Importance of size of exit pupil (1 Viewer)

Dear all!

One thing that surprises me with this excellent forum is the little amount of discussions about the need of a large exit pupil. This is always pointed out in theory and advertisments.

I am to buy a pair of good binos and have looked through binoculars mainly in shops and on photo exhibitions, that is, in good light and, of course, at a steady ground and with no wind. I have looked mainly at 8x30 and 8x42 binoculars from Zeiss, Swarovski and also Leica. Everything looks good in those conditions due to the forgiving conditions where the pupils of the eye are small.

What are your thoughts on this matter when using binoculars in practice, especially at dawn and dusk? I exclude usage in a boot at sea and in strong winds where it is difficult to hold the binoculars steady. I am mainly talking about "good" viewing conditions when you can place and keep the eye in a good position relative to the eye pieces of the binoculars.

To clarify what I mean, I have read some posts about the Nikon SE-s (with 4 mm exit pupil) where several people concluded that they in low light can see as much with a pair of SE-s as with a pair of top notch 8x40/8x42/8.5x42 binoculars (with around 25 % larger exit pupil which equals about 50 % more light transmission). Most comments are about differencies in physical appearance between the porros and the roof binos, but very few seem to care about the differencies in exit pupil.

According to several sources, the pupil in the human eye dilates to a maximum of about 6 mm in diameter in low light for "younger" persons (perhaps under 30), while for people over 50, the dilation tends to stop at around 4 mm.

I have come to think that people over 50 may not care about if they look through a pair of 8x32-s or a pair of 8x42-s in terms of exit pupil. Perhaps those who are now selling their 7x42-s to buy e.g. 8x42-s or 8x32-s instead have become older and no longer really need more than 4-5 mm in exit pupil?

One conclusion is then that the Nikon SE binos are better suited for those over about 40 and to a lesser degree for me (who is 34) if the intention is to find binos for low light usage.

Any thought on this? Is there anyone with deeper insights in human vision who might share their expertise?

An excellent question, gunvald. I am planning to buy a pair of 8x32 SEs. I'm 33. I wonder if I'll have problems with its exit pupil diameter in low light situations.

I sincerely hope a knowledgable somebody will shed some light on this!

gunvald said:

Dear all!

One thing that surprises me with this excellent forum is the little amount of discussions about the need of a large exit pupil. This is always pointed out in theory and advertisments.

I am to buy a pair of good binos and have looked through binoculars mainly in shops and on photo exhibitions, that is, in good light and, of course, at a steady ground and with no wind. I have looked mainly at 8x30 and 8x42 binoculars from Zeiss, Swarovski and also Leica. Everything looks good in those conditions due to the forgiving conditions where the pupils of the eye are small.

What are your thoughts on this matter when using binoculars in practice, especially at dawn and dusk? I exclude usage in a boot at sea and in strong winds where it is difficult to hold the binoculars steady. I am mainly talking about "good" viewing conditions when you can place and keep the eye in a good position relative to the eye pieces of the binoculars.

To clarify what I mean, I have read some posts about the Nikon SE-s (with 4 mm exit pupil) where several people concluded that they in low light can see as much with a pair of SE-s as with a pair of top notch 8x40/8x42/8.5x42 binoculars (with around 25 % larger exit pupil which equals about 50 % more light transmission). Most comments are about differencies in physical appearance between the porros and the roof binos, but very few seem to care about the differencies in exit pupil.

According to several sources, the pupil in the human eye dilates to a maximum of about 6 mm in diameter in low light for "younger" persons (perhaps under 30), while for people over 50, the dilation tends to stop at around 4 mm.

I have come to think that people over 50 may not care about if they look through a pair of 8x32-s or a pair of 8x42-s in terms of exit pupil. Perhaps those who are now selling their 7x42-s to buy e.g. 8x42-s or 8x32-s instead have become older and no longer really need more than 4-5 mm in exit pupil?

One conclusion is then that the Nikon SE binos are better suited for those over about 40 and to a lesser degree for me (who is 34) if the intention is to find binos for low light usage.

Any thought on this? Is there anyone with deeper insights in human vision who might share their expertise?

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Maybe others are too busy birding to discuss this, but you make interesting points. There were some discussions about this on the Astromart forum. Apparently it is not a foregone conclusion that old people have small dilated pupils, and 7mm or more is not uncommon. Similarly young people can have small dilated pupils. I will try and find a link to the graph that I once saw.

I compared the Nikon 8x32 SE with the Swaro 8.5x42 EL and cannot distinguish between them in terms of brightness at dusk and on the night sky. However, I am 40 years of age, and I have not had my pupils measured, so it is possible that I have pupils that do not dilate more than 4mm.

I do find that I prefer to use the Swaros in low light, not because they show more - they don't - but because the larger exit pupil makes it easier to align the binoculars with my eyes. I suspect that a 7x42 binocular would be even better though these seem to be not so popular these days.

I suspect if you ask Scampo what size his pupils are he will say "between 5 foot and 6 foot 5 inches". That's Slough humour. (Scampo is a teacher.)

Dilated pupil size with age

There's a nice graph on page 7 of the following PDF:

http://www.btow.com.au/pdf/agwa/agwa-September-2003.pdf

It confirms that as you say the tendency is for the dilated pupil size to shrink with age, but there is also a large spread.

If you use your binocular at dawn and dusk, I would say that you require a minimum of 4mm exit pupil diametre from your binoculars.

7mm is the maximum size that the human pupil will achieve and this is found (as a general rule) in younger people. So if you are for instance in your mid 50s (I just picked an age, so lets not make an issue) and that person has an eye pupil of 5.6mm, you would not receive the full benefit if you owned say an 8x56 binocular (exit pupil of 7mm).

You would receive the full benefit if you owned a 10x56 (exit pupil of 5.6mm). If the person had a pupil of + 6mm then the 8x56 might be a better option.

As a guide, should you wish to purshase binoculars for use at dawn and dusk then look at these two parameters.
1: Geometric light gathering of no less than 16 (exit pupil of 4mm x 4mm = 16) and
2 An objective lens of no less than 40mm.

This is only a guide as it does not say anything about the manufacture, prism system, coatings or glass types.

It all sounds very interesting. It is in woodland at dusk that this is most likely to be of any importance - and I cannot say that I have noticed much difference between high quality bins in those conditions. The Nikon 8x42, Swaro 8.5x42 and Optolyth 10x40s all seem to my now 50-year-old eyes to give a similarly bright view - even thought the Optolyths shouldn't. Odds bodikins!

I doubt that the pupils in the eyes reach their maximum size until it is far too dark to watch for birds, though.

An interesting thing, Leif - the Nikon scope and the Nikon HGs are surprisingly similar in the qualities of their images - as are the Swaro bins and my son's Swaro 65 scope. The manufacturers must work hard on this, I should think.

scampo said:

I doubt that the eyes pupils reach their maxima until it is too dark to watch for birds, though.

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Most probably.

It's a very important point, but I think the reason you may have heard little about it, is, as this thread suggests, that the variables have become incalculable. I say "become" because exit pupils were relatively easy to factor in through the 1970s, at least.

The math and physics haven't changed, but the technology has.

By the time you purchase any very good optics now--and I am speaking here of only the best--you realize there's a point you step across where you simply believe that this manufacturer does indeed produce the "sharpest" optics--and that is a combination of trust and subjective impression.

As far as I know, the only way to test for your own personal benefit from the light-gathering virtues of various binoculars or telescopes is to actually test them in end-of-day lighting conditions.

That is not easy, but neither is it impossible, as a small birding group of fairly elite birders might well be equipped with a pretty impressive group of fairly elite optics. I was able to borrow a companion's binoculars for half an hour in the field before I bought the same model.

Might I suggest an outing with others interested, just to compare equipment? I would think there are as many people willing to show off their optics as there are people interested in purchasing very good optics. Naturally, some people prefer never to lend; and that must be respected.

But also, if you do happen to drop a pair of borrowed binoculars worth as much as your new car, this might be an excellent moment to remember the debt your best friend never repaid you, and give his name as yours while promising to pay for all damages.

If you stick with theory, though, I'm afraid you might cover several sheets of paper with figures, and you WILL become very knowledgeable, but, really, your conclusions will be as blurry as a pair of bad binoculars.

Meet up? Leif suggested the same - gosh Leif and you could meet half way - say, Iceland. Can I come, too? Could be a good day out!!

(-:

scampo said:

An interesting thing, Leif - the Nikon scope and the Nikon HGs are surprisingly similar in the qualities of their images - as are the Swaro bins and my son's Swaro 65 scope. The manufacturers must work hard on this, I should think.

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I know what you mean. When I tried the Nikon scope I was struck by the similarity of the image in terms of its 'feel' to that produced by the 8x42 HG. I noticed a similar thing for Swaro and Leica. It could simpy be the use of the same coating technologies and glass types, or maybe they each have their own idea of what constitutes an ideal image?

scampo said:

gosh Leif and you could meet half way - say, Iceland. Can I come, too?

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I think I'll stick to the local RSPB reserve, and InFocus field days if that's alright with you!

Carson said:

If you stick with theory, though, I'm afraid you might cover several sheets of paper with figures, and you WILL become very knowledgeable, but, really, your conclusions will be as blurry as a pair of bad binoculars.

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Very true. I don't think it's hard for most people in the UK to find a suitable place to test and compare a couple of pair of bins or scopes. Many shops - for example LCE, Kay Optical, InFocus, Cley Spy, RSPB spring to mind - provide field days and/or excellent viewing facilities. I don't think it's too hard to buy in Winter, and try the items at both mid-day and sunset, though the salesmen at some shops might be a little puzzled. They will be smiling if and when you walk out the door with a new toy.

Thanks for your thoughts and thanks to Leif for the interesting pdf file you supplied.

Carson said:

As far as I know, the only way to test for your own personal benefit from the light-gathering virtues of various binoculars or telescopes is to actually test them in end-of-day lighting conditions.

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I prefer to make my decisions in a two-step manner, with one somewhat theoretical first step (this puts some people off) where information is collected and my candidates are selected. Then a practical hands-on assessment comes next, where I know better what to look for, this is also when the subjective "feel" factor comes in. In my point of view this strategy takes patience and time, but I find it to be a good way to a good decision, since first or even second impressions might lead you wrong. At least this is the case in selection of, say, a medium format camera system if you are used to 35 mm cameras.

Anyway, now time has come to act. A pair of used Zeiss 7x42-s are on their way. I am also going to order a pair of Nikon SE 8x32-s. I have the right to return within a week and should be able to do some good comparative tests. Yes, these binos are indeed different, and I migh end up keeping both, one pair rather light-weight for my camera bag and one for good viewing in low light. At first, I planned to buy a pair of 8x42-s and even had a pair of Nikon HG-s on order, but then started to think in new directions.

I have read about eye dilation decreasing with age, usefulness of large exit pupil consequently being wasted on older eyes, etc., for years. As Leif says, there has been much discussion of this on the Astromart forums, specifically on the forum, "Birding Optics, No Pictures." That forum should actually be called, "Birding Optics, No Birding," because it is predominantly written by people who stargaze and tends to be more theoretical than practical. After months of finding no useful information on it I quit reading it.

In situations where one is using binoculars in open sunlight, notably birding, how could one expect their eyes to be fully dilated? I have to believe that focusing a light source on the eye causes the pupil to constrict. Stephen Ingraham has mentioned this indirectly in several reviews--that a 4mm exit pupil will provide a sort of optimum daytime light level. I don't know if he has scientific reasons to propose this; I suspect he learned it from experience. I find this to be true using various binoculars. An 8x20 definitely provides a dimmer image in open sunlight than an 8x32, but a 7x42 does not provide a brighter image than an 8x32. My eyes dilate to at least 6mm in dim light, so they are capable of using the larger exit pupil.

It seems to me that the usable exit pupil concept derives from astronomical use of optics, and not from daytime use. I welcome any clarification.

Yes, Jonathan, a local astronomer told me he is quite sure it takes the human pupil a good half hour to open fully. He suggested it opens gradually, but closes, or begins to close, quickly in reaction to a sudden blast of light ("light pollution"--such as headlights--to astronomers).

Similarly, traditional slide shows, clicking from one slide to the next, cause the audience's pupils to do pushups (!) as they continually begin to open and close in response to the dark-light-dark-light changing of the slides. Your audience's sleepiness is not just because you've shown too many pictures of Aunt Bertha's children--it's also because of all this wearisome eye exercise.

-- If you would like to improve your slide show, you need two projectors and a dissolve unit. The slides alternate, projector A-B-A-B, via the dissolve. There is no half-second of blackness between pictures. Dissolve times can be set by the projectionist, but the point is that each slide dissolves into the next without undue change of illumination on the screen. This is a MUCH more pleasing slide-show, and your audience stays much more awake. --Um, unless you insist on still showing too much of Aunt Bertha and the kids.

A cheap pair of 7x35 binoculars, despite their 1:5 powerbjective lens ratio, will admit way less light than very fine optics in, say, an expensive pair of 10x42 binoculars. You could actually drop to a 1:4 ratio or worse, and, if the glass were very high quality, you would still have more light. This confounds the numerical comparison.

People reading this thread, who have both cheap and high quality binoculars, may find this interesting: hold your binoculars in the usual position, but about 2 feet out from your eyes, in good light. Your expensive binoculars will show, within the eyepieces, exit pupils that are perfect, brilliant, uninterrupted circles.

-- Now do the same thing with your cheap binoculars. The inside edges of your exit pupils will be interrupted by a very pale gray straight-line "cut" representing a triangle. Look carefully, and you will see it. This is the relatively small prism disallowing the exit pupil its true, theoretical size.

Because the prisms are smaller, the binoculars are cheaper, and they also weigh less. Larger prisms make good binoculars somewhat heavier.

We have been discussing high-priced optics in this thread. If, however, you are quietly reading this and lamenting your modest budget, take heart: an older pair of 8x40 binoculars will still let you become an expert birder, and will give you beautiful views. But DO test out the focus wheel (knob) carefully: in cold weather, if the focus becomes very stiff, your binoculars will work against you. You may also find a good buy second-hand, but you must look for binoculars that have either been used very seldom or else have been pampered. If they are dirty or their exterior indicates rough use, they may have been knocked or dropped. Once the alignment of the lenses inside has been jolted out of true, these binoculars will never be the same again--and, after repairs, they will no longer be a cheap pair of binoculars. If you are buying on a low budget, take your time. I have seen fabulous buys made by people who haunted thrift sales and such for perhaps a year or so.

Apologies for my too-lengthy postings. Someone else may wish to explain the Interesting Case of the More Distant Duck, in which a wonderful principle of physics makes the far end of a picnic bench, viewed through binoculars, flare out wider--or the same-sized bird BEHIND seem to be larger than the one in front! (But I'm bowing out. Thank you.)

I've found optical quality to be far more important than exit pupil size, although as mentioned, I certainly would not go below a 4mm exit pupil.

I bird a lot after work, and so prefer binoculars that can be used at dusk up to the point where I go home because I can no longer see clearly with the naked eye. Cheap 8x40 binoculars are just not up to the job and give up the ghost long before my eyes. Most 8x32 binoculars aren't up to the job either. The Nikon 8x32 SE and 8x32 HG are the exceptions. My preference for low light use is a high quality 8x42 binocular with its 5mm exit pupil that makes for comfortable viewing. However, as with all things, there's a lot of subjective judgements here, and some people will come to different conclusions, prefering perhaps the Leica 8x32 BN or the Zeiss 8x56 Victory!

Low light is when you realise that there is a reason why you paid a small fortune for a pair of binoculars.

The factor which I find most important in exit pupil size is the degree of shake/wind vibration etc you can take without reducing the quality of the image...

I have Leica 8 x 20's and 8x42's In good light and with the bins resting on a surface you'd be hard pushed to tell them apart..... but in a NW10, looking for petrels out of a wind buffeted car its blindingly obvious!

Jane Turner said:

The factor which I find most important in exit pupil size is the degree of shake/wind vibration etc you can take without reducing the quality of the image...

I have Leica 8 x 20's and 8x42's In good light and with the bins resting on a surface you'd be hard pushed to tell them apart..... but in a NW10, looking for petrels out of a wind buffeted car its blindingly obvious!

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I agree. And the reason why this is important is because a larger exit pupil allows for some movement between yourself and your optical device (binoculars or scope) without losing sight of that magnified image. For this reason, a large exit pupil even helps when your own eyes' pupils can't open up that much any more.

But with a dilated pupil, the eye's ability to see detail falls dramatically. The retina gives of its best only when the pupil is small.

Scampo, I didn't know that, and I find it fascinating. The smaller the aperture (ie. f-stop) in a camera lens, the greater the depth of field. In wildflower photography, a flash is often used in daylight to allow for f16 or f22, and hence a very detailed picture for technical botany use. If the same picture were taken at f8 (at which the lens is much more "open"), much less of the flower would be in critical focus.

Scampo, are you saying, then, that our eyes work along exactly this same principle? Is it simply a dictate of physics?

I think it fair to view the eyes as simply a lens with a diaphragm (i.e. the iris), so all the rules apply.

scampo said:

I think it fair to view the eyes as simply a lens with a diaphragm (i.e. the iris), so all the rules apply.

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I heard that when the iris dilates irregularities in the shape of the cornea become more apparent. I know from first hand experience that the cornea is remarkably sensitive. A year ago I got some small splinters in one cornea, that caused it to swell, producing marked astigmatism.

I seem to recall that when the eye dark adapts, it produces a chemical that aids light detection. Exposure to bright light destroys the chemical and hence dark adaptation is lost. I guess this level of low light sensitivity is not that relevant to birding, though last night well after sunset I was surprised to hear a swan fly past. Goodness knows how it sees pylons and HT cables.