Comparing 10x32 NL with 8x42 NL, some (not ground breaking) thoughts... (2 Viewers)

[laurencejackson]

I had the opportunity to compare the 10x32s with my own pair of 8x42 NLs at an optics dealer this morning. I have always enjoyed the 10x32 format having previously owned the EL SV version back in 2016, which I stupidly sold...They are a perfect small bin for travel without giving up anything at all optically.

Anyway, it wasn't the best of days for comparing, sunny blue skies, no clouds and not many shadows to look into. However I was surprised in the comparison to fond the 10x32 'appearing' to be brighter that the 8x42, which is tiotally wrong. They also appeared to have more pop in the view, to the point where I checked that the lenses were clean and they were! In comparison the 8x42 were not quite as clear and sharp. I know that the greater magnification can play games on me when comparing two different configs, a 10x somehow often appears more immersive and I feel I 'see' more detail as a result. I was taking that into consideration though whilst comparing the two.

I can only think that the clear, bright sunny conditions effectively ironed out many of the more subtle differences that you may expect between a x42 and x32. As expected the x42 were easier on the eye placement and I didn't realize that the ocular cups are very slightly smaller on the x32, for some reason I thought they would be the same as AFAIK, the ocular lenses are 25mm diameter on both models. The x42s I could bring up to my glasses without any adjusting wheres the 10x32s required quite a bit of experimenting with the IPD and eyecups, as I ended up with too much eye relief. Concerning eye relief, the 8x, 10x and 12x42 all have the same and for me it's a fraction too generous and I would ideally love an intermediary stop from 0 - 1st notch.

In the hand, they feel better than I expected having read some comments here on BF about the x32s not feeling convincing in the hand compared to the larger models and Els. The focuser was very smooth, on a par with the 8x42 but not as perfect as my 12x42 and my original 2007 8x32ELs which are both the best Swaro focusers I've experienced. There is just a slight notchy feeling when around infinity on both the bins tested today, otherwise perfect. Not Zeiss SF perfect, but still excellent!

Anyway, I ended up buying them! So now I can compare at my leisure... I still think the most perfect bin I've ever used is my original 8x32EL. The barrels, armour (completely intact), focuser, everything still fits beautifully in the hand and optically they are so easy to use. Maybe not as crystal clear as later iterations and the NLs, but less contrived in a way? I sometimes wonder if the NLs, fabulous as they are, are overdeveloped in certain traits but have lost other more fundamental ones along the way.

Sorry, this is all mundane chicken'n chips stuff I know, but it's an interesting and enjoyable exercise for me

PS, I need to sell my Leica 8x42 Ultravid HD-Plus now....if anyone is interested!

 

[claves_10]

The difference between 42mm and 32mm is more tangible at dawn, dusk or cloudy days. 42mm gather just a bit more light which for me I can notice the difference. Big deal....not really. Myself I use the binos when I go to the mountain, that is, whenever does not rain and is sunny or mainly without clouds. In full sunny days I believe your pupil will correct the extra light and contract, the image and colors will be great but hard to discern between both binos and aperture.
laurencejackson
wanted to ask you about the UVs HD+ focuser, how is it compared to EL, NL and of course SF´s.
And moreover Ultravids compared to EL or SF in general image, comfort etc.
Your thoughts and thanks.

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[SalomoneSeverino]

That's a really interesting comparison as I am currently bouncing back and forth between buying a 10x32 and 10x42 NL. Unfortunately I can't test neither of those where I live. So any insights are extremely valuable!

 

[grackle314]

Anyway, it wasn't the best of days for comparing, sunny blue skies, no clouds and not many shadows to look into. However I was surprised in the comparison to fond the 10x32 'appearing' to be brighter that the 8x42, which is tiotally wrong.

I can only think that the clear, bright sunny conditions effectively ironed out many of the more subtle differences that you may expect between a x42 and x32.

In sunny blue sky days a typical human eye pupil is about 2.5 mm diameter. The exit pupil of the 10x32 is about 3.2 mm and the 8x42 is about 5.25 mm. Even though the 42 has larger light gathering at its entrance, the 32 will provide about 50% more illumination into the eye under such conditions. The 10x32 will be brighter than the 8x42 in that circumstance.

Here is a rough approximation why that occurs.

Objective area: 10x32 is 1024 mm squared, 8x42 is 1764.
Binocular exit pupil area: 10x32 is 10.24 , 8x42 is 27.56.
Human eye pupil area in sunny blue sky conditions: 6.25

Relative light going into eye:
10x32 is 1024 x (6.25/10.24) = 625
8x42 is 1764 x (6.25/27.56) = 400
625/400 = 1.56 (about 56% more light gets into the eye with the 32 compared to the 42).

Even though the 42 has greater objective area than the 32, the exit pupil delivers a much smaller fraction of the gathered light into the eye than the 32 on sunny blue sky days.

 

[Jason Bugay Reyes]

I had the opportunity to compare the 10x32s with my own pair of 8x42 NLs at an optics dealer this morning. I have always enjoyed the 10x32 format having previously owned the EL SV version back in 2016, which I stupidly sold...They are a perfect small bin for travel without giving up anything at all optically.

Anyway, it wasn't the best of days for comparing, sunny blue skies, no clouds and not many shadows to look into. However I was surprised in the comparison to fond the 10x32 'appearing' to be brighter that the 8x42, which is tiotally wrong. They also appeared to have more pop in the view, to the point where I checked that the lenses were clean and they were! In comparison the 8x42 were not quite as clear and sharp. I know that the greater magnification can play games on me when comparing two different configs, a 10x somehow often appears more immersive and I feel I 'see' more detail as a result. I was taking that into consideration though whilst comparing the two.

I can only think that the clear, bright sunny conditions effectively ironed out many of the more subtle differences that you may expect between a x42 and x32. As expected the x42 were easier on the eye placement and I didn't realize that the ocular cups are very slightly smaller on the x32, for some reason I thought they would be the same as AFAIK, the ocular lenses are 25mm diameter on both models. The x42s I could bring up to my glasses without any adjusting wheres the 10x32s required quite a bit of experimenting with the IPD and eyecups, as I ended up with too much eye relief. Concerning eye relief, the 8x, 10x and 12x42 all have the same and for me it's a fraction too generous and I would ideally love an intermediary stop from 0 - 1st notch.

In the hand, they feel better than I expected having read some comments here on BF about the x32s not feeling convincing in the hand compared to the larger models and Els. The focuser was very smooth, on a par with the 8x42 but not as perfect as my 12x42 and my original 2007 8x32ELs which are both the best Swaro focusers I've experienced. There is just a slight notchy feeling when around infinity on both the bins tested today, otherwise perfect. Not Zeiss SF perfect, but still excellent!

Anyway, I ended up buying them! So now I can compare at my leisure... I still think the most perfect bin I've ever used is my original 8x32EL. The barrels, armour (completely intact), focuser, everything still fits beautifully in the hand and optically they are so easy to use. Maybe not as crystal clear as later iterations and the NLs, but less contrived in a way? I sometimes wonder if the NLs, fabulous as they are, are overdeveloped in certain traits but have lost other more fundamental ones along the way.

Sorry, this is all mundane chicken'n chips stuff I know, but it's an interesting and enjoyable exercise for me

PS, I need to sell my Leica 8x42 Ultravid HD-Plus now....if anyone is interested!

Glad you like the 10x32 NL! I paired it too with my 12x42 NL and I have the best of both to covered my needs as I lead birding tours in my place from the coast up to the mountains 🤤

When I do dusk and night drive in the rainforest, no problem with both binos for ID for my eyes with the help of torch or spot lights

 

[Dr. K]

In sunny blue sky days a typical human eye pupil is about 2.5 mm diameter. The exit pupil of the 10x32 is about 3.2 mm and the 8x42 is about 5.25 mm. Even though the 42 has larger light gathering at its entrance, the 32 will provide about 50% more illumination into the eye under such conditions. The 10x32 will be brighter than the 8x42 in that circumstance.

Here is a rough approximation why that occurs.

Objective area: 10x32 is 1024 mm squared, 8x42 is 1764.
Binocular exit pupil area: 10x32 is 10.24 , 8x42 is 27.56.
Human eye pupil area in sunny blue sky conditions: 6.25

Relative light going into eye:
10x32 is 1024 x (6.25/10.24) = 625
8x42 is 1764 x (6.25/27.56) = 400
625/400 = 1.56 (about 56% more light gets into the eye with the 32 compared to the 42).

Even though the 42 has greater objective area than the 32, the exit pupil delivers a much smaller fraction of the gathered light into the eye than the 32 on sunny blue sky days.

The pupil reacts to the light in front of it, not just around, in the sky - looking into the eyecups of a bin with less light will cause the pupil to dilate, receiving more of that light you say is lost. Conversely, the pupil will constrict in response to concentrated light, e.g., under the scenario where your pupils on the sunny day are at 2.5mm then concentrate the larger 32mm objective’s sum light gathering into that tiny space, so… I don’t think it’s true that the light from these two configurations differs on the grounds of difference in exit pupil size, under this sunny day scenario. And, perhaps more importantly, has anyone ever experienced a 32mm bin of same size to be brighter than an equal model 42mm? As this would concentrate light in the 2.5mm pupil and be brighter in your scenario even with equal magnifications, right?

 

[William Lewis]

In sunny blue sky days a typical human eye pupil is about 2.5 mm diameter. The exit pupil of the 10x32 is about 3.2 mm and the 8x42 is about 5.25 mm. Even though the 42 has larger light gathering at its entrance, the 32 will provide about 50% more illumination into the eye under such conditions. The 10x32 will be brighter than the 8x42 in that circumstance.

Here is a rough approximation why that occurs.

Objective area: 10x32 is 1024 mm squared, 8x42 is 1764.
Binocular exit pupil area: 10x32 is 10.24 , 8x42 is 27.56.
Human eye pupil area in sunny blue sky conditions: 6.25

Relative light going into eye:
10x32 is 1024 x (6.25/10.24) = 625
8x42 is 1764 x (6.25/27.56) = 400
625/400 = 1.56 (about 56% more light gets into the eye with the 32 compared to the 42).

Even though the 42 has greater objective area than the 32, the exit pupil delivers a much smaller fraction of the gathered light into the eye than the 32 on sunny blue sky days.

Hi Grackle314

I laboured under this misapprehension for a while too.

Providing the transmission of 2 pairs of binoculars is the same and ignoring the perception of brightness being altered by different colour casts etc then different pairs will have the same brightness providing the binoculars exit pupil is smaller or the same size as our own pupil.

The reasoning your applying can lead down a bit of a rabbit hole when for example you look at a 10x25 compared to an 7x42 - which would be brighter if your pupil was dilated to 2.5mm?

 

[claves_10]

Bird forum is the physics wanna be forum.
NO clue if what you mentioned is correct #Grackle314, but I doubt that the 32 mm gathers/transmit more light into the eye in sunny or not sunny day.
I might be wrong, If you have a simpler way of explaining it much appreciated if not... it´s oK.
I would say (sorry writing without having a clue) that the only factor would be objective diameter 42mm vs 32mm. Initially more gathered. The magnification affecting little.
On the other hand exit pupil due to BINO physics, one has 4.2mm exit and the other 3.2mm exit regardless the the light in sunny bright day.
On a sunny days you own eyes pupil will contract to counter balance the light transmitted until it burns out due to too much light. I think in sunny days 42mm objectives binos, you own eye makes such effort that the light gathered with 32mm and 42mm seems the same.

 

[tehri]

Bird forum is the physics wanna be forum.

😋
( No 'mockery' from me, @grackle314 !! Just something funny !! )

 

[claves_10]

So funny Tehri !!! You are a photoshop artist ! (whichever program you use)
Do the Owls have a 42mm pupil?

 

[Patudo]

The pupil reacts to the light in front of it, not just around, in the sky - looking into the eyecups of a bin with less light will cause the pupil to dilate, receiving more of that light you say is lost.

I'm sure that does in fact take place - but have definitely noticed that single-coated binoculars from the 50s/60s are just not as bright as modern multi-coated devices, regardless of any pupil dilation that may take place, in both sunny and overcast conditions. Single-coated binoculars are still perfectly usable, especially on bright summer days - just not as bright.

 

[Dr. K]

I'm sure that does in fact take place - but have definitely noticed that single-coated binoculars from the 50s/60s are just not as bright as modern multi-coated devices, regardless of any pupil dilation that may take place, in both sunny and overcast conditions. Single-coated binoculars are still perfectly usable, especially on bright summer days - just not as bright.

Yes, clearly, when comparing bins with differences in transmission, from coatings or glass composition or whatever, would change things. That is not, however, the focus of the scenario to which I was referring, and responding.

 

[grackle314]

Dr. K makes a relevant point of refinement on the optical brightness having perhaps a different eye pupil diameter when viewing through a binocular compared to direct eye view. Here is an estimate, based on measurements, of the effect Dr. K mentions.

On a sunny blue sky day, the incoming luminance for binoculars is about L = 10**4 candela per square meter.

Many modern binoculars have optical transmission around 90% and people don't detect a few percent difference. Assuming 90% transmission, the luminance coming out of the exit pupil is: L(exit pupil) = 0.9*L*m**2 where m is the magnification.

This means the output luminance from an m=10 binocular will be about L(exit pupil, m=10) = 9x10*6 candela per square meter and from an m= 8 binocular L(exit pupil, m=8) = 0.9(10**4)*64 = 5.76x10**5 candela per square meter.

For luminance around 10**4 to 10**6 candela per square meter, the human eye has pupil size typically about 2.0 mm. See Zeiss figure below from i.Scription by Zeiss: Setting the New Standard of Vision Correction. Also see Holger Merlitz: The Binocular Handbook equation 6.1 and Figure 6.4.

When viewing through a binocular, there is an eye pupil diameter correction factor as pointed out by Dr. K. That factor for the luminance range of 10**4 candela per square meter, and higher, is about 5% for a 50 degree field of vision binocular, see Figure 6.4 referenced above. A 5% increase in eye pupil diameter for the 8x42 compared to the 10x32 would yield a 10% greater eye pupil area for the 8x42.

As both the 10x32 and 8x42 remarked upon by the original poster have exit pupils greater than the eye pupil on a sunny blue sky day, with the refinement of the 8x42 having a 5% larger eye pupil, the 10x32 factor is 56% brighter uncorrected and 42% brighter with this refinement.

As an anecdotal observation, on a sunny blue sky day, I perceive my NL 10x42 as slightly brighter than my Leica UVD8x20 and my Zeiss VP 8x25. The m = 10 increase in candela per square meter compared to the m = 8 is perceptible to me, but if I use them without immediate comparison I don't notice the difference. Eyes have many orders of magnitude detection capability and are not too aware of luminance changes of a few percent.

 

[Patudo]

Yes, clearly, when comparing bins with differences in transmission, from coatings or glass composition or whatever, would change things. That is not, however, the focus of the scenario to which I was referring, and responding.

Actually, that is exactly what I thought you were referencing (that the eye pupil opens up to compensate for less light, rendering small differences in brightness irrelevant). I agree this must happen - the question I'd love to know the answer to is why do multi-coated binoculars appear brighter than single-coated? Is the 10 to 15% difference so great that pupil dilation can't compensate for it (in terms of making the image look comparable in terms of brightness)?

 

[grackle314]

Bird forum is the physics wanna be forum.
NO clue if what you mentioned is correct #Grackle314, but I doubt that the 32 mm gathers/transmit more light into the eye in sunny or not sunny day.
I might be wrong, If you have a simpler way of explaining it much appreciated if not... it´s oK.
I would say (sorry writing without having a clue) that the only factor would be objective diameter 42mm vs 32mm. Initially more gathered. The magnification affecting little.
On the other hand exit pupil due to BINO physics, one has 4.2mm exit and the other 3.2mm exit regardless the the light in sunny bright day.
On a sunny days you own eyes pupil will contract to counter balance the light transmitted until it burns out due to too much light. I think in sunny days 42mm objectives binos, you own eye makes such effort that the light gathered with 32mm and 42mm seems the same.

Hi Claves, good questions. Here are some answers line by line, hopefully helping your understanding.

A 32 mm binocular objective gathers light across the objective lens and transmits about 90% of the gathered light out the binocular exit pupil. "More" light will transmit through the binocular on sunny days compared to dark days, but the fraction of incoming light making it through the binocular is about 90%, sunny or not.

The luminance (brightness) of the light transmitted through a binocular and out the exit pupil depends on the transmission percentage (around 90%) and the magnification. The effect due to magnification can be seen as follows. The exit pupil diameter is equal to the objective diameter divided by the magnification. The area of the exit pupil goes as the square of the exit pupil diameter, (pi/4)*d-squared, and hence the luminance is increased by the ratio of the exit pupil area to the objective area. Luminance out the exit pupil goes as the square of the magnification, m. You can think of this as the light energy is conserved when going through the binocular, if the light is squeezed into a smaller area then the luminance goes up so the energy is conserved. The magnification does play a major factor in the exit pupil luminance.

A 42 mm diameter binocular will collect more light than a 32 mm objective (by the ratio of the areas, ((42/32)**2) = 1.31).

The original poster compared a 10x32 with an 8x42. The 32 mm will have a 3.2 mm exit pupil regardless of how sunny the day. However, the 8x42 will have a 5.25 mm exit pupil (not 4.2 in your note above), which spreads the outgoing luminance over a larger area than the 10x32. While both binoculars will have light output which does not enter the human eye on a sunny sky, the lost light outside the eye pupil of the 8x42 is great enough to overcome the greater light gathering for the 42 mm objective compared to the 32. The main advantage of a 42 over a 32 in this case is the ease and rapidity of getting the image into the eye due to the larger exit pupil.

If both binoculars compared were 10x (10x32 and 10x42) then the output luminance would be the same. For a sunny blue sky day, both binoculars have exit pupils larger than the human eye pupil and would appear similar "brightness", as you note.

 

[grackle314]

Hi Grackle314

I laboured under this misapprehension for a while too.

Providing the transmission of 2 pairs of binoculars is the same and ignoring the perception of brightness being altered by different colour casts etc then different pairs will have the same brightness providing the binoculars exit pupil is smaller or the same size as our own pupil.

The reasoning your applying can lead down a bit of a rabbit hole when for example you look at a 10x25 compared to an 7x42 - which would be brighter if your pupil was dilated to 2.5mm?

Hello William Lewis, here is an answer to your question.

Comparing a 10x25 and 7x42 when the eye is at 2.5 mm pupil.

The 10x25 will have luminance at exit pupil diminished by transmission loss, typically about 90% of light is transmitted, and increased by the square of the magnification since the exit pupil area is decreased by the magnification squared compared to the objective taking in the light. The 10x25 will have exit pupil of 2.5 mm, area of 4.9 mm-squared, the same as the eye pupil in your question. Assuming perfect alignment of eye with exit pupil, the incoming luminance to the eye will have no light lost outside the eye pupil and will collect a luminance filling the eye pupil which is 100 times greater than the incoming light, diminished by the 10% transmission loss. 90% of the light entering the 10x25 arrives into the eye pupil.

The 7x42 will have an exit pupil of diameter 6 mm, area of 28.2 mm-squared. The magnification will increase the luminance at exit pupil by 49 (7-squared, not the 100 of the 10x) and will spread the outgoing light into an area larger than the human eye pupil of 2.5 mm. The 42 mm objective will gather (42/25)**2 = 2.82 times more light than the 10x25, but the 7x42 light is spread out over 28.2 mm-squared versus the eye pupil at 4.9 mm-squared. The 7x42 light getting into the eye is 90% of incoming along with diminishment factor of 4.9/28.2 = 0.17 due to light lost outside the eye. In all the 7x42 collects more and delivers less than the 10x25 when the eye pupil is 2.5 mm diameter; the factor is 2.82*0.17 = 0.49. The 10x25 will deliver twice the light into the eye compared to the 7x42.

 

[agus_m]

Hello William Lewis, here is an answer to your question.

Comparing a 10x25 and 7x42 when the eye is at 2.5 mm pupil.

The 10x25 will have luminance at exit pupil diminished by transmission loss, typically about 90% of light is transmitted, and increased by the square of the magnification since the exit pupil area is decreased by the magnification squared compared to the objective taking in the light. The 10x25 will have exit pupil of 2.5 mm, area of 4.9 mm-squared, the same as the eye pupil in your question. Assuming perfect alignment of eye with exit pupil, the incoming luminance to the eye will have no light lost outside the eye pupil and will collect a luminance filling the eye pupil which is 100 times greater than the incoming light, diminished by the 10% transmission loss. 90% of the light entering the 10x25 arrives into the eye pupil.

The 7x42 will have an exit pupil of diameter 6 mm, area of 28.2 mm-squared. The magnification will increase the luminance at exit pupil by 49 (7-squared, not the 100 of the 10x) and will spread the outgoing light into an area larger than the human eye pupil of 2.5 mm. The 42 mm objective will gather (42/25)**2 = 2.82 times more light than the 10x25, but the 7x42 light is spread out over 28.2 mm-squared versus the eye pupil at 4.9 mm-squared. The 7x42 light getting into the eye is 90% of incoming along with diminishment factor of 4.9/28.2 = 0.17 due to light lost outside the eye. In all the 7x42 collects more and delivers less than the 10x25 when the eye pupil is 2.5 mm diameter; the factor is 2.82*0.17 = 0.49. The 10x25 will deliver twice the light into the eye compared to the 7x42.

Okay, I think I now understand what you are saying. In another example, 8x42 vs 10x42, if our pupils are 2.5mm, the 10x bino would squeeze the light a bit more so it doesn't matter if both binos collect the same amount of light, with the 10x, a greater percetage of this light actually goes to the retina.

In the unrealistic scenario that we are watching the night sky and our pupils are 2.5mm, thanks to the 10x, we would get more light of the 42mm objectives than the 8x, and thus it will allow us to see more stars.
But whenever the object is extended, the "additional" light is extended over a larger surface area (or rather a solid angle). The appeal of big exit pupils is that we can see extended objects almost as bright as we could see them with naked eye up close.

So yeah, in daylight a 10x bino tends to deliver more light to our eyes than a 8x when looking at an object, because it appears larger.

 

[William Lewis]

Hello William Lewis, here is an answer to your question.

Comparing a 10x25 and 7x42 when the eye is at 2.5 mm pupil.

The 10x25 will have luminance at exit pupil diminished by transmission loss, typically about 90% of light is transmitted, and increased by the square of the magnification since the exit pupil area is decreased by the magnification squared compared to the objective taking in the light. The 10x25 will have exit pupil of 2.5 mm, area of 4.9 mm-squared, the same as the eye pupil in your question. Assuming perfect alignment of eye with exit pupil, the incoming luminance to the eye will have no light lost outside the eye pupil and will collect a luminance filling the eye pupil which is 100 times greater than the incoming light, diminished by the 10% transmission loss. 90% of the light entering the 10x25 arrives into the eye pupil.

The 7x42 will have an exit pupil of diameter 6 mm, area of 28.2 mm-squared. The magnification will increase the luminance at exit pupil by 49 (7-squared, not the 100 of the 10x) and will spread the outgoing light into an area larger than the human eye pupil of 2.5 mm. The 42 mm objective will gather (42/25)**2 = 2.82 times more light than the 10x25, but the 7x42 light is spread out over 28.2 mm-squared versus the eye pupil at 4.9 mm-squared. The 7x42 light getting into the eye is 90% of incoming along with diminishment factor of 4.9/28.2 = 0.17 due to light lost outside the eye. In all the 7x42 collects more and delivers less than the 10x25 when the eye pupil is 2.5 mm diameter; the factor is 2.82*0.17 = 0.49. The 10x25 will deliver twice the light into the eye compared to the 7x42.

Hi Grackle.

As a little thought experiment consider that by your maths a higher magnification will almost always lead to a brighter image as long as your not going below the eyes pupil size?

What you may like to consider is the magnification as a "crop" factor, i.e your cropping a section of the gathered light and magnifying it when you go from say a 7x42 to a 12x42. It will still be as bright as long as the eye pupil is above 3.5mm in this case with all other things being equal. You may well see more detail but it will not be because it's brighter.

Will

 

[claves_10]

Grackle314​

Thanks for explaining It, I learn about birds and much more !!!

 

[grackle314]

Okay, I think I now understand what you are saying. In another example, 8x42 vs 10x42, if our pupils are 2.5mm, the 10x bino would squeeze the light a bit more so it doesn't matter if both binos collect the same amount of light, with the 10x, a greater percetage of this light actually goes to the retina.

In the unrealistic scenario that we are watching the night sky and our pupils are 2.5mm, thanks to the 10x, we would get more light of the 42mm objectives than the 8x, and thus it will allow us to see more stars.
But whenever the object is extended, the "additional" light is extended over a larger surface area (or rather a solid angle). The appeal of big exit pupils is that we can see extended objects almost as bright as we could see them with naked eye up close.

So yeah, in daylight a 10x bino tends to deliver more light to our eyes than a 8x when looking at an object, because it appears larger.

Hello agus_m, your understanding for an 8x42 and 10x42 with 2.5 mm eye pupils is correct. The 10x will deliver more light into the eyes, it squeezes the incoming light into a smaller circle coming out of the exit pupil and hence the light is more intense in that smaller circle than for the 8x. With the same objective areas the increase of the 10x over the 8x is the square of the magnification ratio (10/8)-squared = 100/64 = 1.56. A 56% increase in luminance is observable by me when doing back-to-back comparison, but not when handed one or the other with a big time gap in between sightings.

The night sky consideration is different. Our eyes have complicated internals. The rods and cones are not uniformly distributed. For example, for very faint objects at night I look to the side slightly to detect the faintest light. I am less certain of overall binocular performance in night sky conditions. However, in terms of collecting light into the eye, the eye pupil dilates as darkness increases. Depending on the individual, eye pupils over 4 mm diameter would gather more light from the outer areas of exit pupils which are over 4 mm. In my case, my eye pupils do not dilate much over 4 mm so binoculars which put light outside 4 mm exit pupil waste that light on me. For my personal use I favor binoculars with about a 4 mm exit pupil or less.

Your last point about 10x delivering more light to our eyes in daylight (for your 8x42 and 10x42 example) is correct conclusion. Your association with "because it appears larger" is correct in the sense that when it appears larger it does have greater luminance due to the magnification.