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Pupils and aperture stop

  1. Dec 1, 2011 #1
    Dear Forum,

    the aperture stop AS is one the apertures (not necessarily the smallest one) in an optical system.
    the images of the AS are called the entrance and exit pupils.

    What is it important to know the location of the pupils?

    Is it useful when we want to interconnect multiple optical systems and do not want light to be wasted?
    Does the entrance pupil of one system need to be located exactly where the exit pupil of another system is to ensure that all the light is transmitted?

    Do we always want the size of the pupils to be as large as possible?

  2. jcsd
  3. Dec 2, 2011 #2
    The idea is to match the instrument exit pupil diameter to the diameter of the observer's eye pupil. In this case no light will be wasted.
    So you do not always want the exit pupil to be as large as possible.
    For example, using binoculars at night you would want the exit pupil to be about 7mm as that is the probable diameter of the observer's dark-adapted eye pupil. More than that would mean wasted light.
    In daylight you would only need about a 4mm diameter exit pupil to fill a typical eye pupil.
  4. Dec 2, 2011 #3

    Andy Resnick

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    Also, lens aberrations typically increase with aperture size, so in practical terms there is a trade-off between numerical aperture/f-number and cost of the lens.
  5. Dec 2, 2011 #4


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    Assuming your eyes aren't directly in contact with the exit pupil, why does the size of the exit puple matter (ignoring edge diffraction effects), as long as the image size corresponds to the expected viewers input pupil size based on the expected distance from the exit pupil to the viewer's pupil?
  6. Dec 4, 2011 #5


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    If the exit pupil is too big then part of the light cone simply hits outside of your eye's pupil and is lost. If too small then you introduce effects like vignetting.

    The key thing to understand is that incoming light to your eye is NOT focused yet. An eyepiece in an optical device will focus the light not into a point where your eye is at, but into a cone that your eye then focuses.

    See here: http://en.wikipedia.org/wiki/Exit_pupil
  7. Dec 4, 2011 #6
    What seems to matter is that the light entering the optical system must be equal to the light exiting it, otherwise some light will be wasted...

    Entrance and exit pupils sizes and locations help with that.

    In a telecentric system both or only one of the pupils is located at infinity....I don't understand the physical meaning of that. Sure, It means that the aperture stop is on the focal point.

    But how do we interpret the statement that the pupils are located at infinity? Can we draw any useful conclusion in terms of light gathering into the system and light exiting the system?

  8. Dec 4, 2011 #7


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    I was thinking of the case where the image is inverted interally (normally inverted twice for human viewing, using multiple prisms and/or lenses), and expanding as it leaves the exit pupil, a combination of exit pupil size and distance from exit pupil to viewer's retina. In this case, what's imporant is the size of the image on the viewers retina. For a non-inverted image (is this the norm for binoculars which have use a pair of prisms for each eye? what about telescopes?), the image is expanding at it leaves the exit pupil, in which case the exit pupile size does matter, as explained in the wiki article.
    Last edited: Dec 4, 2011
  9. Dec 4, 2011 #8


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    It means that the light "cone" at the pupil is actually a light "cylinder". The rays are parallel rather than convergent.
  10. Dec 5, 2011 #9


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    Ideally you want to have light rays leaving the exit pupil of the eyepiece parallel to each other. This keeps the eye from having to work to focus the image. AKA it looks far away to your eye in regards to focusing. Conversely, staring at a nearby object, such as a monitor, strains the eye because it has to work to focus the diverging rays. Inverted and non-inverted images have no difference in the way the light is converging or not converging. Telescopes rarely have extra mirrors or prisms to invert the image, at that would reduce the amount of light and result in a dimmer image. Other than the required optical surfaces, only an angled flat mirror is used to redirect the light up so you don't have to kill yourself trying to look at something in an eyepiece that is angled almost straight down sometimes.
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