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# Brightness in Exit Pupil

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### #1 EdZ

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Posted 29 January 2006 - 06:07 PM

This excerpt is from my most recent binocular review titled Binocular Tests and Comparisons. It should help answer Kenny's question why some exit pupils appear brighter than others. It may not be the only answer, since there will be spin-off discussion about field of view. But this answers the basic question, even if Apparent field of view is constant.

Brightness of the image
A binocular with a larger exit pupil will have a brighter image. Well, most of the time. Often we read of a user comparing the apparent brightness of the image in the exit pupil from one model binocular to another. Assuming equal size aperture, sometimes it seems difficult to understand how binoculars with equal size exit pupil may differ in brightness or why it might be that a smaller exit pupil binocular may appear to have a brighter image. Standard calculation tells us the larger exit pupil provides a brighter image. But that is the simple calculation, and all is not always what it may simply appear to be.

Assume for example two different 100mm binoculars one with a 4.5mm exit pupil and the other with a 4mm exit pupil. Everything tells us the binocular with the 4.5mm exit pupil should give a brighter image. Why then might it be possible for the 4mm exit pupil to give a brighter image than the binocular with a 4.5mm exit pupil.

In addition to the potential light lost to the exit pupil based on the quality of the coatings and the number of uncoated or single coated surfaces, the following may also contribute to light lost from the exit pupil. Studies have shown that not all binoculars deliver all the light expected based on the simple calculations of aperture area and exit pupil. The brightness of the exit pupil is considerably affected by vignette in the binocular system, especially in the prisms. Even for some of the best binoculars, vignette is present. The amount present can cause considerable differences between models of the same sizes and between varying sizes of objectives producing the same size exit pupil. Total brightness cannot simply be assumed based on expected transmission figures or light gathering area and exit pupil.

If all the light that enters the front prism hole does not exit the back prism hole, it is bouncing around inside the prisms. This does several things. It causes what is known as vignette and it reduces contrast in the image. It will also reduce the brightness of the image in the exit pupil. So it is possible to have two equal sized binoculars with equal size exit pupil and yet one binocular can have a brighter image than the other. Although present even in closely sized and quality binoculars, it may not be as apparent as it sometimes is between high quality and cheap binoculars. But it is a factor.

end

I think we perceive brightness in concert with contrast. Two binocular that have the same lens area, same magnification and therefore same size exit pupil, even so far as to say the same quality coatings, may not appear equally as bright if one looses more contrast than the other. There are many ways that can happen internally and any loss of contrast will cause one image to appear somewhat less desirable than the other.

edz

### #2 EdZ

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Posted 29 January 2006 - 08:30 PM

This link discusses brightness of the exit pupil as affected by field of view

Thread in the Lunar Forum - High power--wide field vs. small FOV

very important to note here this is noted as "pertains to Lunar veiwing only".

edz

### #3 DJB

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Posted 30 January 2006 - 04:22 AM

Hi Ed,

Well put and concise--thanks.

However, you baffle me on one point (only one this time). I understand about prism size and internal scattering of light "rays." But, I do not quite see the relationship between scattering and the vignette (-ing).

A genuine vignette, it seems to me, should descibe the gradual falloff of light at the EDGES of the FOV, possibly due to too-small prisms or baffles, for example. I can't particularly correlate this to light scatter. Perhaps I am not understanding your text, but a sentence or two of clarification I will appreciate. Thanks.

Regards,

Dave.

### #4 Guest_**DONOTDELETE**_*

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Posted 30 January 2006 - 09:37 AM

It is good to read this. We often make simple arguements to explain why one sees what they see, however...as you have pointed out, things are often not so simple. And that various other factors can, and often will, come into play which alter the outcome from what we expect.

### #5 jkevn

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Posted 15 June 2007 - 10:04 PM

What would cause the exit pupil brightness to be nonuniform (other than being clipped, or vignetted)? One cause, I know, is from BK7 prisms compared with BAK4 prisms. I have noticed in some binoculars the center of the exit pupil appears brighter than the circular region close to the perimeter. These were binoculars with BAK4 prisms (Nikon Action Extreme 10 X 50)

### #6 grbrown

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Posted 16 June 2007 - 05:09 AM

EdZ,

That is a brilliant piece - many thanks.

In the table in my PANORAMIC BINOCULARS, Part 2! topic I have included, where available, the transmitted percentage of light. Steiner claim a remarkable 96% for their XP models, Fujinon 95% for their FMT-SX. It seems these makers have cracked vignetting!

Presumably it is possible to measure light in/light out, which would make an interesting lunch-time laboratory excercise for one of our optical experts?!

### #7 Sgt

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Posted 16 June 2007 - 06:48 AM

May be this is why i see a brightness difference between my 10x56 Victory FL and my 10x50 Leica in gloomy daylight (but not full sunshine)?

### #8 EdZ

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Posted 16 June 2007 - 09:09 AM

Who found this post, Kevin?

What would cause the exit pupil brightness to be nonuniform

Vignette. it is present in almost all systems. The more vignette in the system, the more non-uniform will be the brightness. While central brightness may be equal out to 30 or 40 or even 50%, the outer area will always differ.

Remember the discussions of vignette with the laser test showing the cutoff of exit pupil. That shows the nonuniformity of the light distribution in the exit pupil.
Exit Pupil Profile of Oberwerk Ultra 15x70, Fujinon FMT-SX 16x70 and Garrett Gemini 20x80

Exit Pupil Profile of Nikon SE 12x50, Fujinon FMT-SX 10x70 and Oberwerk LER 15x70.

the exact same effect is discussed also in this article published several years ago.
Affect of Eye Pupil on Binocular Aperture
and this addresses the comment above

May be this is why i see a brightness difference between my 10x56 Victory FL and my 10x50 Leica in gloomy daylight

from the article:
Why is it we may not be able to see the difference in brightness?
Consider this. If you have been using binoculars with an exit pupil larger than your eye pupil, all along, your eye pupil has been creating an effective aperture. You cannot get a greater amount of light (a wider beam of light) into your eye than can be allowed thru your pupils. If youâ€™ve been using a 10x70 binocular with a 7mm exit pupil and your eyes dilate to only 5mm, you have never received a larger beam than 5mm into your eyes. Therefore, all along, the binocular has been performing as if it were a 10x50.

So you might ask then, in bright daylight viewing, if eye pupils dilate to only about 2.5mm, would it not be the same to use a 10x25 as opposed to a 10x50?

What differences can be seen in terrestrial viewing?
Let's assume a terrestrial viewer is using a 50mm binocular with a 5mm exit pupil (10x50). Now assume the observerâ€™s eyes in daylight can dilate to only a maximum of 2.5mm. The difference between the full aperture exit pupil of 5mm and the effective aperture exit pupil as a result of the 2.5mm eye pupil, for a 50mm binocular, observing an object at a distance of 100 feet is 5.5 arcseconds of angular resolution, a linear dimension on the object of interest of only 0.03 inches, approximately the width of a thin vein on a leaf. While this is a very real number, it is not likely something most people are able to notice.

What happens in low light terrestrial viewing where many seem to prefer a larger exit pupil because it seems brighter?

Now we need to go back to the cardinal rule that states exit pupil controls effective aperture. There would be no more light delivered by either binocular if eye pupil is 2.5mm, BUT, as seen through a 2.5mm eye pupil, there may seem to be a difference in the two binoculars, and you might be able to see this difference. The 10x25 binocular is using the entire objective lens diameter and we have seen from vignette studies, many (if not all) binocular systems block (vignette) portions of the light from the periphery of the objective lens. The 10x25 is using the entire diameter of the lens to light the exit pupil and the light from the outer portions of the objective is reduced in the exit pupil due to vignette. Not all of the light gathered by the 10x25 reaches the eye.

Most vignette studies show that approximately the central 50% of an objective lens provides 100% illumination of the exit pupil. The light delivered from the area outside the central 50% of the objective does not all reach the exit pupil. In the 10x50 binocular, stopped down by the daylight contracted eye pupil to 10x25, only the central 50% of the objective diameter is putting light into the eye pupil. The key is this 2.5mm exit pupil is 100% illuminated from the central portion of a stopped down 50mm lens. A stopped down 10x50 is providing a lot more illumination to the same size 2.5mm exit pupil than a full aperture 10x25. That would account for a significant difference in apparent brightness between the two, even though both situations have a 2.5mm exit pupil. As a bonus, that same 10x50 binocular will serve the user better under lower light conditions when eye pupil is enlarged to possible 3mm or 4mm.

### #9 1stScope

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Posted 16 June 2007 - 10:22 PM

'Color transmission' also impacts 'brightness', as it's essentially the transmitted spectrum. One would think that transmitting the full visual spectrum so that an image would be color neutral is desirable, but some makes transmitt a spectrum with a blue peak. I'll guess that makers transmitt as much light as possible and if it's not neutral they turn a design problem into a 'feature' and advertise it, like Zeiss does with their binoculars that have peaks in the blue part of the spectrum. It works as the subjective impression is that their binoculars get comments about being bright even when the measured light transmission is less than others.

### #10 Mark9473

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Posted 17 June 2007 - 03:49 AM

1stScope, what I've seen from measured light transmissions, is that Zeiss scores higher than the others...

### #11 Sgt

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Posted 17 June 2007 - 10:43 AM

Interesting, so the image restricted by the daylight eye pupil could have a larger proportion that is fully illuminated by the larger 10x56 than in the 10x50. As light levels increase and the pupil shrinks, the effective aperture of both reaches a level where they both produce an image that's fully illuminated. Sounds like the most plausible explaination for what I see. I wonder how the two different types of prisms in the leica and the zeiss vary in the way they illuminate the exit pupil.

### #12 1stScope

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Posted 17 June 2007 - 01:57 PM

I imagine the post below has been previously discussed. In any case after all of these years roof prism binoculars are starting to get close to what good porro prism models have enjoyed all along. This is in spite of what birders and others have prefered, and what I found before buying some FMT-SX binoculars a few years back.

A friend recently looked at some of the data developed to evaluate binoculars for US Gov't use. Allot of the data refers to immersibility, shock resistance etc. But of particular interest to me was the "light transmission efficiency" rating as a % of total wideband light projected through an aperture 2.5mm x 2.5mm. The ratings for the best binoculars were the Fujinon FMTR-SX and Nikon Prostar (I didn't have a Nikon model # so I don't know if this is the current ED version or previous model) at 97.4% and 97.5%. Then came many, many other binocular models like the Fujinon M-22 (96.2%). I guess the Sandia, Lawrence Livermore, Los Alamos, JPL mandate was to test just about every binocular they could find. But the highest rated roof prism model was the Swarovski SL 10x50 at 93.3%, with the Zeiss FL 10x56 T* right behind at 93.2%.

### #13 Mark9473

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Posted 17 June 2007 - 02:04 PM

### #14 EdZ

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Posted 18 June 2007 - 04:55 AM

I imagine the post below has been previously discussed.

No I would imagine that post to which you linked has not been discussed, although there are members of this board that took part in that discussion.

BTW, are you quoting your own posts from that thread? If not you need to credit her.

Also I'll point out that while numerous other factor were mentioned in that thread, it evolved it seems to be mostly about coating transmissions ["the number of lens surfaces and the quality and completeness of the multicoatings are the dominant factors."]. Worth mention, the individual you quoted, several times posts errors in miscalculations of total transmission based on the transmission of a single coated surface. It seems she neglects the compounding factor for number of surfaces and instead simply adds.

"On the other hand this seems to account for statements in places like CloudyNights.com where they claim that a multicoated 2 surface lens (cemented or not) will transmit 99+% of the light coming to it, so even with 10 surfaces there may only be 1% light loss total for the binocular system."
That statement is incorrect. there are others just like it.

What's also interesting is immediately following this mis-statement, she points to one of my articles here on CN; an article in which I know the information is not miscalculated. So not only is the coatings information miscaclutated but it is mis-referenced.

Anyway, back to my point, they are discussing coatings. This post is discussing vignette and what happens when you don't compare the entire exit pupil or what happens when you compare a portion of a larger exit pupil to a whole smaller exit pupil. I think the only person in that thread to recognize the significance of that concept may have been Henry Link where he said, "In tests using only the center 2.5mm of the binocular objective "real world" factors like optical aberrations, internal reflections and vignetting would play no significant roll in the measurements, but of course these things do effect contrast and apparent brightness for an actual observer."

So what's more important here is, while coatings and all other build aspects and aberrations are important to the brightness seen, that is not what is singled out for explaination in this thread. There are numerous other threads on each of those other topics that can be found by links through the Best Of thread. Vignette of light across the exit pupil is the basis of this explanation. Not only did no one mention that in the BF thread, but none of the manufactuter technical test data they referred to mentioned it either. Just a thought, but seems lot's of people may really be missing some pieces of the puzzle.

edz

### #15 1stScope

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Posted 18 June 2007 - 09:44 AM

My impressions are that 'brightness' is like the blind men describing the elephant, where vignetting (one needs a larger, heavier optic to enable larger stops, prisms, etc. to minimize vignetting), coatings (better coatings enable more light transmission in general), and color transmission (obviously related to light transmission but looking at the visible spectrum) needs to be considered. I thought that the color transmission is more interesting as some optics with lower light transmission are perceived to be brighter due to the subjective impressions of the color transmission.

One can imagine the exit pupil being measured like the field on a camera lens in a typical technical review, where light falloff towards the edge of the field is measured, and where the transmitted spectrum is measured in visual instruments. Optics like camera lenses will tend to be better corrected for color as errors won't be tolerated as they are in visual instruments.

### #16 EdZ

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Posted 18 June 2007 - 03:52 PM

My impressions are that 'brightness' is like the blind men describing the elephant, where vignetting (one needs a larger, heavier optic to enable larger stops, prisms, etc. to minimize vignetting), coatings (better coatings enable more light transmission in general), and color transmission (obviously related to light transmission but looking at the visible spectrum) needs to be considered. I thought that the color transmission is more interesting as some optics with lower light transmission are perceived to be brighter due to the subjective impressions of the color transmission.

I don't disagree. However all these aspects need not be discussed at the same time in one thread. You will find detailed discussions on every one of these topics by following down thru the Best Of links. Often times people try to discuss too many topics in one breath and other people can become very confused as to the causes or the weight of each issue. That is not to say all things should never be discussed together, but only to say that sometimes when a topic is broken out as stand alone and focused on it becomes much easier to understand it's implications.

edz

### #17 msc

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Posted 26 May 2008 - 04:59 PM

> A friend recently looked at some of the data developed to evaluate binoculars for US Gov't use. Allot of the data refers to immersibility, shock resistance etc.

1stScope, any idea where this data can be accessed? I would like to read it.....TYIA

### #18 KennyJ

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Posted 26 May 2008 - 05:12 PM

msc ,

Nice to see your first post of 2008 here !

Even more of a stranger here is the member 1stScope , who hasn't posted here for the best part of a whole year now .

Of course , he or she could be a regular lurker , and reply within minutes for all I know , but just an alert so as you are not too disappointed if your question remains unanswered !

Clear skies
Kenny

### #19 msc

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Posted 26 May 2008 - 05:18 PM

Thanks Kenny, I realize it was on old thread.... but figured I would take my chances.

### #20 ronharper

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Posted 27 May 2008 - 01:02 AM

Sgt,
Fearing dizzyness, I don't dare actually plot the path of light through roof-type prisms. But I have read that the Abbe-Konigs in your Zeiss totally internally reflect at every surface, while the Schmidt-Pechans in the Leica have one pesky surface that must be coated to reflect, and that the reflectivity of that single surface is a major driver of the light transmission of the entire system.

I don't know what Leica you have, but I have a Trinovid, in which that surface is coated with silver, about 93% reflective, bringing the system percent transmission down to the low 80s. The next generation, the Ultravid, went to a dielectric coating, for 99% reflectivity at that surface, and many have remarked that the 6% improvement is noticeable. Oh well, the old Tri is sure sharp at least.

Maybe this effect, coupled with a bit of vignetting, which is less an issue in daylight with the Zeiss's larger exit pupil, account for it's appearance of greater brightness than your Leica. There may a spectral difference too, as 1stScope suggests, adding to the mix, although I don't see why bluer should look brighter.

Still, no roof manages the 95% of even the old-coating FMT-SX. No telling what the new-coating FMT's, obviously less reflective, put out. Fujinon's advertising department apparently hasn't a clue, has gone slam out of business, or does not get along well with R&D. The new Fujinon coatings, however, make me gasp. Even the prism reflects only the deepest weird dim rosy colored light. In daylight, even I can see that the surface brightness shown by my Leica is exceeded by that of my new-coating 10x50 Fujinon, despite the 8x42's slightly larger exit pupil. Of course the Fujinon's big totally internally reflecting Porro prisms also help it. So, I choose the Fuji when I need bruising light-crunch power, like distant owls at dusk, or astronomy of course. Otherwise, it's brightness is not a big factor, and is overwhelmed in most cases by the otherwise excellent images, and slick convenience of the Leica.
Ron

PS, thanks Ed for starting this with a good reason for brightness differences, and to others for ideas on other contributors to this important optical quality.

### #21 GlennLeDrew

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Posted 19 June 2008 - 12:38 AM

I would guess that the transmission percentages supplied by manufacturers are obtained by measuring the central region of the exit pupil. In this way the losses due to the unavoidable vignetting are largely avoided. In other words, only those rays on-axis or nearly so are sampled. In one way I think this is valid, as it allows to assess the efficiency of coatings and glass types used. On the other hand, it can give *potentially* inflated indications of total transmission through the entire pupil. But then, in high-end specimens vignetting is probably not so terribly different among various manufacturers' wares as to render the *comparative* transmissions an unreliable indication of performance.

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