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Comparing Two Wide field Constellation Binoculars- Orion 2x54mm versus Kasai 2.3x40mm

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#51 Dr. Wm

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Posted 17 December 2024 - 08:27 AM

"The objective diameter has nothing to do with the exit pupil size." 

 

Hmm??  In that case why can't I buy a pair of 120mm binoculars with an exit pupil of only 5mm and a nice widefield low magnifying power of, say, 6X? Those would certainly be a gas to use for all kinds of applications!  But as I learned when I first got interested in optics as a kid of 12 or so, such binoculars, with optical systems capable of actually "concentrating" their light cones cannot be built.  For ANY optical system, one can only lower the magnification at the expense of increasing the diameter of the exit pupil.  Thus the binocular system just described, which has very large 120mm objectives, could only render a power as low as 6x with an exit pupil of 20mm -- since 120/6=20.  This is simply an optical limitation we must all live with.  Thus, despite what you may have heard or read elsewhere; "The objective diameter has EVERYTHING to do with the exit pupil size."

 

"The whole point is field of view - not brightness, which is a very slight bonus."

 

Well, yes, we certainly all love an ultra-wide virtual field of view, and one can purchase -- generally very expensive binoculars that have apparent fields of up to around 80+ degrees.  Such binoculars will also provide significantly more"brightness," i.e. total light throughput, than a regular non-wide field glass of the same magnification.  In fact, low power ultra wide field binoculars, with exit pupils up to around 7mm gain their amazing characteristics by being able to take any small region of the view and render it as bright as is physically possible. 

 

This is why, mutatis mutandis, a 14x50 binocular, although it may have up to 2x the resolving power of a much more common 7x50 binocular, can, by the laws of optical Physics, never be more than 1/4 as bright per region viewed as the latter.  Should one then step up to a 28x50 binocular, the resolving power could almost double again -- although in practice 1.5X is closer to the mark -- but the brightness would drop to 1/4 of the 14x50, and a mere 1/16th of the original 7x50s!  10x50 and 12x50 binoculars thus still make good sense to mass produce, since they have exit pupils of only 5mm -- for us now visually-challenged old farts -- or ~4.14mm respectively. 

 

The aforementioned 14x50s, with exit pupils of only 3.57mm could, and do, also perform very well with higher resolution images still bright enough to satisfy for most purposes.  That binocular size is, however, considerably rarer than the 12x50s, 10x50s or 7x50s.  That probably has more to do, I would assume, with the simple fact that 14x binocs, with whatever diameter their objectives may be, are invariably quite difficult for most people to hold steady enough with their hands alone to be profitably used.

 

If one has ever viewed through a cheapo "ultra wide field" designated binocular, one will generally notice immediately that the image is considerably dimmer, and far less satisfying to view, than it is through any reasonably well-built binocular of similar power.  Thus "brightness" is not "a slight bonus," but an absolutely essential component of any superior binocular.  



#52 DVexile

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Posted 18 December 2024 - 04:49 PM

"The objective diameter has nothing to do with the exit pupil size." 

 

Hmm??  In that case why can't I buy a pair of 120mm binoculars with an exit pupil of only 5mm and a nice widefield low magnifying power of, say, 6X? Those would certainly be a gas to use for all kinds of applications!  But as I learned when I first got interested in optics as a kid of 12 or so, such binoculars, with optical systems capable of actually "concentrating" their light cones cannot be built.  For ANY optical system, one can only lower the magnification at the expense of increasing the diameter of the exit pupil.  Thus the binocular system just described, which has very large 120mm objectives, could only render a power as low as 6x with an exit pupil of 20mm -- since 120/6=20.  This is simply an optical limitation we must all live with.  Thus, despite what you may have heard or read elsewhere; "The objective diameter has EVERYTHING to do with the exit pupil size."

 

Again, this is simply wrong because the binoculars being discussed are a Galilean design, not a Keplerian design.  What you write would be correct of any Keplerian telescope, and that's what most people are familiar with.  The 2x54s are not Keplerian, they are Galilean and so applying Keplerian analysis them just results in erroneous conclusions.

 

Here is a brief overview of the two different designs:

 

https://brunelleschi.../eesplora2.html

 

And this is a detailed explanation of the exit pupils in each with figures:

 

https://www.scielo.o...n4/v50n4a13.pdf

 

I'd recommend reading and understanding both of those if you want to make a useful contribution to the discussion.


Edited by DVexile, 18 December 2024 - 06:33 PM.

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#53 NinePlanets

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Posted 18 December 2024 - 06:19 PM

Dr. Wm,

 

If you don't like them, you are certainly free to not use them.

By the same token, those of us who like them are certainly free to use them.

 

Good day, sir.



#54 Y0_OY

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Posted 11 January 2025 - 04:06 PM

I own the 2x54 under the Omegon brand. I bought them because I was hoping that I could see some more stars under my Bortle 9 metropolitan sky. Well, obviously that was a bit naive, because as the review says, light pollution is a problem with their aperture.

At first I would regret spending money on them but I have since learned to love them. I enjoy sweeping the nightsky with the when I travel to darker skies. Checking on T CrB over the summer has been so easy with them.

So, yeah, they are a bit niche but they complement every setup nicely.

Thank you for the well written review.

I thought about your comment and I guess it depends on the individual because I also live under a Bortle 9+ sky and the Omegon help me a lot. I have very poor night vision, so I can't see some objects that might be visible to the naked eye on a moonless night, like the Seven Sisters. My wife sees them, but I don't, and it was so frustrating. lol.gif Now I rely on my Omegon and I'm so grateful that they work so well. But of course this is not Atacama. cool.gif



#55 Y0_OY

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Posted 11 January 2025 - 04:19 PM

Until recently, I haven't been keeping track of the DSO I've seen with my 2x54's.  So far, my "official" observations through these binos have been:

 

- M31

- NGC 457 (ET Cluster)

- M39

- M13

- M15

- Double Cluster

- Melotte 20

- M34

- M45

- Hyades

- Collinder 39 (Coathanger)

 

It'd be interesting to keep a running tally of which DSO can be seen through these.  

 

Mike

Wow, thank you for sharing ! I wish I lived under better skies but it is very good to know it is possible. Yes, please, keep running the list !


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#56 jgroub

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Posted 24 January 2025 - 02:42 AM

In a previous discussion/review of the 2.3x40 binoculars and the 2.1x42s, I recall reading that because of the Galilean design, the increase in light gathering ability is NOT related to the size of the objectives, but is simply stated by the magnification factor - which applies both to actual magnification as well as amplification of the amount of light received by the eye.  

 

So, in that discussion/review, the reviewer of both said that the 2.3x binos went noticeably - but not hugely - deeper than the 2.1s, even though the objective sizes were the reverse of that. 

 

First of all, am I remembering correctly?  And if so, does that mean that these 2x54s are even dimmer - even if only slightly so - than the 2.3s and 2.1s? 

 

It seems counterintuitive, but does anyone know?  I'm just curious if anyone's done a side-by-side to compare that.  I mean, sure, going from a 28-degree view to a 36-degree view is worth it, but I was just wondering about the slight decrease in brightness.  

 

Thanks.  



#57 DVexile

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Posted 24 January 2025 - 06:31 AM

Yes, in theory the 2.3x will see a bit deeper than the 2.0x for these Galilean designs.  That’s mostly because of the slightly higher magnification rather than the slightly larger effective entrance pupil.  (In binoculars their magnification has a larger effect on stellar limiting magnitude than their aperture).

 

The difference is likely around 0.2 magnitude for the 2.0x compared with the 2.3x.  Not huge, but akin to something like the difference between 7x35 and 8x42.  But this is the predicted difference, not a direct comparison!

 

Binocular limiting magnitude is a complex subject:

 

https://www.cloudyni...ts/limiting.pdf


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