Daft question about low-light performance (2 Viewers)

[Sheepwatcher]

I apologize in advance for asking a question that I feel I should know the answer to, but I'm confused about the interplay of objective diameter and exit pupil in determining the low-light performance of a pair of binoculars. For the sake of the argument, let's assume that the viewer has pupils that dilate to 5mm and a bunch of binoculars with identical optical characteristics. I feel relatively confident that a 10x42 will have poorer low-light performance than a 12x50, since they have similar exit pupil diameters so the objective diameter determines performance. Am I ok so far? But then it gets hazy for me...what about a 10x50 vs the 12x50? Is the determinant of low-light performance simply the objective diameter, or does the larger pupil (matching the maximum size of the viewer) of the 10x50 give better performance into dusk? Then how about the more extreme examples -- 8x50 or 7x50? Would these give the viewer a better view into the gloom than the 10x or 12x, even though the exit pupil diameter exceeds the viewer's pupil size? Help!! And thank you very much!

Milton

 

[Conndomat]

I feel relatively confident that a 10x42 will have poorer low-light performance than a 12x50, since they have similar exit pupil diameters so the objective diameter determines performance. Am I ok so far?

Hi Milton,

that is not correct, what matters is the eye pupil and not the lens opening!

If your assumption were correct, a 12 inch (approx. 305 mm.) telescope would have to be super bright at 73x magnification (exit pupil identical to 12x50), but it isn't!
In fact, the telescope with its own exit pupil is no brighter than 12x50 binoculars, what increases is the resolution and thus the recognition of details.

But then it gets hazy for me...what about a 10x50 vs the 12x50? Is the determinant of low-light performance simply the objective diameter, or does the larger pupil (matching the maximum size of the viewer) of the 10x50 give better performance into dusk? Then how about the more extreme examples -- 8x50 or 7x50? Would these give the viewer a better view into the gloom than the 10x or 12x, even though the exit pupil diameter exceeds the viewer's pupil size?

Simply put, the larger pupil shows flat objects brighter, while the higher magnification is better for seeing details.
If the exit pupil of the binoculars is larger than the pupil of the eye, there is no additional gain for the observer.

Andreas

 

[Sheepwatcher]

Hi Milton,

that is not correct, what matters is the eye pupil and not the lens opening!

If your assumption were correct, a 12 inch (approx. 305 mm.) telescope would have to be super bright at 73x magnification (exit pupil identical to 12x50), but it isn't!
In fact, the telescope with its own exit pupil is no brighter than 12x50 binoculars, what increases is the resolution and thus the recognition of details.

Simply put, the larger pupil shows flat objects brighter, while the higher magnification is better for seeing details.
If the exit pupil of the binoculars is larger than the pupil of the eye, there is no additional gain for the observer.

Andreas

Andreas, thank you for your reply. If I understand you correctly, an 8x32, 10x42, and 12x50 all have roughly equivalent low-light performance in terms of how bright an object appears at dusk? This is very counterintuitive to me!

Milton

 

[Conndomat]

Andreas, thank you for your reply. If I understand you correctly, an 8x32, 10x42, and 12x50 all have roughly equivalent low-light performance in terms of how bright an object appears at dusk?

At least flat objects are roughly the same brightness, but the higher level of detail recognition is of course helpful.

Andreas

 

[John A Roberts]

Hi Milton,

The short answer is 'It’s complicated'. As it depends on a variety of factors, including:

a) The lighting levels, so whether an observer’s eyes are using cones, rods or a combination;
b) The age of the observer, in terms of the maximum diameter of pupil dilation, and;
c) The instrument characteristics.

Unsurprisingly, a lot of experimental work was done in the first half of the 20th century in relation to military applications,
including by Max Berek of Leitz in the 1940’s (published in German). And usefully, Holger Merlitz has much more recently
published in English about the work of Berek and others.

See posts #21 and 22 for links and some pdf's at: Vortex Razor UHD 18x56 ?
Along with some of the following comments.

- - - -
One example from Holger: Binocular performance for 'an average' below 30 yo, compared to 'an average' about 60 yo:

. . . 🤯


John


p.s. Gijs has also published an overview in English of various considerations.
See 'Test colour vision, brightness, resolution and contrast in binocular image' at: Verrekijker testen | House Of Outdoor & Optics


p.p.s. In relation to the graph of 'Binocular Efficiency' by Vukobratovich in post #21 in the above link,
it should by correctly displayed as shown in post #26 at: Swarovski slc 15x56 or el 12x50

 

[[email protected]]

Andreas, thank you for your reply. If I understand you correctly, an 8x32, 10x42, and 12x50 all have roughly equivalent low-light performance in terms of how bright an object appears at dusk? This is very counterintuitive to me!

Milton

When you are discussing low light performance in a binocular, you are concerned with two separate parameters. Brightness and Twilight Factor.

1) Brightness First is brightness, which is a function of the size of the binocular's exit pupil and the binocular's transmission.

EP or exit pupil is the cone of light coming out of the binocular that hit's your cornea and then passes through the vitreous humor of the eye to reach the retina which has cone and rod cells that receive the incoming light signal then interpret that information and sends it to your optics nerve which transmits the signal and then sends it to your brain.

The bigger the cone of light, the brighter the image is going to seem to your brain, but the cone of light has to get through your pupils which change in size with varying levels of light and the amount they can change varies with your age. If you are younger or less than 30 years old, your pupils can dilate to as large as 7mm, but if you are over 60 years old your pupils might only be dilating to 5mm.

So a binocular with a 7mm EP may not be noticeably brighter than a binocular with a 5mm EP if you are over 60. Another factor that comes into play is the transmission of the binocular, or what % of light that enters the objective of the binocular that actually reaches your cornea.

Take two binoculars with identical EP sizes like a 8x42 but one having 95% transmission and the other with 90% transmission.

The binocular with 95% transmission will appear brighter because even thought the EP is the same in both binoculars, that 5mm cone of light hitting your cornea is brighter than the binocular with 90% transmission.

It is like two different flashlights that have the same beam size, but one flashlight has a brighter or more intense beam.

In daylight your pupils only open to about 4mm so that is about the maximum amount of light your eye can take in from the binocular so you can see a binocular with a larger EP doesn't do you a lot of good for daytime use, but one with higher transmission will because that 4mm EP is more intense or brighter.

So in the daytime, transmission is more important than EP size after you reach the limiting factor of about 4mm and when you are over 60 years of age transmission also becomes more important than EP size because you can no longer receive an EP size of maybe anything larger than 5mm because your pupils will only dilate to about 5mm.

So all these factors interplay in how bright a binocular seems to you.

2) Twilight Factor Twilight Factor is the Square Root of the Objective Lens Diameter in mm times the magnification of the binocular, and the higher the number, the more detail you can see under low light.

What it basically means is higher magnification helps you see detail under low light the same way it helps you see detail in bright light. It has nothing to do with brightness, but rather is just describing how easy it is to see detail under low light.

It can produce irrational numbers if you plug in abnormal binocular magnifications and objective sizes, but it is still meaningful with normal binocular formats like a 8x42 and a 10x42.

3) Conclusion Both these factors interplay and will determine how bright a binocular seems and how much detail you can see with any given binocular.

In the three binoculars you mention above if they all have roughly equal transmission they would all be about equal in brightness because their EP is about the same size but the 12x50 would show the most detail under low light with the 10x42 next and the 8x42 would show the least detail due to Twilight Factor.

What all this means is that if you are young, a binocular like an SLC 8x56 with a 7mm EP and 93% transmission will appear brighter to you but if you are over 60 or during the daytime a binocular like the Habicht 7x42 with a 6mm EP and 96% transmission will appear brighter to you.

So your age determines what kind of binocular you should choose for low light, and you should look for one with about a 6mm EP and the highest transmission you can get if you're over 60. So for most of us old folks, the best low light binocular is something like the Habicht 7x42.

What you might find odd is that a binocular like the Habicht 8x30 W with its 95% transmission will appear brighter in the daytime than most 8x42's even the alphas with their 92% transmission because your eye's pupil is only opening to about 4mm so the Habicht maxes it out with EP size, but it hits your cornea with a more intense or brighter cone of light because of the higher transmission.

 

[Maljunulo]

I believe that you are overthinking a bit.

For a given magnification, the glass with the largest objectives will perform best at lower light levels.

The size of the exit pupil is an incidental consequence of the relationship between objective diameter and magnification.

 

[[email protected]]

I believe that you are overthinking a bit.

For a given magnification, the glass with the largest objectives will perform best at lower light levels.

The size of the exit pupil is an incidental consequence of the relationship between objective diameter and magnification.

Not necessarily. It depends on how dilated your pupils are. If you are older and your pupils are only dilating to say 5mm or if you are using your binocular in the daytime when your pupils only dilate to about 4mm, the larger objective will not make any difference because the biggest EP your can accept is 4mm.

Under these conditions' transmission will make more difference than aperture because the 4mm EP entering your eye will be brighter and more intense. A bigger aperture is only advantageous in low light or when you are younger and your pupils are dilating fully to 7mm, so the eye can receive the bigger EP.

Think of two flashlights, one with a 7mm diameter beam and one with a 4mm diameter beam. The flashlight with the 4mm diameter beam has higher transmission like a Swarovski Habicht 8x30 and the other one with the 7mm diameter beam has a bigger aperture but lower transmission like a Swarovski SLC 8x56.

If your eye is only dilating to 4mm as in the daytime or when you are older, the biggest diameter beam that will enter your eye is 4mm. So the 7mm beam diameter of the SLC is now reduced to 4mm and the EP of both binoculars is now equal in size.

But the 4mm EP of the Habicht is BRIGHTER because it has higher transmission than the SLC, so the light that reaches your retina is going to be brighter.

It is not overthinking, it is a fact. Just think about it for a while. A bigger aperture doesn't do you much good in the daytime or when you are older to a point. Transmission makes more difference because of the reduced size of your eye's pupil.

If you are older or in the daytime, a Swarovski Habicht 7x42 with 96% transmission will be brighter than a Swarovski SLC 8x56 with 93% transmission, even though it has a bigger aperture than the Habicht.

Aperture size and magnification aren't the only important factors in low light performance in binoculars. Transmission can be equally or more important depending on eye dilation size.

You're oversimplifying low light performance in binoculars. It is not just the relationship between objective diameter and magnification. It is the relationship between objective diameter, magnification, transmission and eye pupil dilation size.

 

[Rg548]

I think we worry too much about all this low light stuff.
I currently have 8x42 HT's which are super bright..... amazing, one of the best I've had in this regard....but when the light is gone, it's gone!
My 10x42 Meostars really weren't 'THAT' far behind even with smaller exit pupil and lower transmission figures.
It's generally minutes of difference, not much more... the 10x42's were perfectly good, I could see plenty.
I did own Zeiss FL 8x56's but the itch wasn't worth the scratch.
Sure they were bright, but big and heavy, and the brightness wasn't enough to make me keep them.
I had no problems going to 10x42's straight after... they were fine.
I'm settled on the HT's and I do enjoy their brightness/ low light performance..... but it's not the be all and end all in my opinion.

 

[Sheepwatcher]

I believe that you are overthinking a bit.

For a given magnification, the glass with the largest objectives will perform best at lower light levels.

The size of the exit pupil is an incidental consequence of the relationship between objective diameter and magnification.

Yes, that makes sense, but I am trying to understand the relationship between bins of different magnifications as well.

 

[Sheepwatcher]

I think we worry too much about all this low light stuff.
I currently have 8x42 HT's which are super bright..... amazing, one of the best I've had in this regard....but when the light is gone, it's gone!
My 10x42 Meostars really weren't 'THAT' far behind even with smaller exit pupil and lower transmission figures.
It's generally minutes of difference, not much more... the 10x42's were perfectly good, I could see plenty.
I did own Zeiss FL 8x56's but the itch wasn't worth the scratch.
Sure they were bright, but big and heavy, and the brightness wasn't enough to make me keep them.
I had no problems going to 10x42's straight after... they were fine.
I'm settled on the HT's and I do enjoy their brightness/ low light performance..... but it's not the be all and end all in my opinion.

I agree that we can chase diminishing returns. This is more of an intellectual curiosity for me. I realized that I understood less than I thought I did about this, and that always makes me uneasy, wanting to learn more.

 

[Maljunulo]

The same laws of physical optics apply to, and work the same for all binoculars great and small.

The glass with the largest objectives will still be brighter and have a larger exit pupil, for any comparison at a given magnification.

A 10X50 will always have a bigger exit pupil than a 6X30, and a 6X42 will always have a bigger exit pupil than a 10X50.

I hope this helps, because I can’t think of any other way to say it.

ADDENDUM: All I can think of to say now is “Oops!”

 

[Binastro]

5x10 = 50

5x6 = 30

Usually.

B.

 

[Sheepwatcher]

5x10 = 50

5x6 = 30

Usually.

B.

I was wondering if I was missing something...

 

[[email protected]]

The same laws of physical optics apply to, and work the same for all binoculars great and small.

The glass with the largest objectives will still be brighter and have a larger exit pupil, for any comparison at a given magnification.

A 10X50 will always have a bigger exit pupil than a 6X30, and a 6X42 will always have a bigger exit pupil than a 10X50.

I hope this helps, because I can’t think of any other way to say it.

True, but you also have to consider how bright the exit pupil is and that is a function of the binocular's transmission and if your pupils will dilate enough to accept the larger exit pupil.

 

[normjackson]

We're in this together

 

[Elkhornsun]

The objective's area determines how much light enters the binocular or scope. A 50mm objective provides a surface area that is 41% greater than a 42mm objective. How much of the light reaches your eyes depends in part on the quality of the coatings on the lens surfaces inside.

What we perceive as sharpness is a function of both image magnification and image contrast. A 12x provides 50% greater image magnification than a 8x and this makes it easier to see details in a subject. In low or flat light the contrast decreases and I can compensate in large part with a higher magnification binocular.

Switching from 8x to 10x binoculars helped me a great deal. For distant subject I use a 12x50 much of the time. For casual walking around a 10x25 has been adequate and at 12 ounces having it hanging from my neck for hours has not been a problem.

 

[Gijs van Ginkel]

Maljunulo, post 12 and some post by others,
If one talks about the laws of physiscs it helps if the information supplied is correct.
The exit pupil of a 10x50 binocular is 5 mm and the exit pupils of 6x30 and 7x35 or 8x40 are all 5 mm. That is rather basic and can be found in many books or information material about binoculars.
Gijs van Ginkel

 

[Maljunulo]

Maljunulo, post 12 and some post by others,
If one talks about the laws of physiscs it helps if the information supplied is correct.
The exit pupil of a 10x50 binocular is 5 mm and the exit pupils of 6x30 and 7x35 or 8x40 are all 5 mm. That is rather basic and can be found in many books or information material about binoculars.
Gijs van Ginkel

Gijs;
You are quite correct, of course.

Yes, I have posted a very embarrassed addendum to post 12, which consisted of "Oops!"

I could have done what a very well known poster does, and deleted the post, but chose not to.
Richard

 

[Patudo]

There have probably been a gazillion threads about low light but what very few attempt to define in them is - what does low light actually mean? Civil twilight? Nautical twilight? Virtual blackness? Or failing that, what sorts of situations are we talking about? That can be every bit as important as objective diameter, magnification etc.

I don't do a great deal of observing in really low light, but from what I've seen, the darker it gets, to get a real improvement over the standard 8x42 or 10x42 you need a pretty serious step up in capability ie. 10x56 or thereabouts.