Author Topic: help understanding exit pupil  (Read 86 times)

presarersweet

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help understanding exit pupil
« on: December 25, 2017, 09:48:53 AM »
So, I have noticed that there's a great deal of discussion about exit student and what size range is usable/ideal.  While studying these discussions I have come to the conclusion that I must be misunderstanding some key facet.  When I consider exit student I imagine a tiny disk hovering in space where the image is projected.  Obviously, just if something like an eye is at the location to receive the photons is the disk realized.  The photons keep moving and spreading of the exit pupil.

If people talk of too big an exit pupil they talk as though the "surplus" photons are dispersed throughout the disk, thus assessing the image compared to what it would be if able to get all the photons.  But why would they be dispersed like this?  Why doesn't the ring of photons bring about a decrease in the view whilst keeping the remaining view at the same brightness?

Hopefully this makes sense.  And hopefully I'm not overlooking anythingoverly obvious.  Thanks!



Antonio Stanton

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Re: help understanding exit pupil
« Reply #1 on: December 30, 2017, 11:39:55 AM »
The focal plane is a disk where it all reaches focus.  Focal planes can be thought of as disks. 
The exit pupil is a cone streaming out of the scope.  One slice of it is the focal plane.  Your mission is to place your head where your pupil intersects the exit pupil and then to adjust focus so that it best suits the need of your eye. 
If the exit pupil is so large it exceeds the size of your pupil, the light splashes uselessly on your iris.  If the exit pupil is very narrow it is harder to acquire. 
Wide exit pupils and narrow exit pupils each have ways of revealing different eye defects. 
Exit pupil allows us to talk about magnification relative to the instrument without reference to the scope.  0.5 mm exit pupil is high magnification on any scope.  It is 200x on my 4 inch refractor and and 700x on my c14, i.e. it standardizes magnification per inch of aperture.  200x is high magnification on the refractor and medium low on the c14.
200x on the c14, at 2 mm exit pupil, is in the physiologically optimal 1 to 2 mm range for the human eye.  The same spot on my refractor is 50x.
Hope that helps, Greg n

Donnell Keown

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Re: help understanding exit pupil
« Reply #2 on: December 31, 2017, 09:54:05 AM »
There are smarter people here than me, but that cone of light that comes out of the eyepiece
has a diameter called the exit pupil, and I believe it is measured at the place
where it enters your eye when you hold your eye in the proper position for seeing.
If the exit pupil diameter is bigger than the opening of your iris, than
anything that is outside your iris does not go in your eye. Hence wasted light.

Most young adults have an iris opening of 7mm when their eye is fully dialated.
For example I have a 80mm f/5 refractor telescope and a 42mm eyepiece.
There are two ways to calculate the exit pupil Either the eyepiece focal length / f-ratio.
Or the objective diameter / telescope system magnification
In this example I would be 42/5 = 8.4 or 80 / ( 400/42) = 8.4
Either way the exit pupil here is bigger than my iris opening so that light cone cannot completely
enter my eye.

Now generally as we age that maximum iris diameter shrinks. That is why older people
have problems seeing in the dark. I have noticed it for me and I am in my 50's.
I think there is a thread on this site as to how you can measure
your iris diameter.

On the other end of things minimum exit pupil. Generally you cannot go under .5mm
When that happens not enough of your retina is illuminated and you start to see "junk"
in your eye. I'm not sure why this is. It may be some eye/brain system thing where
if enough of your retina is illuminated the system can congatively remove the floaters.

David Collins

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Re: help understanding exit pupil
« Reply #3 on: January 04, 2018, 04:26:36 AM »
I think your confusion is because you believe the exit pupil is the size if the image. If that were the case, then your idea would be correct. however the Exit Pupil does not measure the size of the image; it measures the diameter of the beam of light which can then befocussedto produce the image (normally on your retina). Without getting too technical, I believe every point in the exit pupil beam could in principle be focussed to produce a complete but very very faint image. If you collect up a large portion of the exit pupil beam then you add together all the 'micro point' images to make one brighter image. If the exit pupil is larger than the pupil of your eye - say the exit pupil beam is 10mm in diameter, but your eye's pupil is only 7mm in diameter then you only collect (7/10)*(7/10) of the available light - 49%. In which case you are wasting over half the light and you would be better using a smaller exit pupil which allowed you to collect all the light.

I think.

consmagestma

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Re: help understanding exit pupil
« Reply #4 on: January 04, 2018, 12:39:16 PM »
When outside at night, the human eye becomes dark-adapted over a span of time, and the eye's pupil enlarges, allowing more light to enter, with the maximum diameter ranging from 7-8mm in youth, to 4-5mm in old age. Dark-adapted pupil diameter decreases as we age.

To find the exit-pupil of any eyepiece, you simply divide the eyepiece's focal-length by the f-ratio of the telescope...

Your 25mm Plossl ÷ f/6(of your XT8) = 4.2mm

...or more simply, 25 ÷ 6 = 4.2mm

Your 34mm has an exit-pupil of 5.7mm. If the diameter of your dark-adapted pupil meets or exceeds 5.7mm, then you're good. If not, then the part of the light that exits the eyepiece will fall onto the iris instead of into the pupil; thereby light is wasted, and in the pupil not receiving all of the light gathered by the primary mirror of the telescope.

Take this 2" 50mm eyepiece, for example... http://agenaastro.co...w-eyepiece.html

50 ÷ 6 = 8.3mm

That's a considerably large exit-pupil, and also a very low power. A 50mm would give a low magnification of only 24x with your XT8, but if the diameter of your dark-adapted pupil is less than 8.3mm, then some light will be lost; again, falling onto the iris instead of entering the pupil. It would be like observing with a 6" or 7" telescope even though one has an 8".

However, some don't mind wasting a little light, if they can have that very low power in its place.

People generally choose 50mm eyepieces, and 60mm, too, for their f/10 Schmidt-Cassegrains, and their f/12-f/15 Maksutovs, resulting in exit-pupils of 5mm/6mm for f/10, 4.2mm/5mm for f/12, and 3.3mm/4mm for f/15, and all well within the limits of most people, and regardless of age.

As one goes up in magnification, like with your 10mm(120x), the smaller the exit-pupil becomes(1.7mm), and everybody's happy.

revenade

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Re: help understanding exit pupil
« Reply #5 on: January 07, 2018, 11:52:54 PM »
Quote
So, I have noticed that there is a lot of discussion about exit pupil and what size range is usable/ideal. While reading these discussions I have come to the conclusion that I must be misunderstanding some key aspect. When I think about exit pupil I imagine a little disc hovering in space on which the image is projected. Of course, only if something such as an eye is at that location to receive the photons is the disc realized. Otherwise the photons keep moving and spreading out from the exit pupil.

When people speak of too large an exit pupil, they speak as though the "excess" photons are distributed across the disc, thus dimming the image compared to what it would be if able to receive all the photons. But why would they be distributed like this? Why doesn't the unusable ring of photons result in a reduction of the view while keeping the remaining view at the same brightness?

Hopefully this makes sense. And hopefully I'm not overlooking anythingtoo obvious. Thanks!


Deb:

Your question is a good one and it's a question that is fundamental. You are wondering why the field of view does not change if your eye does not admit all the light from the objective.

The key to understanding this is to realize that each pencil, each tiny part of the exit pupil contains the entire field of view. From a practical point of view, you can see this by simply placing your hand in the light path, you still see the same field of view, it is just dimmer.

Analytically you can see that it must be true, field of view calculations include the eyepiece parameters and the focal length of the telescope but never the aperture of the telescope. A 100mm F/10 and a 200mm F/5 provide the same field of view with the same eyepiece because they have the same 1000mm focal length, the difference is that the 200mm scope provides an exit pupil that is twice as large. If that light does not enter the eye, it has the same effect as masking the aperture.

Imagine you were looking through the two scopes in question with a 40mm eyepiece and that your dark adapted eye only opened to 4mm. The 100mm would provide a 4mm exit pupil and all the light would enter your eye. The 200mm scope would provide an 8mm exit pupil (40mm F/5) and only 4 mm of the exit pupil would enter your eye. The images would be indistinguishable from one another.

Conceptually, one can bundle these thoughts together. Any part of the objective (assuming no vignetting) provides an image of the entire field of view at the focal plane. If you block part of that the objective, there will actually be a hole in the exit pupil, you can see this easily by inspecting the exit pupil of a scope with a central obstruction. But that does not affect the field of view, it affects the image brightness, the effective aperture.

I hope this helps.

Jon

Donald Jansen

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Re: help understanding exit pupil
« Reply #6 on: January 08, 2018, 02:48:24 PM »
Quote
To find the exit-pupil of any eyepiece, you simply divide the eyepiece's focal-length by the f-ratio of the telescope...

Your 25mm Plossl ÷ f/6(of your XT8) = 4.2mm

This is one correct formula to calculate. But it is not excessively intuitive.

One that I find to be more understandable is

EP = Diameter of objective/magnification.

Thus, a 300 mm objective working at 200X provides an exit pupil of 1.5 mm. For my viewing perception, anywhere from 1.5 - 2 mm EP is a sweet spot for galaxies.

The smaller the EP, the "darker" and also more "contrasty" the image is.

I do NOT subscribe to the general wisdom that too large anEP is "wasteful" or "bad". I have had absolutely stunning views of many many objects at 37X with my 40mm Paragon TMB and an EP about 8.1 mm (much bigger than my 61-year old pupil should theoretically be able to dilate to)!

Travis Kuhlman

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Re: help understanding exit pupil
« Reply #7 on: January 10, 2018, 02:29:01 AM »
Just to add to these good explanations. Most people find that a 2mm exit pupil is very comfortable and used a lot for general observing. I usually start of with a 20mm in my 8" SCT that is f/10, so 2mm exit pupil. For more power when observing the planets and the moon, I'll go as low as a 1mm exit pupil......when seeing allows it. With a 10mm EP I'm at 200x and a lot of times it may be too much power because of poor seeing. On the other end, for large star clusters and nebula I'll use a exit pupil that gives me 4-5mm exit pupil.

One other thing to consider is, light pollution. I also have a 10" Dob that is f/5. I have 2" wide angle EP from 28-40mm and I use them in my 8" f/10, but have to be careful when in the Dob, f/10 because of light pollution in my backyard. If I go too high with an exit pupil, the sky background will start to go gray, which is not pleasing to me. My 24mm, 68* will give me 4.8mm, which is ok for objects up at zenith, but when I start moving to objects lower in the sky, it will start going gray, so I'll switch to an EP that is under 3mm exit pupil.

housletica

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Re: help understanding exit pupil
« Reply #8 on: January 10, 2018, 06:34:21 AM »
Quote
The focal plane is a disk where it all reaches focus. Focal planes can be thought of as disks.
The exit pupil is a cone streaming out of the scope. One slice of it is the focal plane. Your mission is to place your head where your pupil intersects the exit pupil and then to adjust focus so that it best suits the need of your eye.
If the exit pupil is so large it exceeds the size of your pupil, the light splashes uselessly on your iris. If the exit pupil is very narrow it is harder to acquire.
Wide exit pupils and narrow exit pupils each have ways of revealing different eye defects.
Exit pupil allows us to talk about magnification relative to the instrument without reference to the scope. 0.5 mm exit pupil is high magnification on any scope. It is 200x on my 4 inch refractor and and 700x on my c14, i.e. it standardizes magnification per inch of aperture. 200x is high magnification on the refractor and medium low on the c14.



Quote

To find the exit-pupil of any eyepiece, you simply divide the eyepiece's focal-length by the f-ratio of the telescope...



To experiment with the question of very large exit pupil observing, last month I put a 63mm eyepiece in the focuser of a 115mm f7 refractor and observed the star fields that include M24. That eyepiece gave a 9mm exit pupil, larger than any normal human eye's maximum pupil dilation. I had observed the same field immediately prior with a 30mm eyepiece giving 4.3mm exit pupil and 40mm eyepiece that yields a 5.7mm exit pupil. The outlandishly large exit pupil showed the beautiful field full of stars sharply and as satisfyingly as the others but the overall image was simply dimmer. Obviously and noticeably so. I have heard the effect of observing with such oversize exit pupils described as appearing as though through a smaller aperture scope and when I saw what I saw i thought that a very good description.

And yes, yours is a very good question about a critical concept in visual astronomy. Thanks for asking it.

Harry Smull

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Re: help understanding exit pupil
« Reply #9 on: January 10, 2018, 11:25:51 PM »
Quote
Quote

Deb:

Your question is a good one and it's a question that is fundamental. You are wondering why the field of view does not change if your eye does not admit all the light from the objective.

The key to understanding this is to realize that each pencil, each tiny part of the exit pupil contains the entire field of view. From a practical point of view, you can see this by simply placing your hand in the light path, you still see the same field of view, it is just dimmer.

Analytically you can see that it must be true, field of view calculations include the eyepiece parameters and the focal length of the telescope but never the aperture of the telescope. A 100mm F/10 and a 200mm F/5 provide the same field of view with the same eyepiece because they have the same 1000mm focal length, the difference is that the 200mm scope provides an exit pupil that is twice as large. If that light does not enter the eye, it has the same effect as masking the aperture.

Imagine you were looking through the two scopes in question with a 40mm eyepiece and that your dark adapted eye only opened to 4mm. The 100mm would provide a 4mm exit pupil and all the light would enter your eye. The 200mm scope would provide an 8mm exit pupil (40mm F/5) and only 4 mm of the exit pupil would enter your eye. The images would be indistinguishable from one another.

Conceptually, one can bundle these thoughts together. Any part of the objective (assuming no vignetting) provides an image of the entire field of view at the focal plane. If you block part of that the objective, there will actually be a hole in the exit pupil, you can see this easily by inspecting the exit pupil of a scope with a central obstruction. But that does not affect the field of view, it affects the image brightness, the effective aperture.

I hope this helps.

Jon
Jon,

Thank you for this very clear explanation. Having gone through each of your points I can see how every day experience and math say that it must be true that each "pencil" contains the image. However, this does conjure some questions concerning these "pencils."

I probably need to think about this more, as I'm having trouble verbalizing my questions... but let me try.

1. Would it be consistent with this model to say that above some threshold, pencils of any size would contain the image with the difference being in resolution provided by individual pencils; but collectively the same resolution at whatever scale is being used. I think this has to be the case.

2. What is the minimum size of a pencil? I guess that's really the question. It has to be bigger than a photon, but would it be described by area or number of photons? I think area, but below a certain number of photons one would not have enough information to "see" the object. Or maybe it is photon-sized -- either present or absent, with a probabilistic presence across the field; I think I'm leaning towards this model.

Clearly this is of purely of theoretical interest with no relevance to eyepieces!

Cleo Wickware

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Re: help understanding exit pupil
« Reply #10 on: January 12, 2018, 08:00:19 AM »
Deborah,
I admire your desire to understand this beyond the merely superficial!

The exit pupil is basically the image of the objective as projected by the eyepiece a small distance behind the eyepiece. If you place a piece of translucent material (wax paper, or 'frosted' Scotch tape) exactly at the exit pupil, you see a sharply focused image of the objective. If the scope has a secondary obstruction, it too will be sharply focused with the objective. (To be strictly correct, the secondary will be focused at a slightly different distance than that of the objective, but this is a small difference and we can ignore it for now.) If you move the translucent screen inside or outside the actual exit pupil distance, the disk of light has an unfocused edge, becoming ever more blurred the farther from the exit pupil it's located.

For every one image point contained in the field of view (and we can say there are of order a million), there is one cylindrical bundle of light exiting the eyepiece whose diameter equals that of the exit pupil, and which, like every other cylindrical bundle, passes through the exit pupil. Indeed, the exit pupil is the place where all those cylindrical beams overlap as they cross each other.

If the eyepiece has an apparent FOV of, say, 60 degrees, the cylinders of light for image points at the very edge of the FOV exit the eyepiece at a 30 degree angle with respect to the optical axis. For image points ever nearer to the field center, their cylindrical bundles exit the eyepiece at progressively smaller angles with respect to the optical axis.

There is no image as such located at the exit pupil. Because the eyepiece must send essentially parallel, unfocused light to your eye in order that your eye's lens do the actual focusing upon the retina, all light streaming through the exit pupil is merely the superposition of a multitude of cylindrical bundles of parallel or nearly parallel light.

The eyepiece is really an optical coupler which effectively 'compresses' the larger objective down to the size of your eye's iris, plus or minus. Equally correctly, the eyepiece expands your smaller iris to the size of the objective, plus or minus. Just as your hand placed on or near the objective will have its image focused upon the exit pupil along with the objective, so will some object located at the exit pupil be projected upon the objective.

For instance, if the objective aperture is 200mm, your hand width is 100mm, and the exit pupil is 4mm, your hand placed just in front of the objective will be projected at a width of 2mm in that 4mm exit pupil.

Conversely, for the same objective aperture of 200mm and exit pupil of 4mm, if a 2mm wide stick is placed across and at the exit pupil, that stick's image will be projected upon the objective to a width of 100mm.

In the foregoing examples, the 200mm aperture and the 4mm exit pupil tells us the magnification is 50X. Note that the 'shrinking' of the hand held before the objective is a de-magnification of 50X, and the 'expanding' of the stick at the exit pupil is also 50X. Beautiful symmetry, via proportionality of the relevant triangles.

If the exit pupil exceeds your iris, the outer annulus of light not passing through your smaller iris only illuminates the iris. In such case, your iris edge would be projected upon the objective as smaller in the same proportion as the iris : exit pupil diameter.

Well, that should be quite enough background material of the more fundamental kind for now, which I hope facilitates a deeper understanding.

steviselath

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Re: help understanding exit pupil
« Reply #11 on: January 13, 2018, 03:28:58 AM »
Glenn's description of the exit pupil as an image of the objective is correct.
But it's also important to realize that the image behind the eyepiece is not FOCUSED by the eyepiece.
The eyepiece merely compresses the focused image of the objective into a size we can receive into the eye.

In reality, the image behind the eyepiece is afocal, meaning it is in focus at all distances from the eyepiece once you have brought the eyepiece
to focus (essentially making the focal plane of the eyepiece coincident with the focal plane of the objective).
Back away from the eyepiece and the field of view gets smaller in the same way it would if you looked at a distant building through a progressively longer and longer pipe.
But, just as with the pipe, the image doesn't change its focus at any distance from the eyepiece.

So it is important to NOT think of the exit pupil as a focal point for the eyepiece, because it is not. It is an image of the telescope's objective's field shrunk to the size of
a small disc (which is often not completely flat) that can fully enter the eye. And that image is bounded by the iris at the focal plane in the eyepiece (called the Field Stop).

Another way to look at it is that the telescope forms a focused image at its focal plane and the eyepiece is merely a small magnifier to examine a small section of that focal plane.
Higher power eyepieces merely blow the image of the telescope up to a larger size.

Mario Evans

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Re: help understanding exit pupil
« Reply #12 on: January 13, 2018, 05:06:15 AM »
deb, the intricacies of exit pupil are a form of church here on CN, so enjoy the journey of learning.

the fundamental fact about the EP is simply this: point your scope at the day sky, and the exit pupil is just the reduced view image of your illuminated mirror or lens, in other words the opening in the end of your telescope tube.

therefore, anything that can get through the front of your telescope tube will get through the exit pupil.

there is an explanation of the exit pupil, why we calculate it the way we do, and its significance for visual astronomy, here. there's discussion of "wasted light" and "wasted magnification", two of the stranger ideas to come from exit pupil accolytes.

you might also find some value in the preceding section that discusses magnification and magnification recommendations.

litgeschsappa

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Re: help understanding exit pupil
« Reply #13 on: January 23, 2018, 02:07:26 AM »
Quote
Deborah,
I admire your desire to understand this beyond the merely superficial!

The exit pupil is basically the image of the objective as projected by the eyepiece a small distance behind the eyepiece. If you place a piece of translucent material (wax paper, or 'frosted' Scotch tape) exactly at the exit pupil, you see a sharply focused image of the objective. If the scope has a secondary obstruction, it too will be sharply focused with the objective. (To be strictly correct, the secondary will be focused at a slightly different distance than that of the objective, but this is a small difference and we can ignore it for now.) If you move the translucent screen inside or outside the actual exit pupil distance, the disk of light has an unfocused edge, becoming ever more blurred the farther from the exit pupil it's located.

For every one image point contained in the field of view (and we can say there are of order a million), there is one cylindrical bundle of light exiting the eyepiece whose diameter equals that of the exit pupil, and which, like every other cylindrical bundle, passes through the exit pupil. Indeed, the exit pupil is the place where all those cylindrical beams overlap as they cross each other.

If the eyepiece has an apparent FOV of, say, 60 degrees, the cylinders of light for image points at the very edge of the FOV exit the eyepiece at a 30 degree angle with respect to the optical axis. For image points ever nearer to the field center, their cylindrical bundles exit the eyepiece at progressively smaller angles with respect to the optical axis.

There is no image as such located at the exit pupil. Because the eyepiece must send essentially parallel, unfocused light to your eye in order that your eye's lens do the actual focusing upon the retina, all light streaming through the exit pupil is merely the superposition of a multitude of cylindrical bundles of parallel or nearly parallel light.

The eyepiece is really an optical coupler which effectively 'compresses' the larger objective down to the size of your eye's iris, plus or minus. Equally correctly, the eyepiece expands your smaller iris to the size of the objective, plus or minus. Just as your hand placed on or near the objective will have its image focused upon the exit pupil along with the objective, so will some object located at the exit pupil be projected upon the objective.

For instance, if the objective aperture is 200mm, your hand width is 100mm, and the exit pupil is 4mm, your hand placed just in front of the objective will be projected at a width of 2mm in that 4mm exit pupil.

Conversely, for the same objective aperture of 200mm and exit pupil of 4mm, if a 2mm wide stick is placed across and at the exit pupil, that stick's image will be projected upon the objective to a width of 100mm.

In the foregoing examples, the 200mm aperture and the 4mm exit pupil tells us the magnification is 50X. Note that the 'shrinking' of the hand held before the objective is a de-magnification of 50X, and the 'expanding' of the stick at the exit pupil is also 50X. Beautiful symmetry, via proportionality of the relevant triangles.

If the exit pupil exceeds your iris, the outer annulus of light not passing through your smaller iris only illuminates the iris. In such case, your iris edge would be projected upon the objective as smaller in the same proportion as the iris : exit pupil diameter.

Well, that should be quite enough background material of the more fundamental kind for now, which I hope facilitates a deeper understanding.

Glenn,
Thank you so much for this in-depth explanation! I needed to spend a little time to see if I could make sense of it. I think I'm starting to get it, but I do have a question: do the bundles of light pass through the exit pupil with literal simultaneity? Or in extremely fast succession?

Fast succession is easier to grasp, as it would simply require that the brain quickly layer all the incoming information and form a picture, which on some level at least is certainly what happens. While the "surface" of the exit pupil would be dimensionless in the sense that it's the surface that receives the information (retina), but it could also be thought of as having a thickness -- if we could stop time and see all the incoming bundles that would contribute to a given image they would be spread out very, very slightly.

Literal simultaneity on the other hand... For this to work I would have to stop thinking of light as being made up of distinct particles following Newtonian-like laws, which is how I've been thinking about it, but just don't see how it would work in this scenario. Is this a productive avenue for thinking about this?

Thanks again for the input!

Tommy Evans

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Re: help understanding exit pupil
« Reply #14 on: January 23, 2018, 05:11:42 AM »
Quote
When outside at night, the human eye becomes dark-adapted over a span of time, and the eye's pupil enlarges, allowing more light to enter, with the maximum diameter ranging from 7-8mm in youth, to 4-5mm in old age. Dark-adapted pupil diameter decreases as we age.

To find the exit-pupil of any eyepiece, you simply divide the eyepiece's focal-length by the f-ratio of the telescope...

Your 25mm Plossl ÷ f/6(of your XT8) = 4.2mm

...or more simply, 25 ÷ 6 = 4.2mm

Your 34mm has an exit-pupil of 5.7mm. If the diameter of your dark-adapted pupil meets or exceeds 5.7mm, then you're good. If not, then the part of the light that exits the eyepiece will fall onto the iris instead of into the pupil; thereby light is wasted, and in the pupil not receiving all of the light gathered by the primary mirror of the telescope.

Take this 2" 50mm eyepiece, for example... http://agenaastro.co...w-eyepiece.html

50 ÷ 6 = 8.3mm

That's a considerably large exit-pupil, and also a very low power. A 50mm would give a low magnification of only 24x with your XT8, but if the diameter of your dark-adapted pupil is less than 8.3mm, then some light will be lost; again, falling onto the iris instead of entering the pupil. It would be like observing with a 6" or 7" telescope even though one has an 8".

However, some don't mind wasting a little light, if they can have that very low power in its place.

People generally choose 50mm eyepieces, and 60mm, too, for their f/10 Schmidt-Cassegrains, and their f/12-f/15 Maksutovs, resulting in exit-pupils of 5mm/6mm for f/10, 4.2mm/5mm for f/12, and 3.3mm/4mm for f/15, and all well within the limits of most people, and regardless of age.

As one goes up in magnification, like with your 10mm(120x), the smaller the exit-pupil becomes(1.7mm), and everybody's happy.

It doesn't get to 8mm, even in small children and infants.