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Optimal exit pupil size for city light pollution

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#76 MartinPond


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Posted 23 July 2021 - 11:52 AM

Strangely, I've found 5 and 6 mm exit  pupil binos that still have fussy eye placement. A shrine manon  and a universe brand (both 7x35 10 DEG) come to mind- unless you have the IPD set right and position the eye just so- meh , pretty crappy view. Take the time and get everything just right and quite rewarding. I put those anomalies down to eyepiece design but can't prove it. .  Pat

Sounds like an EOFB/kidney-beaning issue is in there.

True; a little extra exit pupil does help with that.

#77 ECP M42



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Posted 23 July 2021 - 01:09 PM

It's all about ease of eye positioning.


Let's take an extreme case of a small exit pupil, say 1mm.   If your eyes are positioned to be off by 1mm, you see nothing.   It should be obvious that a large exit pupil of 6mm is much more forgiving as far as eye positioning is concerned.

But you wear glasses! The "only" explanation ... 


The larger the exit pupil, the easier it is to center your own eyes within the "sweet spot"


A larger exit pupil then 7mm isn't needed, but it would make the binocular very comfortable to look through.

Do you wear glasses you too? Because, this only applies to those who wear glasses ... or those who do not decently adjust the IPD of their binoculars. 


I have done dozens of tests with many different binoculars and have never found the statement that "with a wider pupil it is easier to observe" to be true. 

I only noticed that there are more comfortable eyepieces and more uncomfortable eyepieces, regardless of the exit pupil, and that my preferences never perfectly match the preferences of other users, with whom I have been confronted on this issue.  


I also use binoculars with sunglasses (and I often use 10x25 too), so I can understand that it is easier and more tolerant to use large pupils during the day using glasses, but even here I have several reservations, as many binoculars remain difficult to observe, regardless of their larger exit pupil. 


In my opinion, before making certain statements that seem absolutist and that must be valid for everyone, and above all to avoid confusing newbies who do not use glasses, it should always be specified that glasses are used and that this is the main cause of choices a little out of logic. 


Did I say it right?  flowerred.gif  smirk.gif

#78 ECP M42



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Posted 23 July 2021 - 01:33 PM

For me, under messy skies,

more power cannot increase contrast that doesn't exist.

The assumption that the grey level is almost black is invalid.

   At least,,,,,here.   It just isn't true.


I find the best brightness, around 5mm exit pupil,

   helps my optic cortex do the signal processing to see 

   more subtle colors and details.

Martin, we are all a little different, even if basically we are all the same, and here it is possible to see our differences. Years ago I discovered that (under the same sky) there are people who can see the Orion Nebula with the naked eye and others who cannot.

There is nothing bad or negative about this, but is simply evident a difference in retinal sensitivity (main cause). And so, if you prefer (like others) large pupils even under polluted skies, in my opinion and deduction, it means that you need them (it's your need), because most likely your retina is less sensitive than others. 

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#79 MT4


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Posted 23 July 2021 - 05:47 PM

ECP was right that we're all different in our preferences for things, binoculars and exit pupils included.


I was merely sharing my findings that I prefer 6mm exit pupil for daytime use because it helps make it easy to see the view while walking about whereas for nighttime use under brightly-lit skies my eyes prefer smaller exit pupils for a darker sky background.  As such, for daytime use I prefer my Nikon EDG 7x42 to my EII 8x30 but for nighttime use I've reached for my Ell 8x30 more often than my EDG 7x42.


For me, a key factor in my differing exit-pupil preferences is that during the day I tend to walk around and for nighttime use I tend to stay put or move very slowly.  When staying put, or using binoculars on a tripod, small exit pupils don't pose as much of a challenge for eye positioning compared to when walking about, provided that one takes care in properly setting up the IPD.  (If anyone's tried using tiny binoculars, say 8x20 or 10x25, on a rocking boat, you'd know what I am talking about here.  There's a reason why the Navy generally prefers the 7x50 format.)


Having said all that, we're all different so there are no absolutes here.


Also, as I've stated multiple times in this thread, my local skies are very well-lit with SQM readings being somewhere around 17 (17.5 at best), probably much worse than most observer's.  As such, the findings that I've shared in this thread probably don't apply very well to many observers though I would hope that there are bits here and there that do apply well enough.

Edited by MT4, 23 July 2021 - 07:33 PM.

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