Join for FREE
It only takes a minute!
Zeiss - Always on the lookout for something special – Shop now

Welcome to BirdForum.
BirdForum is the net's largest birding community, dedicated to wild birds and birding, and is absolutely FREE! You are most welcome to register for an account, which allows you to take part in lively discussions in the forum, post your pictures in the gallery and more.

Importance of size of exit pupil

Reply
 
Thread Tools Rating: Thread Rating: 5 votes, 5.00 average.
Old Friday 5th March 2004, 18:16   #1
gunvald
Registered User

 
Join Date: Jan 2004
Location: Linkoping
Posts: 32
Importance of size of exit pupil

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?
gunvald is offline  
Reply With Quote
Old Friday 5th March 2004, 18:29   #2
oathkeeper
Registered User

 
Join Date: Feb 2004
Location: USA
Posts: 127
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!
__________________
Manendra Pedris

"If God had consulted me before embarking on Creation,
I would have suggested something simpler"
- Alfonso X, King of Castile and Leon (1221-1284) commenting on the Ptolemaic planetary system.
oathkeeper is offline  
Reply With Quote
Old Friday 5th March 2004, 18:40   #3
Leif
Registered Member

 
Join Date: Mar 2003
Posts: 2,959
Quote:
Originally Posted by gunvald
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?
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.)
Leif is offline  
Reply With Quote

BF Supporter 2004 2005 2006 Support BirdForum With A Donation

Old Friday 5th March 2004, 20:57   #4
Leif
Registered Member

 
Join Date: Mar 2003
Posts: 2,959
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.
Leif is offline  
Reply With Quote

BF Supporter 2004 2005 2006 Support BirdForum With A Donation

Old Friday 5th March 2004, 21:31   #5
mak
Registered Member

 
Join Date: Feb 2003
Location: UK
Posts: 347
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.

Last edited by mak : Friday 5th March 2004 at 22:00.
mak is offline  
Reply With Quote

BF Supporter 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Support BirdForum With A Donation

Old Friday 5th March 2004, 21:34   #6
scampo
Steve Campsall
 
scampo's Avatar

 
Join Date: Aug 2003
Location: Leicestershire, UK
Posts: 6,273
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.
__________________
Steve
"...when the cities lie at the monster’s feet there are left the mountains."
Robinson Jeffers, "Shine, Perishing Republic"

Last edited by scampo : Saturday 6th March 2004 at 07:54.
scampo is offline  
Reply With Quote
Old Friday 5th March 2004, 21:37   #7
mak
Registered Member

 
Join Date: Feb 2003
Location: UK
Posts: 347
Quote:
Originally Posted by scampo
I doubt that the eyes pupils reach their maxima until it is too dark to watch for birds, though.
Most probably.
mak is offline  
Reply With Quote

BF Supporter 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Support BirdForum With A Donation

Old Friday 5th March 2004, 22:05   #8
Carson
Registered User
 
Carson's Avatar

 
Join Date: Feb 2004
Location: Vancouver, Canada
Posts: 100
Wink

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.

Last edited by Carson : Friday 5th March 2004 at 22:23.
Carson is offline  
Reply With Quote
Old Saturday 6th March 2004, 07:56   #9
scampo
Steve Campsall
 
scampo's Avatar

 
Join Date: Aug 2003
Location: Leicestershire, UK
Posts: 6,273
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!!

(-:
__________________
Steve
"...when the cities lie at the monster’s feet there are left the mountains."
Robinson Jeffers, "Shine, Perishing Republic"
scampo is offline  
Reply With Quote
Old Saturday 6th March 2004, 10:26   #10
Leif
Registered Member

 
Join Date: Mar 2003
Posts: 2,959
Quote:
Originally Posted by scampo
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.
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?

Quote:
Originally Posted by scampo
gosh Leif and you could meet half way - say, Iceland. Can I come, too?
I think I'll stick to the local RSPB reserve, and InFocus field days if that's alright with you!

Quote:
Originally Posted by Carson
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.
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.
Leif is offline  
Reply With Quote

BF Supporter 2004 2005 2006 Support BirdForum With A Donation

Old Saturday 6th March 2004, 13:24   #11
gunvald
Registered User

 
Join Date: Jan 2004
Location: Linkoping
Posts: 32
Thanks for your thoughts and thanks to Leif for the interesting pdf file you supplied.

Quote:
Originally Posted by Carson

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.
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.
gunvald is offline  
Reply With Quote
Old Saturday 6th March 2004, 14:11   #12
Jonathan B.
Registered User

 
Join Date: Nov 2003
Location: United States
Posts: 363
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.
Jonathan B. is offline  
Reply With Quote

BF Supporter 2004 2005 Support BirdForum With A Donation

Old Sunday 7th March 2004, 04:38   #13
Carson
Registered User
 
Carson's Avatar

 
Join Date: Feb 2004
Location: Vancouver, Canada
Posts: 100
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 power:objective 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.)

Last edited by Carson : Sunday 7th March 2004 at 05:20.
Carson is offline  
Reply With Quote
Old Sunday 7th March 2004, 07:42   #14
Leif
Registered Member

 
Join Date: Mar 2003
Posts: 2,959
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.
Leif is offline  
Reply With Quote

BF Supporter 2004 2005 2006 Support BirdForum With A Donation

Old Sunday 7th March 2004, 09:13   #15
Jane Turner
Registered User
 
Jane Turner's Avatar

 
Join Date: Oct 2003
Location: Hoylake, Merseyside
Posts: 22,899
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 is offline  
Reply With Quote

BF Supporter 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Support BirdForum With A Donation

Old Sunday 7th March 2004, 12:39   #16
Swissboy
Registered User
BF Supporter 2019
 
Swissboy's Avatar

 
Join Date: Dec 2003
Location: Sempach, Switzerland
Posts: 3,593
Quote:
Originally Posted by Jane Turner
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!
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.
Swissboy is offline  
Reply With Quote

BF Supporter 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 Support BirdForum With A Donation

Old Sunday 7th March 2004, 13:58   #17
scampo
Steve Campsall
 
scampo's Avatar

 
Join Date: Aug 2003
Location: Leicestershire, UK
Posts: 6,273
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.
__________________
Steve
"...when the cities lie at the monster’s feet there are left the mountains."
Robinson Jeffers, "Shine, Perishing Republic"

Last edited by scampo : Sunday 7th March 2004 at 14:00.
scampo is offline  
Reply With Quote
Old Sunday 7th March 2004, 16:57   #18
Carson
Registered User
 
Carson's Avatar

 
Join Date: Feb 2004
Location: Vancouver, Canada
Posts: 100
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?
Carson is offline  
Reply With Quote
Old Sunday 7th March 2004, 19:16   #19
scampo
Steve Campsall
 
scampo's Avatar

 
Join Date: Aug 2003
Location: Leicestershire, UK
Posts: 6,273
I think it fair to view the eyes as simply a lens with a diaphragm (i.e. the iris), so all the rules apply.
__________________
Steve
"...when the cities lie at the monster’s feet there are left the mountains."
Robinson Jeffers, "Shine, Perishing Republic"
scampo is offline  
Reply With Quote
Old Sunday 7th March 2004, 19:33   #20
Leif
Registered Member

 
Join Date: Mar 2003
Posts: 2,959
Quote:
Originally Posted by scampo
I think it fair to view the eyes as simply a lens with a diaphragm (i.e. the iris), so all the rules apply.
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.
Leif is offline  
Reply With Quote

BF Supporter 2004 2005 2006 Support BirdForum With A Donation

Old Sunday 7th March 2004, 19:50   #21
Jay Turberville
Registered User

 
Join Date: Aug 2003
Location: Fountain Hills, AZ
Posts: 409
Quote:
Originally Posted by scampo
I think it fair to view the eyes as simply a lens with a diaphragm (i.e. the iris), so all the rules apply.
Yep. But that means you have to get down to the nitty gritty and work out the details of what is the optimal f-number for the human eye. There may be a point where an even smaller sized iris begins to hurt image sharpness. This will depend on the spacing of rods and cones and how well the eye's lens is corrected. If we know the density of the sensors on the retina, we could compute the iris size where our eye would become by diffraction from the iris. Optimal would probably be an iris size somewhat large than that depending on how well corrected the person's particular vision is.

The optimal f-number for a camera or camera-like imaging system will vary depending on factors such as these. Most 35mm camera lenses will not stop down past f22 because they are diffraction limited at around that point. Most give optimal resolution and sharpness at between f5.6 and f11. Shooting at f22 may give a greater DOF, but it does so at the cost of image sharpness. The "sweet spot" varies depending on the quality of the lens.

Typical digital cameras don't stop down further than f10. This is because they become diffraction limited at around f10 - a different f-number than for 35mm cameras. For instance, my CP5000 won't stop down further than f8. Digicams have a very much narrower "sweet" spot for optimal sharpness. Probably around f5.6.

http://www.jayandwanda.com/digiscope...cope_calc.html
If you plug in 1X on the calculator, you can calculate optimal f-numbers for the camera by itself. We can see that a CP4500 becomes diffraction limited at around f10. Optimal will be somewhat below f10.

The typical focal length of a human eye is 17mm. So a 4mm iris is a bit wider than f-4. If it is 2mm in bright light, then we typically see at around f8. If we knew the density of the eye's receptors, we should be able to enter the eye as a camera in the calculator and figure out when it is diffraction limited. Our optimal iris size should be a bit larger than it size when diffraction limited.
__________________
Jay Turberville
www.jayandwanda.com

Last edited by Jay Turberville : Sunday 7th March 2004 at 19:55.
Jay Turberville is offline  
Reply With Quote
Old Sunday 7th March 2004, 20:02   #22
oathkeeper
Registered User

 
Join Date: Feb 2004
Location: USA
Posts: 127
WOW! A whole new dimension of thinking...
__________________
Manendra Pedris

"If God had consulted me before embarking on Creation,
I would have suggested something simpler"
- Alfonso X, King of Castile and Leon (1221-1284) commenting on the Ptolemaic planetary system.
oathkeeper is offline  
Reply With Quote
Old Sunday 7th March 2004, 20:40   #23
mak
Registered Member

 
Join Date: Feb 2003
Location: UK
Posts: 347
[quote=Carson]
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.[quote]

I dont know about cheap, but I have seen a £300+ pair of binoculars, with this feature, even a diamond shape within the exit pupil. A classic case of "shaved" prisms, and it is a sign of poor manufacture. £300, not worth £25.
mak is offline  
Reply With Quote

BF Supporter 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Support BirdForum With A Donation

Old Sunday 7th March 2004, 21:05   #24
Jay Turberville
Registered User

 
Join Date: Aug 2003
Location: Fountain Hills, AZ
Posts: 409
Quote:
Originally Posted by scampo
I think it fair to view the eyes as simply a lens with a diaphragm (i.e. the iris), so all the rules apply.
I found this as a follow-up.
http://www.spie.org/web/oer/october/oct97/eye.html

Using the supplied spacing for cones, I entered the info in my calculator and the typical human eye does become diffraction limited at around a 2mm iris or f8. This would be 200 lp/mm or almost twice what the above author shows for a 3.5mm iris in his model. So it seems that the optics of our eyes is the main limiter and that "stopping down" further - probably all the way to f8 will continue to improve perceived sharpness and resolution.
__________________
Jay Turberville
www.jayandwanda.com
Jay Turberville is offline  
Reply With Quote
Old Sunday 7th March 2004, 21:16   #25
Swissboy
Registered User
BF Supporter 2019
 
Swissboy's Avatar

 
Join Date: Dec 2003
Location: Sempach, Switzerland
Posts: 3,593
Quote:
Originally Posted by Leif
IGoodness knows how it sees pylons and HT cables.
Well, they often don't. Even owls get caught by cables. These kinds of things - just like window glass - are not part of the original nature. And thus, no adaptations have been able to occur.

Last edited by Swissboy : Sunday 7th March 2004 at 21:18.
Swissboy is offline  
Reply With Quote

BF Supporter 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 Support BirdForum With A Donation

Advertisement
Reply


Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Advantages with 7x binos gorank Binoculars 45 Thursday 5th February 2004 13:05

{googleads}

Fatbirder's Top 1000 Birding Websites

Help support BirdForum

Page generated in 0.38087797 seconds with 37 queries
All times are GMT. The time now is 11:15.