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Exit Schmexit..... Are you an exit pupil snob?

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#1 Sky_LO

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Posted 13 April 2021 - 03:49 PM

Hello, 

 

I know I will most likely be hammered for this but......

I don't entirely buy the "too big an exit pupil wastes light" theory ! 

 

I understand the concept that too big an exit pupil EP...causes light to fall on the iris and not into the pupil.  Thereby wasting light

and / or stopping down the view.  

 

Here is my counter 'theory'.  

1) I believe the retina is many many times as wide as our pupils - in all directions.  ( Peripheral vision  )  

The human eye can see OUT through the pupil at much wider angles than just seeing the light that comes straight in. So therefore light from a wider angle eyepiece is visible to the eye, and we don't just  see light at only the size of the pupil area.   

 

2) Most astronomers will agree that if you like to use a nebula filter (which darkens the view) that a bigger exit pupil eyepiece should be used.  (the bigger exit pupil EP lets in more light so the filter works better than in a smaller aperture eye piece) 

Doesn't this show that a larger aperture eyepiece lets in MORE light (i.e. works better with a filter) 

If it lets in more light, then at the same time I am to believe it lets in less light due to exit pupil limits.

How can both be true? 

 

3) I just don't care.  In order to get a huge field of view, I need a larger aperture eye piece. 

Don't you want to see the full pliedes?   All of the beehive?  The double cluster with the star field around it? 

Andomeda in its three degree glory? 

 

I observed with a guy that REFUSES to even look into an eyepiece that does not have a proper exit pupil.  He literally took three steps back, turned his back on me and said, "that EP you are using won't match anyone's exit pupil".   I just got exit pupil snobbed.       

Yikes, don't be an exit pupil snob please.  Most objects where I want the large field of view are bright objects, so any potential "stop down" is mostly inconsequential.   I am typically going to use that low magnification / big FOV eyepiece only a few times a night on some really big targets. 

 

Just enjoy the view and please don't snob me about it.   I am not sure I buy the theory anyway. 

Yes I have seen the math.   I believe the math does not take into account the shape and capacity of the human eye.  I believe the math is an oversimplification.   In my opinion, "the exit pupil limit" at a minimum makes the view the same brightness as a smaller exit pupil or matching exit pupil eyepiece, but it certainly is not dimmer?

 

So there you go.  Please be understanding to those of us who just want a wide view now and again.       

 

-Lauren  

 

PS - if you feel like responding with the math.....do a self check for exit pupil snobbery.... lol.gif


Edited by Sky_LO, 13 April 2021 - 04:16 PM.

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#2 siriusandthepup

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Posted 13 April 2021 - 04:26 PM

 

I observed with a guy that REFUSES to even look into an eyepiece that does not have a proper exit pupil.  He literally took three steps back, turned his back on me and said, "that EP you are using won't match anyone's exit pupil".

Silly observer!

 

You ARE stopping down the telescope. You are stopping it down to the size that is richest field for your eye - at the magnification you are now using.

 

Nothing wrong with that!! <eyes twinkle> You should be proud of doing that and automatically accomplishing a wide RFT field without doing any calculations at all.

 

I have an old Rini 63mm 2" eyepiece that I keep specifically for that purpose.

 

So nice to be able to get both sides of the Double Cluster in one field of view in your f/4.5 Dob.

 

waytogo.gif


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#3 Astrojensen

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Posted 13 April 2021 - 04:29 PM

 

 

1) I believe the retina is many many times as wide as our pupils - in all directions.  ( Peripheral vision  ) 

The human eye can see OUT through the pupil at much wider angles than just seeing the light that comes straight in. So therefore light from a wider angle eyepiece is visible to the eye, and we don't just  see light at only the size of the pupil area.

This is not how it works... You can bend the theory all kinds of ways in your head, but that doesn't change the facts. The fact is that the pupil in our eye determines how much light that enters it. Field of view plays no role in this. 

 

 

 

2) Most astronomers will agree that if you like to use a nebula filter (which darkens the view) that a bigger exit pupil eyepiece should be used.  (the bigger exit pupil EP lets in more light so the filter works better than in a smaller aperture eye piece)

Doesn't this show that a larger aperture eyepiece lets in MORE light (i.e. works better with a filter)

If it lets in more light, then at the same time I am to believe it lets in less light due to exit pupil limits.

How can both be true?

*facepalm* 

 

What you apparently don't understand is that the image, even if the exit pupil of the eyepiece is much larger than that of your eye, will never get fainter than what the eye sees at that exit pupil (unless the telescope has a central obstruction, but let's ignore that for a moment). Some of the light, that the telescope has collected is lost, true, but the image will be as bright as it can ever get at that magnification. You could get the same result with a smaller telescope, at the same magnification, producing the maximum exit pupil your eye can handle. 

 

 

I observed with a guy that REFUSES to even look into an eyepiece that does not have a proper exit pupil.  He literally took three steps back, turned his back on me and said, "that EP you are using won't match anyone's exit pupil".

That's just stupid. The low-power view in a telescope, with an eyepiece giving an oversized exit pupil, can be fine. 

 

*

 

About central obstruction: If your telescope has one, especially if it has a large one (like an SCT) then you should be careful about using oversized exit pupils, because the central obstruction will show up as a large, diffuse spot in the middle of the field of view, and then you will experience visible brightness loss, compared to a magnification that fits your pupil. 

 

https://www.televue....n=Advice&id=102

 

 

Clear skies!

Thomas, Denmark


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#4 junomike

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Posted 13 April 2021 - 04:30 PM

I don;t find a huge exit pupil an issue unless in an OTA where I can see the central obstruction (mainly Newt)

I do realize that there are trade off to the wider FOV but for me it's sometimes worth it.

 

Also, just because you can't see it, doesn't mean it's not there....


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#5 ButterFly

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Posted 13 April 2021 - 04:46 PM

I use a 41 Pan on my 80mm f/6 refractor during the day.  The exit pupil is very much larger than my eye.  It is the brightest view I can get at that magnification, and I get all the benefits of a high f-ratio scope for color and field.  If you don't like bright views and excellent color correction and saturation, then no, it's not for you.

 

My dob has a central obstruction.  That grows with the exit pupil.  That's what limits its ability to accept larger and larger exit pupils.  Once the exit pupil exceeds my eye's pupil, the growing of the obstruction means less light is getting in, until the obstruction is as big as my pupil.  It's a specialist eyepiece in the dob for visual use with a balancing act.


Edited by ButterFly, 13 April 2021 - 04:59 PM.

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#6 siriusandthepup

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Posted 13 April 2021 - 04:47 PM

OK - here are my real world calculations for everyone to dissect (shields from rocks thrown).

 

16" f/4.5 Dob = 1838mm fl. 63mm eyepiece.

 

1838mm/63mm = 29x  magnification.

 

16" = 406mm diameter.

 

406mm/29x = 14mm exit pupil.

 

Assuming 7mm "ideal" exit pupil then you have cut aperture by half.

 

8" telescope now (forget the diagonal size here - a trivial matter).

 

An 8" telescope - 200mm diameter.

 

200mm/29x = 7mm exit pupil. Nothing wrong with that.

 

This is just good telescope utilization. IMHO

 

63mm eyepiece has about 50 deg field of view(or a little more - don't remember exactly right now).

 

50deg apparent / 29x = 1.7 deg field of view

 

Sweet!


Edited by siriusandthepup, 13 April 2021 - 04:47 PM.

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#7 SeattleScott

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Posted 13 April 2021 - 04:48 PM

There is a time and place for oversized exit pupil, like Pleiades, Beehive, etc. Yeah you are giving up some aperture but these targets don’t need aperture. I find I can do 8mm without seeing secondary shadow. Normally I prefer 5-6mm just to keep background sky darker; more aesthetically appealing. But certain targets call for max FOV. Nothing wrong with your approach.

Scott
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#8 Jon Isaacs

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Posted 13 April 2021 - 05:03 PM

Here is my counter 'theory'. 

1) I believe the retina is many many times as wide as our pupils - in all directions.  ( Peripheral vision  ) 

The human eye can see OUT through the pupil at much wider angles than just seeing the light that comes straight in. So therefore light from a wider angle eyepiece is visible to the eye, and we don't just  see light at only the size of the pupil area.

 

 

The field of view is not determined by the pupil diameter.  You can perform a simple experiment to prove this:

 

Look though a telescope and then using your hands, block part of the objective. The image dims but the field of view is unchanged.

 

2) Most astronomers will agree that if you like to use a nebula filter (which darkens the view) that a bigger exit pupil eyepiece should be used.  (the bigger exit pupil EP lets in more light so the filter works better than in a smaller aperture eye piece)

Doesn't this show that a larger aperture eyepiece lets in MORE light (i.e. works better with a filter)

If it lets in more light, then at the same time I am to believe it lets in less light due to exit pupil limits.

How can both be true?

 

 

An exit larger than your dark adapted pupil does not let in less light, it just doesn't let in more light.  The advice by myself and others is to use larger exit pupils with filters but not to exceed the diameter of your dark adapted pupil.

 

As Thomas said, in a scope with a central obstruction, there will be slightly less light due to the fact that the aperture is reduced but the shadow of the secondary is not.

 

3) I just don't care.  In order to get a huge field of view, I need a larger aperture eye piece.

Don't you want to see the full pliedes?   All of the beehive?  The double cluster with the star field around it?

Andomeda in its three degree glory?

 

 

Eyepieces do not have apertures like telescopes. They have focal lengths and field stops. The focal length of the eyepiece and the focal ratio of the scope determine the exit pupil. The field stop and the focal length of the telescope determine the true field of view.

 

Sometimes it is an advantage to use an exit pupil larger than the dark adapted pupil just to get that wider field that might come with a longer focal length eyepiece.

 

You friend who refuses to use exit pupils larger than his dark adapted pupil, how accurately does he know his dark adapted pupil diameter?

 

It must be measured to know..  individuals vary greatly..  there are people 70 years old with dark adapted pupils larger than 7.0 mm, people under 50 with dark adapted pupils less than 5.0 mm

 

Jon


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#9 Jon Isaacs

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Posted 13 April 2021 - 05:08 PM

I use a 41 Pan on my 80mm f/6 refractor during the day.  The exit pupil is very much larger than my eye.  It is the brightest view I can get at that magnification, and I get all the benefits of a high f-ratio scope for color and field.  If you don't like bright views and excellent color correction and saturation, then no, it's not for you.

 

 

At night, with an exit pupil larger than your pupil, you'll get the brightest views of extended objects like galaxies and nebulae but the stars will be dimmer since their brightness is determined by the aperture of telescope.

 

Jon


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#10 Mike W

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Posted 13 April 2021 - 05:12 PM

I'm 68 and anything above my 24mm pan and I get a small amount of astigmatism. I have an eye exam scheduled for next week to confirm it. I noticed this in my 27 pan last year and my XW30 this year. Sucks to get old!


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#11 rob1986

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Posted 13 April 2021 - 05:25 PM

At night, with an exit pupil larger than your pupil, you'll get the brightest views of extended objects like galaxies and nebulae but the stars will be dimmer since their brightness is determined by the aperture of telescope.

Jon


sometimes dimmer views aren't a bad thing, but it probably results in brighter background noise too

#12 dan_h

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Posted 13 April 2021 - 05:26 PM

 

I don't entirely buy the "too big an exit pupil wastes light" theory ! 

 

I understand the concept that too big an exit pupil EP...causes light to fall on the iris and not into the pupil.  Thereby wasting light

and / or stopping down the view.  

 

You got it. That's all there is to it, nothing more. 

 

Nobody is saying too big an exit pupil causes any other problems except maybe that the shadow of any secondary mirrors can get in the way at large exit pupils. But that is not part of the "wasted light theory."

 

".....too big an exit pupil EP...causes light to fall on the iris and not into the pupil."    You said it. 

 

It is just one item that should be considered when choosing an eyepiece. It is a reasonable guideline. In practice it has limited impact if not followed religiously. It is not going to hurt you or get you arrested or end your career or create any other disaster in your personal life.  

 

dan



#13 TOMDEY

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Posted 13 April 2021 - 05:36 PM

Hi, Sky LO!

 

Your #1 and #2 are incorrect --- but you more make up for it with your brilliant point #3! A "larger than my eye's pupil" is fine, at times even desirable, especially if the system has no central obstruction. The guy that you referenced is an idiot. >>>

 

~I observed with a guy that REFUSES to even look into an eyepiece that does not have a proper exit pupil.  He literally took three steps back, turned his back on me and said, "that EP you are using won't match anyone's exit pupil".   I just got exit pupil snobbed.~

 

His dopiness is in still not grasping or realizing that a larger than his precious personal pupil is fine. He incorrectly references ~anyone's exit pupil~ what zoomed right over his shriveled little brain is that eyes don't not have exit pupils --- they only have entrance pupils. Little distinction --- critical distinction! So off he went, refusing to enjoy the magnificent view. Kinda like the self-appointed, ingrown, exclusive, elitist snobs who boycotted John Dobson (?!) in his early ingenious Star-Party nights...because his ~junky light buckets~ weren't even worthy of their copping a peek. Every generation has its snobs --- in that sense, amateur astronomy hasn't really changed all that much over the centuries. Now-a-days, if you even look into a telescope (rather than image) you're blown off as a Neanderthal. Go figure. ~Neanderthals of the World --- Unite!~   Tom

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#14 25585

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Posted 13 April 2021 - 05:40 PM

No I'm not. While I accept the theory, there are much more obvious impediments to concentrate on overcoming or tolerating.

 

Exit pupil behaviour/steadiness, however (along with effective eye relief) is very important to me in eyepieces.

 

Seeing, optics including my eyes, weather are most important, comfort & a zen viewing experience. Mental arithmetic does not feature. I swap various eps around, but its more from curiosity than calculation. Time and opportunities are too short otherwise.


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#15 ButterFly

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Posted 13 April 2021 - 06:13 PM

At night, with an exit pupil larger than your pupil, you'll get the brightest views of extended objects like galaxies and nebulae but the stars will be dimmer since their brightness is determined by the aperture of telescope.

 

Jon

Take that Alnitak!


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#16 DavidSt

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Posted 13 April 2021 - 06:38 PM

What I don’t get about the too large an exit pupil thing is that if you deal with astigmatism or even if you don’t all day, what’s a little light loss with increased magnification? You’re still probably seeing more or better. It may not be the best view you can buy but it may be the best view you’re going to get at the moment. Tell your friend that if he’s so freaking uppity, you’ll be happy to take his donation for a lower focal length eyepiece with wider view followed by a thank you letter to follow. On the other hand, he might just be ignorant and just doesn’t understand.
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#17 turtle86

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Posted 13 April 2021 - 07:24 PM

 

3) I just don't care.  In order to get a huge field of view, I need a larger aperture eye piece. 

Don't you want to see the full pliedes?   All of the beehive?  The double cluster with the star field around it? 

Andomeda in its three degree glory? 

 

I observed with a guy that REFUSES to even look into an eyepiece that does not have a proper exit pupil.  He literally took three steps back, turned his back on me and said, "that EP you are using won't match anyone's exit pupil".   I just got exit pupil snobbed.       

 

 

Totally agree.  I've always had a 40mm class eyepiece in my kit (first a 40mm Paragon and now a 41mm Pan) and I've never hesitated to use it in my f/4.3 18" Dob (really about 4.9 with a Paracorr) when I wanted an extra wide FOV.  It's also a real pleasure to use the 41mm Pan in my NP101 for sweeping the Milky Way at a dark site.  When I'm getting a flat field of nearly 5 degrees, I can easily live with a few of the photons hitting my iris instead of my retina.


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#18 Tropobob

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Posted 13 April 2021 - 07:39 PM

I had a 12.5" F5 Dob and almost as a joke, tried it with a 50mm Plossl.  The joke was on me, for the view was stunning.  I felt that I had been totally misinformed by all the rules of thumb re exit pupils. 

 

I eagerly showed this to other people in an Astronomy Group that I was involved in and found that younger people enjoyed the view; whereas older people thought it was terrible. As I aged, I moved across to the 'terriblies'. 

 

So if U are in your thirties or younger, try it out.  However, I predict that your opinion will change when the flexibility of your eyes (called accommodation) weakens.   


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#19 MartinPond

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Posted 13 April 2021 - 08:31 PM

Approaching your pupil size, of course, you

    have lower power and more light.

 

Beyond your pupil size, there is one main benefit:  

   it is faster and simpler to find the best eye placement.


Edited by MartinPond, 13 April 2021 - 08:32 PM.

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#20 Jon Isaacs

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Posted 13 April 2021 - 09:29 PM

I had a 12.5" F5 Dob and almost as a joke, tried it with a 50mm Plossl.  The joke was on me, for the view was stunning.  I felt that I had been totally misinformed by all the rules of thumb re exit pupils. 

 

I eagerly showed this to other people in an Astronomy Group that I was involved in and found that younger people enjoyed the view; whereas older people thought it was terrible. As I aged, I moved across to the 'terriblies'. 

 

So if U are in your thirties or younger, try it out.  However, I predict that your opinion will change when the flexibility of your eyes (called accommodation) weakens.   

 

No one has a 10 mm dark adapted pupil..

 

What was stunning about the view? 

 

For someone with a 7mm dark adapted pupil, it could be the fact that they were looking though an 8.75 inch F/7.2.  Much less coma, easier on the eyepieces. 

 

I do something similar with my 12.5 inch F/4.06, I use the 41mm Panoptic. Despite being a senior citizen,  my dark adapted pupil is, or at least was a few years ago, closer to 8mm than 7mm, with the 41mm Panoptic, it's a 9.6 inch F/5.3 to my eye. Not too bad but I prefer the 12.5 inch with a Paracorr. 

 

It shouldn't have anything to do with accommodation, it should be just opposite.  At slower focal ratios, the depth of focus is increased which is an advantage for those with less visual accommodation. 

 

There is no rule against oversized exit pupils but it's good to understand the consequences.  

 

Jon


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#21 stevew

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Posted 13 April 2021 - 10:10 PM

Hello, 

 

I know I will most likely be hammered for this but......

I don't entirely buy the "too big an exit pupil wastes light" theory ! 

 

You think it's just a theory.... LOL....

It's very real and is easy to see the difference. Get to a dark sight and experiment a little and see for yourself...


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#22 HellsKitchen

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Posted 13 April 2021 - 10:34 PM

No, I just use whatever eyepiece gives my subjective best view of the object. I do keep the exit pupil at 6mm or less, because I pay for whole aperture, I use whole aperture.


Edited by HellsKitchen, 13 April 2021 - 10:35 PM.

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#23 Mitrovarr

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Posted 14 April 2021 - 12:54 AM

Nah, I don't have much trouble with it. All the telescopes I use much are F/5 or shorter and my longest FL eyepiece I use much is 30mm. That's a 6mm exit pupil, and that's fine.

 

I think modern widefield eyepieces have helped with this a lot, since you don't have to go to crazy long FLs to get low magnifications.



#24 hoof

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Posted 14 April 2021 - 01:29 AM

Approaching your pupil size, of course, you
have lower power and more light.

Beyond your pupil size, there is one main benefit:
it is faster and simpler to find the best eye placement.

Yup, having an oversized exit pupil is great for maximizing apparent brightness of extended objects. Why? Because we have a heck of a time holding steady! We sway naturally a bit, so if your eye is 7mm dilated and the exit pupil is 7mm, most of the time the pupils are misaligned, thus you’re not maximizing the brightness. An 8 or 9mm exit pupil lets you sway a millimeter or so without the brightness being affected (for an unobstructed scope). Similar with an undersized exit pupil a millimeter or two smaller than your eye’s pupil.

IIRC some old navy binoculars have upwards of 20mm exit pupils, simply to keep brightness at maximum in rough seas when searching for the enemy visually at night. Being able to sway half a centimeter without affecting the view can be useful! :)

About an hour ago I was comparing M51 visually to night vision with my Mod3 on my 15” F/4.1 dob with my TV 67mm plossl. No problems with the view visually, though the view of the galaxy pair with NV was amazing. But it depends on the ‘scope. My 6” F/4 astrograph with my 52mm Erfle is next to useless visually, due to its 43% obstruction, for example, unlike the 15”.

Edited by hoof, 14 April 2021 - 01:33 AM.


#25 rob1986

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Posted 14 April 2021 - 01:52 AM

No one has a 10 mm dark adapted pupil..

What was stunning about the view?

For someone with a 7mm dark adapted pupil, it could be the fact that they were looking though an 8.75 inch F/7.2. Much less coma, easier on the eyepieces.

I do something similar with my 12.5 inch F/4.06, I use the 41mm Panoptic. Despite being a senior citizen, my dark adapted pupil is, or at least was a few years ago, closer to 8mm than 7mm, with the 41mm Panoptic, it's a 9.6 inch F/5.3 to my eye. Not too bad but I prefer the 12.5 inch with a Paracorr.

It shouldn't have anything to do with accommodation, it should be just opposite. At slower focal ratios, the depth of focus is increased which is an advantage for those with less visual accommodation.

There is no rule against oversized exit pupils but it's good to understand the consequences.

Jon


i would say that oversized exit pupils do not perform identicaly to the stopped down system.


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