giving the
    brighter image. Is it credible that the fact that I don't see a
    difference is due to my aging eyes not being able to use the 8mm exit
    pupil (if that is in fact what I measured) ?

    Or am I completely at sea ?

  2. #2
    Bill Tschumy's Avatar
    Bill Tschumy Guest

    Default exit pupil

    On Mon, 22 May 2006 18:36:14 -0500, Tom Rauschenbach wrote
    (in article <>) :

    Yes, the exit pupil will be given by the Objective diameter divided by the
    magnification. The diameter of the eyepiece lens is generally unrelated to
    the exit pupil.

    If all other things are equal (lens coatings, prism loss, etc) then the
    instrument with the larger exit pupil will provide a brighter image for
    non-stellar objects. That doesn't necessarily mean you will see as much
    through it because magnification also plays a big role.

    If you are doing this in daylight then your pupils will be significantly
    smaller than your maximum. You are definitely not sing the full exit pupil.
    The fact that you eyes are well back from the "eye point" of the instrument
    will also be a factor although I'm not sure exactly how that would play out.

    No, I think you are well on land <g>.

    Bill Tschumy
    Think Astronomy -- Austin, TX

  3. #3
    Willie R. Meghar's Avatar
    Willie R. Meghar Guest

    Default exit pupil

    On Mon, 22 May 2006 19:36:14 -0400, Tom Rauschenbach
    <> wrote:

    Yes to the first question.

    For the second question -- a qualified yes: The diameter of the exit
    pupil will change when the focus is changed. The difference is
    negligible for a telescope; but for some binoculars, particularly
    lower powered binoculars, it can be quite noticeable -- and
    measurable. Therefor (for astronomical purposes) you should focus the
    binoculars on the most distant object in sight when measuring exit
    pupil diameters.

    This is a confusing realm for most people -- even for some of the
    self-proclaimed 'experts'. For example (assuming the object being
    observed fits wholly within both binocular fields of view):

    One person might point the two pair of binoculars at the same extended
    celestial object and proclaim: "The 100mm binoculars show greater
    image brightness." This person's reasoning is base on a comparison of
    the total amount of light concentrated into the object's ENTIRE image.

    Another person might perform the identical experiment and proclaim:
    "The 70mm binoculars show greater image brightness." This person's
    reasoning is based on the total amount of light concentrated within
    equal "apparent size areas" within the images of the object. (By
    "apparent size", angular extent on the background sky is NOT being
    referred to. Instead, equal apparent angular extents as seen by the
    observer's eye while looking through the binoculars is being referred

    Within their contexts, each of the above individuals would be correct!

    Yet, in the real world, observing under a starry sky, all one need do
    is take a glance at any extended, nebulous, celestial object that will
    entirely fit within the higher powered binoculars' field. It will
    become quite obvious which binocular is better for observing such
    objects. (Aging eyes will not change the outcome of this experiment).

    Numbers and mathematics, even in the absence of any and all
    mathematical errors can and do mislead many people -- not just amateur
    astronomers When in doubt, do a real world experiment!

    So long and
    Thanks for all the fish!



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