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Affect of Eye Pupil on Binocular Aperture

Exit Pupil is the beam of light exiting out thru the eyepieces. The diameter of that beam can be found by dividing objective diameter size by magnification. Larger Exit Pupil provides increased BRIGHTNESS. But there are some very important points for consideration.

exit pupil 7mm vs 5mm
If you have a 7mm eye pupil then you get all the light from say a 7x50. But if your eye pupil does not get that large, and many don't, then the effective aparture that you are getting out of the binocular is reduced. So, a person with a 6mm eye pupil, when using a 7x50, it actually only getting 7x42. A person with a 5mm eye pupil is getting effectively a 7x35.

7mm exit puplis will provide a much brighter image than a 5mm exit pupil, BUT ONLY IF you have very very dark skies. Then you would have good contrast. But if you have any amount of light pollution, let's say you have mag 5 skies or worse yet mag 4 skies, then that large 7mm exit pupil may work against you. The brighter sky background will be brighter in the binocular with a 7mm exit pupil and it will tend to wash out the contrast for all objects. In this case a 5mm exit pupil might be better.

I would seldom recommend a 7x50 binocular to anyone as it is a specialty binocular far more affected by limitations of exit pupil and sky conditions and in most instances will not perform as it was intended.

I would never recommend a 7mm exit pupil 7x50 as a first astronomy binocular.

edz

Several threads have embedded discussions on exit pupil. Following are discussions and links to the original threads.

One question that a beginner often asks about is exit pupils. There is so much talk about exit pupils that it can be thoroughly mind-boggling. This primer will set you on the right path. When you begin to yearn for more information visit the Best Of link dedicated to exit pupil.
Binocular Primer - An explanation of Exit Pupil, Eye relief and Field of View

Another member asked "I think these (7x50 celestron ultima) are great binoculars...BUT... I find they amplify sky glow or produce it? ... Is this because of the rather large exit pupil(7.1mm)? I don't see this glow in my 15x70's and I'm guessing that it's because of the 4.4mm exit pupil or the higher magnification."
Exit Pupil Affects the Performace of Your Binocular - Make the Right Choice

This discussion took place here several years ago.
Q
Is the difference between a 5mm and 7mm exit pupil really that significant (I've been looking at the Pentax 7x50 PCF WP's)? The 7x50's give a much wider FOV (324ft/1000yds vs. 261ft/1000yds), which I would think is more important for bino star gazing...Am I missing something fundamental here?

A
Generations of binocular designers who have forgotten more about optics than I will ever know can surely not all be wrong or stupid to have designed so many 7 x 50 , 8 x 56 , 9 x 63 , 10 x 70 and 11 x 80 binoculars over so many years.

"Accepted theory" seems to indicate that, generally speaking, people over around 50 years of age have maximum pupil dilation of only around 5mm in darkness. I bravely pronounce that this assumption is somewhat flawed if not complete poppycock. A noted regular contributor to this wonderful site, let alone forum , who is over the age of 50 , has recently had his dilated pupils measured at 7mm --and that not in COMPLETELY dark -adapted circumstances .

There may well be very good arguments that a binocular with an exit pupil as large as 7mm is performing nowhere near it's maximum potential , but that is not to say that the same binocular will not provide a much more "bright, easy and comfortable" image than would the same (aperture) with higher power eyepieces.

Because terms such as "brightness" "ease of view" and "comfort" have no measurable units, they are unlikely to stand-up well in any technical tug of war against the advantages of higher power (and thus smaller exit pupils) that CAN be measured in the very significant and universally accepted and very real properties of contrast and resolution.

So it is the old argument of "swings and roundabouts" , another example of the fact that no matter how good any particular binocular is for any specific intended purpose, there is always a compromise.

When you gain in power you lose in brightness and field of view, when you gain wider fields of view you tend to lose "flatness" -- a desired quality for stargazing.

Another "con" of a large exit pupil is that it amplifies any human eye astigmatism , making it virtually impossible for anyone with such afflictions and NOT wearing glasses or lenses to correct it, to attain "crisp ,sharp, clear" images , which again are terms not to be found in an optics expert's vocabulary but which are more "artistic" terms ... intended to convey just about EVERYTHING desirable about a magnified image.

I happen to prefer very wide fields of view with hand-holdable low power and "easy to see with " 6mm exit pupils, which is why I wouldn't swap my Zeiss 7 x 42 for any binocular in that size or price range.

All this said , I would estimate that 90% of the time a 5mm exit pupil is ENOUGH IF NOT TOO MUCH for binocular astronomy pleasure and performance.

Regards -Kenny.

Tom,
First, if you do not have eye pupils that dilate beyond 5mm, then 7mm exit pupil does you no good. [With the Exception that Larger exit pupils do make it much easier for your eyes to acquire and hold the image]. But if you do have eye pupils that dilate beyond 5mm, then there are applications where the 7mm exit pupil will show greater performance. That is primarily on broad diffuse extended objects. But only if you can get to dark enough skies.

What do you give up to get 7mm exit pupils? Image size. But if the image is broad and diffuse, do you need a larger image scale? Maybe not.

The binocular that would be best for broad diffuse extended objects is not the same binocular that would be best for most other objects. For most everything else, a higher magnification (that produces a smaller exit pupil) provides a larger image and a bit deeper limiting magnitude that allows seeing more. More stars in wide-open clusters, more resolved stars in dense clusters, a little bit more definition in doubles.

All that is probably why many of us own more than one binocular. Kenny even has his terrestrial favorites, I my backpacking favorites.

It's not all in the exit pupil either. A higher contrast, sharper image from a smaller exit pupil can exceed an image from a larger exit pupil if not from an equally high quality binocular.

My 16x70s exceed my 15x70s and my 10x70s.
My 12x50s exceed my 10x50s.

But in either case not in field of view. However, fov can lose some of its relative importance if the image is considerably better.

edz

More Info can be found at:
Original Discussion Embedded in Thread "Nikon 10 X 50 Cf Action Vi Binoculars"

EXIT PUPILS
Q
Are large aperture, low mag binoculars pointless?

It seems that large aperture, low mag. binoculars are wasteful. What is the reason for their creation if the extra aperture serves virtually no purpose?
(Would I be better off using my brother's 15x45 binos... or at least be just as well served by a pair of 11x50's because the other 20mm of aperture are wasted?)... Please explain this to me...

A
Binoculars that provide very large exit pupils are designed with maximum brightness in mind. That is, they will deliver the maximum bright image of faint diffuse objects. This is a benefit when viewing faint diffuse nebula or large comet tails or the extended arms of galaxies like M31, M33 and M101, things of that nature.

For almost all objects other than faint diffuse objects, magnification provides greater benefit. Magnification allows you to see deeper in magnitude, putting more stars in the field. This is a benefit in open clusters and star fields like the M24 star cloud or even scanning the Milky Way.

Magnification also provides larger image scale that allows you to see objects that otherwise would have been too small for the eyes to perceive.

Remember, a larger objective gathers more light, allowing you to see faint extended objects. A larger exit pupil delivers a brighter image to the eye, again helpful for faint extended objects. Magnification is what allows you to see most small detail.

Whether large aperture low power binoculars are the right choice in binoculars depends very much on what you intend to use them for.

edz

More Info can be found at:
Embeded in the Original Thread "Are large aperture, low mag binoculars pointless"

A good point is made here;
There would appear to be a popular misconception , even amongst experienced binocular users , that an exit pupil from a binocular that is greater than the dilation of the human eye -pupil , is completely "wasted".

There are two very good reasons why a binocular benefits from having exit -pupils greater than diameter of the eye pupil.

1.Avoids the necessity for setting the IPD perfectly, and then the need to perfectly center your eyes to the exit pupils.

2. Permits your eyes to swivel in their sockets, without losing the image, as the iris of your eye slides off to one side as you look toward the edge of the field of the binoculars.

However
For a 10x70 binocular with a 7mm exit pupil, if your maximum dilated eye pupil is 6mm, then light gathering thru the binocular will be determined as if deleivered by a 10x6mm eye pupil = 60mm effective aperture.... Resolution will be determined as if delivered by 70mm x (6mmeye pupil/7mm exit pupil) = 60mm lens, same as light gathering. In general, if your eye pupil does not dilate as wide as the exit pupil of the binocular, your eye effectively reduces the aperture of the binocular.

More info can be found at
Original discussion in thread "Relation: aperture-mag when view extended objects"

Edited by EdZ (06/17/07 07:16 AM)

Aperture / Magnification relationship in binocs

Most recently, this discussion took place touching on all aspects of the above mentioned topic. This thread may be best read in it's entirety.

Comment about Exit Pupil Size

Numerous other threads have discussion related to the same topics.

Are large aperture, low mag binoculars pointless?

See the following comments from some of our forum participants:

It seems that large aperture, low mag. binoculars are wasteful. What is the reason for their creation if the extra aperture serves virtually no purpose?
(Would I be better off using my brother's 15x45 binos... or at least be just as well served by a pair of 11x50's because the other 20mm of aperture are wasted?)... Please explain this to me...

I’ve have come to the conclusion that with binoculars, like everything else in life, it all boils down to compromise. From my …experience I’d have to say that large aperture/low mag binocs are most definitely NOT pointless, they just fill another niche. ... it depends on what your going to use them for.

whilst neither low mag nor large aperture binoculars are remotely "pointless" both categories DEFINITELY have their place in the pleasures of astronomy, unless you are viewing from an exceptionally desirable, non light polluted site, then the combination of LARGE (70mm +) AND LOW power (<12x) would seem to make little sense to me.

when you increase the magnification the image gets a little dimmer so if you are looking at some of the larger nebulae in dark skies the 11x is probably more appropriate.

See the complete comments at this location
Are large aperture, low mag binoculars pointless?!

Masking a Binocular Aperture
Does aperture rule in bino land?
masking is one way to see the effects of aperture vs magnification. This post has a discussion related to some of those effects.
Also refer to the article
How to Understand Binocular Performance

Binoculars that provide very large exit pupils (large aperture-low mag) are designed with maximum brightness in mind. That is, they will deliver the maximum bright image of faint diffuse objects. This is a benefit when viewing faint diffuse nebula or large comet tails or the extended arms of galaxies like M31, M33 and M101, things of that nature.

For almost all objects other than faint diffuse objects, magnification provides greater benefit. It allows you to see deeper in magnitude, putting more stars in the field. This is a benefit in open clusters and star fields like the M24 star cloud or even scanning the Milky Way.

Magnification also provides larger image scale that allows you to see objects that otherwise would have been to small for the eyes to perceive.

Most people will argue that the size of the objective lens is what determines the resolution, and yes this is true. But how much of that resolution the eye can see is dependant on how large the resolved image is magnified. Just about every binocular objective size provides more than enough resolution for the eyes. Without sufficient magnification, the eye will not see what has been resolved.

Take an example of an observer with acuity that allows seeing objects as separated when they are magnified to an apparent size of 150 arcseconds. When viewing an object like M36 using 15x binocs, stars that are as close to each other as 10 arcsec will be seen as separate objects. The same observer using 11x binocs will not be able to see stars only 10 arcsec apart as separate objects, they will appear as one. Hence, 11x will not resolve as many stars in M36.

Remember, a larger objective gathers more light, allowing you to see faint extended objects. A larger exit pupil delivers a brighter image to the eye, again helpful for faint extended objects. Magnification is what allows you to see most small detail.

Also, increases in magnification show more stars. If you used 10x, 12x and 15x binoculars on the same star field, you would see 25-50% more stars with the 12x and another 25-50% more again with the 15x. If you tried to get the same star magnitude gains by increasing aperture and keeping power constant, the size increases over 50mm would quickly make the aperture diameter prohibitive.

Whether large aperture low power binoculars are the right choice in binoculars depends very much on what you intend to use them for.

Relation: aperture-mag when view extended objects

In my 15x70mm Binoculars I know that I can see more stars than a smaller pair, but what about things other than stars?

Suppose I'm looking at M42 with my 15x70's. I assume that magnification doesn't matter too much as it's already pretty big, aperture big enough to match the magnification is more important. I will rate the 'brightness' of M42 through this pair of binoculars as 1.

Now suppose I use a pair of 7x50's. I had thought that these couldn't compare to the 15x70mm's, but now I'm not so sure. If I was to look at M42, it would look smaller, but also stand out more, right? I'm only getting half as much light as I was with the 70mm lenses, but I'm also only magnifying it to have an area about 1/4 the area of the 15x70's. So I would give these a brightness rating of 2. Which is twice as good as the 15x70's.

I know that the bigger image scale does make up for the dimmer view somewhat, but how much does this matter?

There's a lot of factors that determine whether an object will stand out more in a lower power larger aperture binocular.

The larger aperture has greater light gathering ability.

Image scale has a lot to do with whether you can see detail in an object or not.

Extended objects, especially faint diffuse objects benefit from greater aperture.

In binoculars, even globular clusters and dense open clusters can be considered extended objects. These will benefit much more from larger image scale provided by magnification.

I've seen the North America nebula with my 15x70s. I've never seen it with my 10x50s. Assume viewing a 1 arcminute square. If you consider the amount of light gathered spread over the area of the magnified image, then the 15x70s vs 10x50s gather 1.96x the light (70x70=4900 vs 50x50=2500, so 4900/2500=1.96) and they spread it over an area 2.25x as large (15x15=225 vs 10x10=100, so 225sqarcmin/100sqarcmin=2.25x). If 1.96x the light is spread over 2.25x the area, you would think the 15x70s would make it more difficult to see, but that is not the case. Larger image scale can sometimes make it easier to see faint light.

edz

Read the full response at this post
Relation: aperture-mag when view extended objects?

EXIT PUPILS

I wanted to test the difference between two perfectly matched binoculars, but with one variable, exit pupil. I tested 16x70 Fujinons and it's twin, the Fujinon 10x70. Not identical twins, but sure from the same family. These two binoculars share exactly the same everything except the eyepieces. So, only power, and hence exit pupil, are different (and fov, but not important here).

Think of the sky as the backdrop in a portrait. Your subject must stand out from the backdrop to be seen. Consider it to be a grey scale. You all understand the affect of dark sky background. Higher magnification in a telescope reduces the extended light of the background until it approaches a completely dark sky, to the point you can no longer see the field stop. Lower magnification with a larger exit pupil delivers more light, not only from your subject, but also from the background. This can be beneficial or detrimental, depends on the condition. Generally, this makes it much more difficult to see the subject, unless you are viewing under very dark skies.

In moderate 4.5-5.5 skies, 10x70s show a background about three or 4 shades brighter on the grey scale than what is delivered by 16x70s. With a 7mm exit pupil, there is so much light coming into the eyes from the extended sky background, it makes it more difficult to see anything and everything. The 16x70s provide such a contrasty view against the darker backdrop, the contrast gain from the higher magnification of the 16x70s was far reaching and unmistakable. These are exactly the same binocular, so the only improvement in grey scale background most definitely comes only from the increase in magnification, and hence a smaller exit pupil.

But on one night, I did get both binocs out under mag6+ skies. Since the sky was already so dark, the backdrop in the 10x70s did not appear so much brighter than the 16x70s. Much more could be seen with the 10x70s in a darker sky than in the bright sky. Faint nebula stood out now that they were being used in their productive element. On this night the difference between the two was not nearly as great. The 16x70s still see more, but on the faintest nebula, the 10x70s did very well.

My point is not to say one is better than the other. For me, in most conditions, the 16x70s will perform better. But those 10x70s, under the darkest skies I can get to, would be just the tool to observe extended objects and deliver that "finishing" light to my eyes. They have their place. But used in the wrong circumstances, they are no match for the right tool.

Every tool has it's purpose.

edz

Exit Pupil, Large or Small

sure...more magnification means more diffusion of the extended object's light, but I'm asking about the explicit relationship between exit pupil and brightness, not magnification and brightness per se. yes, these are related, but they're not the same thing (keep in mind that two different scopes/binos can give you a different exit pupil for the same magnification...or the same exit pupil with different magnifications).

If we were looking at a white diffuse surface with equal instruments except for exit pupil diameters, the one with the larger exit pupils would have a brighter image. One of the big issues with large exit pupils is that under light-polluted skies they transmit the light pollution as well as the celestial objects. The claim is that smaller exit pupils yield more contrast by reducing the impact of the light pollution

In order to determine the brightness of a view through a binocular you not only need to know the exit pupil size, but also the AFoV. A binocular with a larger exit pupil doesn't *always* give more light to the observer.

Well, not according to the official definitions I've read.

AFOV has nothing to do with image brightness. You are confusing the image formed in your eye with the exit pupil. The exit pupil is simply the image of the effective aperture of the system.

How much light a system transmits is related to its effective focal ratio. If you change the magnification of a system, you also change the effective focal ratio. This has nothing to do with spreading light - sky glow has the same luminance value whether you measure a tiny area or a whole slab.

Lets see how this works. We will use telescopes because we can change the eyepice. The OTA has a printed focal ratio. The brightness of the aerial image formed is dictated by the focal ratio of the scope. An eyepiece will be used to make an image we can see - in astrophotography this is known as afocal projection but using a camera rather than an eye.

The effective focal ratio of the system = OTA focal ratio X eyepice projection magnification.

The eyepiece projection magnification = the eye's focal length / eyepiece focal length.

As you can see, the magnification increases as the eyepiece focal length deceases. And with the first equation, the systems focal ratio increases with the magnification. Using shorter focal length eyepieces increases the effective focal ratio of the system and makes darker images. As the focal ratio increases, the exit pupil deceases.

Light output is not controlled by field of view. If I use an eyepiece in my scope that produces a 3mm exit pupil, it doesn't matter if that eyepiece has a 50° Afov or a 68° Afov, the light output is exactly the same. I just get the same light output over a wider area.

So in effect the view is being dimmed by having a wider AFoV? If you were to somehow block off the edge in the 68 degree AFoV so that all I see is 50 degrees I would assume light output is decreased so that it is less than the orginal 50 degree view?

Again, Light output is not controlled by Field of View.

What is "image brightness"? Can anyone give a clear definition?

several links to sites

One definition of "Image Brightness"

The apparent luminance of the image as seen through an optical system. This brightness is determined by the brightness of the object, the transmittance and the diameter of the exit pupil of the system.

Take a specific magnification, say
10. Then in relation to that, the larger objectives (gathering
more light) yield larger exit pupils:

10x30 = 3
10x50 = 5
10x70 = 7

So the brightness is directly expressed by the exit pupil: the
ratio of light-gathering to magnification.

In terms of the brightness, the largest exit pupils can only be
fully used by dark-adapted younger folks on moonless nights
under dark skies, because the sky background is very dark in
that situation. But under moonlight and light pollution, then
having a lower ratio of gathered light in relation to
magnification yields more contrast, which makes fainter stars
easier to see, since the background is darker.

Brightness in Exit Pupil
Why some exit pupils appear brighter than others of equal size.

Ray Diagrams
All the rays from the cone passing thru the objective to the focal point will pass thru the eyepiece. No rays will be lost. The cone of rays from the objective focal point to the eyepiece exit pupil has a different angle cone for every eyepiece. The shape of the cone of rays gets narrower or wider depending on the eyepiece focal length. That's what focal length is all about. Focal length of a lens creates a cone of light with an angle from the apex at the focal length to the base at the lens. A ray diagram showing a cone of light from the focal length of the objective, once the rays pass the focal point, will show rays at the angle of the eyepiece focal length. Of course F obj / F ep = Mag.

Focal length of a lens