Telescope Reviews: Exit Pupils 5mm vs 7mm

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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)

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