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A Basic Exit Pupil Question

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#1 Joe1950

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Posted 29 December 2017 - 10:43 PM

May I get a clarification on the 'exit pupil' topic?

 

• With a 7x50 binocular you have a 7mm exit pupil.

 

• If your eyes are aged and you live in a light polluted area, say your pupils dilate to (at best) 5mm, which is roughly 71% of the full 7mm.

 

• So, under these circumstances, the best you will do with your 7x50s, is the equivalent of a 7x35 (50mm x 71%)?

 

 

Or is there more to it than that?



#2 Richard O'Neill

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Posted 29 December 2017 - 10:58 PM

"Or is there more to it than that?"

 

Nope, you got it.


Edited by Richard O'Neill, 29 December 2017 - 11:10 PM.


#3 J A VOLK

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Posted 29 December 2017 - 11:10 PM

you will still get the increased resolution of a 50mm

#4 Joe1950

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Posted 29 December 2017 - 11:21 PM

Thanks Richard!

And a good point to consider JA.

 

In a way, it's disappointing since at age 67 and living in a light polluted area, I really can't use the full potential of my 11x70s, with a 6.4mm exit pupil. I guess the 8x42s come closer, but I'm just guessing on the 5mm as being attainable.

 

I guess if I wanted to have a 70mm bino, the 15x70 would be a better choice. But I already have the 11x70s from years ago and they do show very nice star images, even though the field is narrow at 4.5°.

 

Some years ago I went to the eye doctor for a check-up and he put those drops in my eyes. I had forgotten my sunglasses in the car and I had to literally feel my way to the car when I got outside, it was so overwhelmingly bright.

 

Getting back to work, I looked in the mirror and my eyes looked very weird with the pupils dilated as they were.



#5 Rich V.

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Posted 29 December 2017 - 11:22 PM

Correct.  Your eye pupil is stopping down the exit pupil to its own size. 

 

Since binoculars aren't fully illuminated to the edge, though, the central 71% of a 7x50 will be better illuminated than 100% of a 7x35, so the view can appear brighter to the edge. Your looking through the sweet spot of the bino.  A 7x50 can be easier to keep a proper eyepoint as well, since being centered up on the 7mm exit pupil is less critical.

 

On the negative side, design concessions generally give up some FOV in most 7x50s compared to what's available in 7x35s.

 

Rich



#6 Joe1950

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Posted 29 December 2017 - 11:31 PM

Okay Rich. That's a good thought. With the 11x70s I am getting almost 80% of the full aperture, at 5mm.

 

I do see a lot of stars even in the narrow field with those binos. As I say, they are virtually pinpoint except at the very edge. Those binos were about $150 back when I bought them and only available through Adorama. My son has had them but wasn't using them at all so I borrowed them back. I can't recall buying them.

 

He also has the Orion Mini Giant 8x56s. I do remember getting them. Again there is that 7mm ep. I'll have to do a side by side some nice clear night and see what I feel best with.

 

Oh well, I guess we use what we have and enjoy the views, whatever they are!

 

Thanks everyone!


Edited by Joe1950, 29 December 2017 - 11:36 PM.


#7 siriusandthepup

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Posted 29 December 2017 - 11:38 PM

Us oldies have to adapt.

 

Here's the basics:

 

Divide the aperture by the mag to get exit pupil.

 

7x50  = a hair over 7mm exit pupil

7x35  = 5mm

6x30  = 5mm

10x50 = 5mm

10x70 = 7mm

14x70 = 5mm

16x70 = 4.4mm

20x80 = 4mm

 

The increase in magnification gives the proper exit pupil. What did you lose? A little wideness of field. Not a big loss really.



#8 Joe1950

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Posted 29 December 2017 - 11:55 PM

Doesn't seem fair, does it, Ed?

 

Good chart!

 

The wide field is mostly for the immersive effect, unless you have the real expensive binos. It's a nice experience, but I try not to focus on the field edge since the image gets soft there, with the kind I buy.

 

The Aculon 8x42s have a nice wide field and are tack sharp in the center with the aspheric EP elements. It's a very nice experience. Daytime views are extraordinary. Again, as long as you don't look directly at the very edge and get picky.



#9 GlennLeDrew

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Posted 30 December 2017 - 01:08 AM

you will still get the increased resolution of a 50mm

Nope. By interfacing a smaller iris to a larger exit pupil one is *truly* utilizing a commensurately smaller aperture. It's *exactly* like installing an aperture mask over the objective. In Joe's example, a person's 5mm iris with a 7x50 renders that bino as a 7x35. If a 35mm mask were to be installed over the 50mm objectives, and one's iris were to be 5mm or larger, the image would be exactly as seen when one's iris is 5mm.

 

Remember, the eyepiece couples the larger objective to the smaller iris. If the iris is smaller than the exit pupil, the eyepiece projects upon the objective an image of the iris enlarged as the magnification. A 5mm iris is projected upon a 7X50 objective as 5mm multiplied by the magnification of 7X, or 35mm. Which has the same effect of a 35mm aperture mask affixed to the objective.

 

But for brighter subjects this is somewhat moot.

 

It is only when the exit pupil gets down to about 1mm do we realize the full resolving power of an optical system. For it is at this point where we begin to resolve the Fresnel pattern of diffraction. At any exit pupil much larger than this we are not pushing the optic to its full potential (assuming aberrations are not the limiter.)

 

And so a 7X bino requires to have an objective of about 7mm in order to deliver a 1mm exit pupil and so work at its maximum resolution. Assuming it has decent quality, it would resolve about as well as would a 7X50--for sufficiently bright objects, of course. Where the subject is not bright, the larger aperture permits to probe to fainter limits for given visual resolving power.

 

And in the end, for a visual instrument at least, the aperture is merely to support the desired magnification. An aperture larger than required to deliver a maximal resolving ~1mm exit pupil is there just to provide a brighter image and thereby improve the signal to (visual) noise ratio.



#10 Erik Bakker

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Posted 30 December 2017 - 03:37 AM

I've found a very workable exit pupil in the 5.6-6 mm range. Gives me the opportunity to combine a largish exit pupil with a somewhat smaller instrument. But my pupils still do reach over 7mm, so I still appreciate what a beautiful bright and calm image that can offer the observer.



#11 garret

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Posted 30 December 2017 - 04:54 AM

 

 

Nope. By interfacing a smaller iris to a larger exit pupil one is *truly* utilizing a commensurately smaller aperture.

???

You are also saying: when you are using a 7x50 binocular under very bright light conditions like on the beach and the pupil is very small (1.5mm) , the view would be considerable less sharp (because of much smaller effective aperture of only 10.6mm) then with large eye pupils.

 

Seriously I have never ever read such claim elsewhere... 


Edited by garret, 30 December 2017 - 05:11 AM.


#12 Jon Isaacs

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Posted 30 December 2017 - 05:51 AM

 

 

 

Nope. By interfacing a smaller iris to a larger exit pupil one is *truly* utilizing a commensurately smaller aperture.

???

You are saying also: when you are using a 7x50 binocular under very bright light conditions like on the beach and the pupil is very small (1.5mm) , the view would be considerable less sharp (because of much smaller effective aperture of only 10.6mm) then with large eye pupils.

 

Seriously I have never ever read such claim elsewhere... 

 

 

The first time I saw this pointed out was in a long thread on Astromart.  Alan French made the point and it took a while for other members to understand the concept. 

 

But, consider this:

 

I make a 10.6 mm aperture mask for my 7x50s.  If my pupil is 1.5mm, how can I determine if the mask is in place or not in place if my pupil is centered on exit pupil?  

 

If one computes the resolving power or the size of the Airy disk of both the binoculars and the eye, one can see that it is only when the full exit pupil is entering the eye that full resolution of the telescope is possible.  To do this, one calculates the angular resolving power of the telescope, of the eye's pupil and divides the resolution of the exit pupil by the magnification.  

 

Rayleigh Criterion (arc-seconds)  = 138(mm)/D

 

For the 7x50's:  130/50 = 2.76 arc-seconds.    Exit pupil = 50/7= 7.14 mm  If the eye is dilated to 7.14mm, it's aperture is capable of resolving:

 

138mm/7.1mm = 19.4 arc-seconds.  Since the binoculars magnify the image 7 times, 19.4 arc-seconds / 7x = 2.76 arc-seconds.  

 

If the eye's pupil is 1.5 arc-seconds, then it can only resolve 138/1.5 = 92 arc-seconds, which happens to be the resolving power of a 10.6 mm aperture if the image is magnified 7 times.  

 

This is theoretical resolving power, the assumption being that the eye is a perfect optic.  However, as Glenn explained, the resolution of the eye is poor at large exit pupils and the resolution of the dark adapted eye is also much reduced. A 50mm objective is capable of resolving less than 3 arc-seconds. That would mean Castor would be an easy split.  

 

But no one resolves Castor in 7x50s, the eye is very poor at a 7mm exit pupil.  The focal length of the eye is about 17mm, that means it's operating at f/2.4, not likely to be sharp.. 

 

Jon



#13 garret

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Posted 30 December 2017 - 06:46 AM

I have made a 10mm circular mask in front of my 10x50, couldn't see definite sharpness difference between left and right under low light conditions (in home).

Unfortunately, here in western Europe we have patent on clouds the last 3 months, testing soon under the stars is unlikely.



#14 Tony Flanders

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Posted 30 December 2017 - 07:03 AM

 

Nope. By interfacing a smaller iris to a larger exit pupil one is *truly* utilizing a commensurately smaller aperture.

???
You are also saying: when you are using a 7x50 binocular under very bright light conditions like on the beach and the pupil is very small (1.5mm) , the view would be considerable less sharp (because of much smaller effective aperture of only 10.6mm) then with large eye pupils.

 

Yes and no.

If your eye's lens was optically perfect, and if your retina contained tiny pixels like the ones in a cell phone, then yes, the view would be less sharp during the daytime. But in fact, the rods and cones in your eye are much bigger than the pixels in a cell-phone sensor. So even if the image on your retina is sharper, you're unable to see that added sharpness.

 

In fact, the whole concept of resolution in the optical-theoretical sense is irrelevant for conventional binoculars, because they operate at such extremely low magnification. To see the increased resolution, you would need to be using 20X or higher, not a measly 7X.

 

Moreover, nobody's eye lenses come close in quality to a well-designed glass lens, and most people's eye lenses fall very far short. So just as stopping down a mediocre photo lens improves the image quality, so using exit pupils smaller than your own eye's pupil tends to improve the image quality. That's why for targets that are both bright and small -- the planets in particular -- masking down the aperture of binoculars often improves the view.



#15 Jon Isaacs

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Posted 30 December 2017 - 08:27 AM

Further thoughts on 7x50 with 5mm pupils.. Some random comments:

 

-  Not all 7x35s are 7x35s,.. with the 7x50s, you can be confident you do have 7x35s.

 

-  Field illumination should be improved.  

 

- Binoculars are generally around F/4.  That means 7x50s have a 200mm focal length, 7x35s around 140mm.  This means less field curvature and it also means the 50mm are operating at F/5.6 which is probably a good thing.

 

- Eye placement is easier, more margin for error.

 

Jon



#16 ks__observer

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Posted 30 December 2017 - 08:28 AM

Loss of apperture seems to be the source of continuing discussion since the start of time.
I recently posted on the subject elsewhere.
I still question loss of brightness for the target being viewed.
Yes, if you want to fully illuminate the FOV you lose apperture.
But we only only care about illuminating the little target in the middle.
I do not see how fov getting clipped at edge affects brightness of target in middle.
Question:
When eye-pupil less than bino/ep exit pupil, will, say, 50mm apperture still produce 50mm brightness for target in middle of fov, or will target in middle now look as if seen through 35mm?
It seems people always focus on question of illuminating total fov as opposed to small target in middle, which is what we care about.
Am i missing something?

#17 DVexile

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Posted 30 December 2017 - 08:56 AM

Yes you are missing something. Shine a laser into a binocular at the center of the FOV and you'll end up with a circle of light the size of the exit pupil out the eyepiece. If your eye pupil is smaller than that you do indeed lose light even though the subject was a point source at the center of the FOV. 


Edited by DVexile, 30 December 2017 - 08:58 AM.


#18 ks__observer

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Posted 30 December 2017 - 09:02 AM

Shouldn't a point source in the front end appear as point source in the back end?



#19 DVexile

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Posted 30 December 2017 - 09:13 AM

No it shouldn't. With just the objective and no eyepiece you'd have a point at the focal plane indeed.  But that's not what a binocular or a telescope is.  They have eyepieces. 

 

There are plenty of websites that outline basic optics of telescopes if you are interested. 



#20 Jon Isaacs

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Posted 30 December 2017 - 09:29 AM

Shouldn't a point source in the front end appear as point source in the back end?

 

It won't be a point,  it will be an Airy disk and it will be dimmer than it would be were you using the entire exit pupil.  The brightness of the star depends on the aperture that collects the light you are looking at. 

 

Partially block one objective with your hand.  The view will be dimmer but the field of view won't change.. 

 

Jon



#21 siriusandthepup

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Posted 30 December 2017 - 09:49 AM

This shrinking of binoculars aperture due to exit pupil contraction is something I've understood for a long time.

 

Here is a shocking consideration for you: If, due to using my 8x42mm Fuji's in the daytime, my exit pupil shrinks fro 5 mm to 1.5mm then I'm only using less than 1/3 of the 42mm aperture.  Hmm - less than 14mm aperture utilization. Suddenly I can see usefulness to those 8x20's for daytime use. (a lot more compact and light too).

 

In this hobby we aren't programed to think that way. We are always looking for ways to squeeze the last photon out of all our gear. We're night hounds.

 

Just a little more salt - this applies to our daytime spotting scopes too.

 

Oh, and then there is Moon viewing with our little exit pupils also.



#22 caveman_astronomer

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Posted 30 December 2017 - 10:09 AM

This shrinking of binoculars aperture due to exit pupil contraction is something I've understood for a long time.

 

Here is a shocking consideration for you: If, due to using my 8x42mm Fuji's in the daytime, my exit pupil shrinks fro 5 mm to 1.5mm then I'm only using less than 1/3 of the 42mm aperture.  Hmm - less than 14mm aperture utilization. Suddenly I can see usefulness to those 8x20's for daytime use. (a lot more compact and light too).

 

In this hobby we aren't programed to think that way. We are always looking for ways to squeeze the last photon out of all our gear. We're night hounds.

 

Just a little more salt - this applies to our daytime spotting scopes too.

 

Oh, and then there is Moon viewing with our little exit pupils also.

The deciding factor could be the FOV and eye relief, sometimes narrower and shorter in the compacts, along with possibility that you might already own a standard-sized binocular that does well in those two areas.



#23 ks__observer

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Posted 30 December 2017 - 10:16 AM

The brightness of the star depends on the aperture that collects the light you are looking at. 

 

If, "The brightness of the star depends on the aperture that collects the light," shouldn't a star at the center of the exit-pupil's image circle be the result of the full OTA objective?



#24 Jon Isaacs

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Posted 30 December 2017 - 11:40 AM

 

The brightness of the star depends on the aperture that collects the light you are looking at. 

 

If, "The brightness of the star depends on the aperture that collects the light," shouldn't a star at the center of the exit-pupil's image circle be the result of the full OTA objective?

 

 

No.. 

 

Each piece or pencil of the exit pupil contains the entire image.  If you do the experiment I suggested,  blocking half the objective with your hand,  you will see the exit is now a half circle.  You still see the entire field of view but it's dimmer. 

 

The center of the exit pupil does not correspond to the center of the image,  it corresponds to the center of the objective. 

 

Do the experiment,  see for yourself.  

 

Jon



#25 ks__observer

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Posted 30 December 2017 - 11:54 AM

 

 

 You still see the entire field of view but it's dimmer. 

 

 

Agree.  Each point on the objective sees the entire FOV.

 

 

Each piece or pencil of the exit pupil contains the entire image. 

 

Not sure I agree here.

I could be wrong:

I think an eyepiece creates an actual full image, the image circle, at the eye-relief point with a diameter equal to the exit pupil.

The brightness of any point on the image circle picture is solely a function of the aperture -- not the end user.

The end user's eye looks at the image as it would look at any image -- and the brightness for any point on the picture that makes it through the eye-pupil is the same for everyone -- for that spot on the picture that makes it through the eye-pupil.




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