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Nikon's theory on exit pupil and image brightness

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#1 supai

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Posted 09 October 2019 - 04:20 PM

hi - I'm quite new, and certainly haven't read all 332 pages on this site, so I may be exhuming ancient history, but is the last part of this text on the Nikon sport optics site correct?

 

https://imaging.niko...ic/basic_05.htm

 

"Why do both large-exit-pupil binoculars and small-exit-pupil binoculars provide the same bright images in bright conditions?

The human pupil normally opens about 2mm in daylight, and 7mm in the dark.
If you use binoculars with an exit pupil of over 2mm in daylight, you won't perceive dark images. Brightness will not vary whether you use binoculars with a 7mm or 2mm exit pupil.
On the other hand, if you use binoculars with a small exit pupil in the dark, the image will not appear as bright as when seen with the naked eyes."

 

If this is true, why does the Orion Nebula (choosing an extended object to avoid discussion of point-like stars) look much brighter in my 10x30s than with my naked eye? I always thought - because a 30 mm aperture collects 302 / 72 (18 times) as much light as an eye that is open to 7 mm. On the other hand, that light is concentrated in a ~3 mm diameter patch on the retina. Is Nikon saying that this reduced area of illumination (compared with the full opening of 7 mm) cancels out the 18 times greater light gathering? Even if that makes any sense, it isn't exactly true arithmetically - 302 / 72  is not equal to 72 / 32 . And common sense suggests it must be wrong.   



#2 hallelujah

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Posted 09 October 2019 - 04:34 PM

Others here will have answers for you.

I recommend that you get a copy of this book,

it will help to answer many of your binocular questions as a newbie.

 

https://www.amazon.c...70656709&sr=1-1

 

Stan



#3 hallelujah

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Posted 09 October 2019 - 04:40 PM

Here's another excellent binocular book for beginners, etc.

 

https://www.barnesan...ASABEgKUNfD_BwE

 

Stan



#4 Jon Isaacs

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Posted 09 October 2019 - 05:45 PM

If this is true, why does the Orion Nebula (choosing an extended object to avoid discussion of point-like stars) look much brighter in my 10x30s than with my naked eye? I always thought - because a 30 mm aperture collects 302 / 72 (18 times) as much light as an eye that is open to 7 mm. On the other hand, that light is concentrated in a ~3 mm diameter patch on the retina. Is Nikon saying that this reduced area of illumination (compared with the full opening of 7 mm) cancels out the 18 times greater light gathering? Even if that makes any sense, it isn't exactly true arithmetically - 302 / 72  is not equal to 72 / 32 . And common sense suggests it must be wrong.

 

 

This is complicated and sometimes counterintuitive.  I will try to explain it the best I can.

 

-  The exit pupil is the beam of light leaving the eyepiece that you look at.  If you look at the eyepiece lens from a distance, you can see the exit pupil as a bright circle that is actually an image of what the objective sees.  As Nikon says, the diameter of the exit pupil is equal to the aperture divided by the magnification.

 

You can verify this by making a ring with your thumb and forefinger and placing it in front of one of the objectives, as you make the ring smaller, you can see the exit pupil get smaller. 

 

- The exit pupil contains the entire image regardless of the exit pupil.  The entire field of view of the retina is illuminated, not just a 3mm diameter, regardless of the diameter of the exit pupil.  When your pupil contracts, the image becomes dimmer but you still see the same image.

 

You can repeat the above experiment but this time look through one side of the binoculars while forming the ring with your thumb and forefinger in front of the objective.  As you make the ring smaller, the field of view remains the same but the image becomes dimmer and dimmer.  

 

If your pupil were open to 7mm, and you had 10x70 binoculars, then the 7mm exit pupil would fill your pupil and the image would be as bright as the night sky. As you constrict the ring to 50mm in diameter, 40mm, 30mm, the image dims because you are collecting less light but that light is being spread over the same area. 

 

The image brightness of a 3mm exit pupil compared to a 7mm exit pupil is related to the area, the 10x30s are 9/49 = 18.4% the brightness of 10x70s.  

 

- It is important to understand the concept of brightness is surface brightness, that's photons per unit area. The 10x30s capture (30/7)= 18.4x the light but it is magnified 10 times so it is spread out over 10= 100 times the area.  Thus each retinal cell receives 18.4/100 = 18.4% of the light.

 

- Why the Orion nebula appears brighter:  This has to do with the workings of eye/brain.  A camera would see the image of Orion in the binoculars as being 10x larger in diameter but about 1/5th as bright.

 

The eye doesn't work that way.  As long as the image is sufficiently bright, the eye is primarily a contrast detector, Contrast is the ratio of the brightnesess in an image. When you magnify the image, the object itself dims but so does the background sky and it is all in proportion so magnifying an image does not change the contrast, it just makes it dimmer.  But the bigger image shows more detail and the eye/brain interpret that as being brighter.  

 

Imagine looking at Orion in a 300mm telescope at 100x.  Your binoculars show a much more detailed image of Orion because the image is 10 times larger even though it's about 1/5th the brigthtness.  With a 300mm telescope at 100x, the image is 10 times larger than in the binoculars and of equal brightness.  

 

This is a complex subject and the concepts are difficult to grasp.  Hopefully you can perform the experiments I outlined and you can get some sort of a handle on things.  

 

Jon


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#5 Yarddog

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Posted 09 October 2019 - 06:02 PM

That is a good explanation.

 

I have experienced that same phenomena in the evening and early morning looking for game. The view through high quality binoculars seems to get brighter. I know this is not possible because there is light loss through every glass element.

 

Now I know why.



#6 supai

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Posted 09 October 2019 - 08:30 PM

Thanks Jon. I think I see that. So in fact, one could argue that Nikon's text is a bit too physics based and insufficiently perception based.



#7 Swedpat

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Posted 11 October 2019 - 01:18 AM

This is an interesting topic!

 

Technically not even the very best optics with highest light transmission can match the naked eye when it comes to brightness. This because every added lens will cause light loss. The brightest binocular(with prisms) can provide 95 or 96% of the naked eye brightness. A galilei binocular even brighter. This SHOULD anyway mean you perceive similar brightness.

Still you sometimes can perceive a brighter image through the binocular.

I think there can be a few reasons more than Jon Isaacs mentions to this situation:

 

*when using a binocular the eyecups block some straylight from the sides which improves the low light performance compared to naked eye.

 

*you mix up light amount with surface brightness.

I have heard people being exalted about the brightness through binoculars("it was like turning on ten lights!")

When looking towards light sources you can be very dazzled through the binoculars compared to naked eyes. This is not because of a brighter image, however. The image brightness can actually decrease because of that the eye pupils will contract due to the high light amount hitting the eyes. Still the light amount will be a lot of times higher than with naked eye. And that may feel like brighter image.

 

Just my two cents.


Edited by Swedpat, 11 October 2019 - 05:29 AM.


#8 PEterW

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Posted 11 October 2019 - 01:56 AM

The amount of light input variability from the pupil expanding is a small fraction of the light sensitivity change you can get from dark adaption (which can take over 30mins to obtain). The eye-brains response to faint objects is maximised for a specific apparent size, which optics can deliver.

Peter

#9 supai

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Posted 11 October 2019 - 05:54 AM

Nice to get some thoughtful responses from experts. I like the idea that our perception relates more to the total light input, or "photons per second", than to the flux or "photons per unit area per second". Why do some people use visual moon filters in telescopes? Because they are dazzled by the image, the more so the larger the telescope, other things being equal.



#10 Rich V.

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Posted 11 October 2019 - 11:09 AM

Nice to get some thoughtful responses from experts. I like the idea that our perception relates more to the total light input, or "photons per second", than to the flux or "photons per unit area per second". Why do some people use visual moon filters in telescopes? Because they are dazzled by the image, the more so the larger the telescope, other things being equal.

Yet the Moon's surface brightness is never greater than your naked eye's view of it; it dazzles because more of your retina is illuminated due to the larger image scale due to magnification.  The brightness will cause your dark adaptation to go away and your eye's pupil diameter will contract.  That contraction of your eye pupil will also decrease the effective aperture of the scope if your eye pupil becomes smaller than the exit pupil of the scope/eyepiece combo.  Decreasing the effective aperture also increases the effective f ratio of the scope.

 

Likewise, during daytime use, a tiny 10x25mm compact binocular will be similarly bright as a big 10x70 bino because if your eye pupil is contracted to 2.5mm by the daylight brightness, it's rendering only 25mm of the 70mm's aperture effective, just like putting as 25mm mask on the objective.  Your pupil is "stopping down" the 10x70's 7mm exit pupil to only 2.5mm effective.

 

Rich


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#11 Jon Isaacs

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Posted 12 October 2019 - 05:46 AM

Nice to get some thoughtful responses from experts. I like the idea that our perception relates more to the total light input, or "photons per second", than to the flux or "photons per unit area per second". Why do some people use visual moon filters in telescopes? Because they are dazzled by the image, the more so the larger the telescope, other things being equal.

 

I have my guesses as why people use moon filters on telescopes: 

 

Primarily it's habit, thinking that dark adaptation is a good thing. Dark adaptation is a good thing viewing faint objects but counter productive viewing bright objects like the moon and the planets.  I just let my eye adapt to the brightness of the moon.  That way I get the full benefit of my photopic vision, color, resolution.  

 

From what I see reading on CN, people with large aperture scopes, 20 inch plus scopes, generally do not use moon filters.  I don't.  At the magnifications/exit pupils appropriate for seeing fine details, the moon is actually quite dim.  

 

The total light input from an object or a given telescope is always the same as long as it fits in the field of view.  Magnification is a trade off between image brightness and image size.  At low light levels the eye has very poor resolution so increasing the size of the object may be necessary in order for the eye to actually resolve it.  

 

Jon




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