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How a big exit pupil can reduce the aperture

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#1 GlennLeDrew

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Posted 24 August 2013 - 11:25 PM

This link refers you to a post I made in the ATM forum. It's perfectly relevant for binoculars, and shows the mechanism behind aperture reduction when one's iris is smaller than the exit pupil.

#2 daniel_h

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Posted 25 August 2013 - 01:06 AM

terrific info

#3 Plan9

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Posted 25 August 2013 - 07:13 PM

Replying to Glenn:

This link refers you to a post I made in the ATM forum. It's perfectly relevant for binoculars, and shows the mechanism behind aperture reduction when one's iris is smaller than the exit pupil.


I've used this is a justification for sticking with smaller aperture bins, at least for 8-12x - my pupils don't appear to go much above 4mm! This has been working well for me.

Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover


What kind of minds discuss binoculars? :)

Bill

#4 GlennLeDrew

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Posted 25 August 2013 - 10:41 PM

The minds which discuss binos are the ones which refuse to be handicapped by the reduction in signal to noise that squinting with one eye imparts.

#5 Gordon Rayner

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Posted 26 August 2013 - 12:06 AM

The exit pupil in your third illustration is shown determined by the too-small iris, as though the rays were travelling from the iris to the objective. That is convenient for illustration, but we know that the rays from the objective form an exit pupil whose diameter is determined by the full aperture of the objective, independent of the size of the iris.

The too-small iris cannot use all the rays in the actual exit pupil, whose diameter is determined by the entire objective.

The eyeball rotates around a center which is behind the iris, so the actual full aperture determined exit pupil provides more room for movement of the iris, as the eyeball rotates while the observer scans the field of a stationary binocular or telescope.

Not all of the objective area contributes to the image, in this case, but the observer will see some light from the outer part of the field, rather than sudden darkening.

Commentary?

#6 GlennLeDrew

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Posted 26 August 2013 - 12:29 AM

Gordon,
Correct. The image of the full objective is formed at the exit pupil, the outer part of which falls outside the iris opening. And this provides 'wiggle room' for the iris to move before pupil clipping occurs (the same applies when the iris is larger than the exit pupil.)

The bottom illustration's rays are those which delineate the light bundle accommodated by the eye, to emphasize the role of the iris. The main point here is the little-known aspect whereby the eyepiece projects the iris onto the objective, which is how the coupling of objective to eye is accomplished.

#7 hallelujah

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Posted 26 August 2013 - 10:51 AM

The minds which discuss binos are the ones which refuse to be handicapped by the reduction in signal to noise that squinting with one eye imparts.


Excellent! :applause: :waytogo:

Stan

#8 Gordon Rayner

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Posted 26 August 2013 - 01:40 PM

Without my reviewing all that you have written about your innovative flashlight test for true working aperture, where does the backward ray trace, in the flashlight test, start? At the center of the exit pupil?( which seems to be the same point in either the larger"true" exit pupil or in the eye iris determined exit pupil)

I realize that more thinking and drawing and reading by me would help, but a few words from you will save time otherwise spent in reinventing the wheel.

#9 Stacy

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Posted 26 August 2013 - 02:41 PM

In the case of binoculars, your effective aperture is double (over that of a telescope).

#10 TomCorbett

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Posted 26 August 2013 - 05:00 PM

Post moved by author to a new thread. Eye pupil size, eye color, DNA, Mydriasis.

#11 GlennLeDrew

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Posted 26 August 2013 - 10:39 PM

Stacy,
Two optical systems do not effectively double the aperture. They increase signal to noise by 41% (the square root of 2), which is equivalent to 0.37 magnitude. This is effectively equivalent to a single objective system whose diameter is 18.7% larger.


Gordon,
In the flashlight test for aperture one is merely sending light through the system in the same manner it passes through when imaging a point source on axis. In normal use, parallel light from a distant target enters the objective and a parallel bundle emerges out the eyepiece.

The flashlight, when at least 10 eyepiece focal lengths behind the eyepiece, sends approximately parallel light into the eyepiece, which is then brought to a near point at the focus. The light entering the eyepiece fills the eye lens, whose diameter is larger than the exit pupil. No matter. Only that light which is passed by the most restrictive aperture (if not the objective edge, then usually the frontmost prism surface) emerges. And so from the perspective of the light which successfully transits the full system, the ray paths encompassing the full bundle exclude light outside the exit pupil.

#12 Stacy

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Posted 27 August 2013 - 12:38 AM

Opps, I meant "area". That should be double.

#13 GlennLeDrew

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Posted 27 August 2013 - 12:45 AM

Actually, the areal increase is the square root of 2, or 1.41, which by diameter is 18.7%. Two equal signals do not integrate so as to simply double in brightness. All that results is an increase in signal to noise, *not any increase in brightness whatsoever.* But we can equate the signal to noise boost as equivalent to increasing brightness, even though it's not strictly correct to do so.






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