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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12517
Loc: Cumberland, R I , USA42N71.4W
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This thread started from the Nikon SE thread but took off on Exit Pupil Brightness and Afov. You can go back to the original here.
Original Nikon SE thread
edz
Quote:
Kenny:
I wouldn't say you were wrong that AFOV is a factor in determining apparent brightness -- it's just not the only factor.
Fiske
I would say the biggest (or at least one of the biggest) factors in determining the brightness of the image is determined by the illumination of the exit pupil. If you remember a while back it was found we could determine at what point light entering on the objective no longer produced illumination of a full exit pupil. In some binoculars, the full diameter of the exit pupil might be illuminated from 40-50% of the aperture while in others it is fully illuminated by only 20-30% of the aperture. Further out, the light entering from the edges in some models illuminates maybe 40% of the exit pupil, while in some models light from the edges illuminates only 10-20% of the exit pupil. In some of the more premium models light does indeed illuminate a greater percentage of the exit pupil.
mention was made of the prism types. There could and probably would be some difference in transmission due to prism types.
Pinewood asked, Can the difference in apparent brightness be attributed to the coatings? All these premium binoculars have premium coatings. While a better coating will provide for greater light transmission, the construction of the binocular provides for the illumination of the exit pupil. I think the percent illumination far outweighs the gain or loss in transmission due to very slight differences in already premium coatings.
Field of view is not a determinant in light gathering or light delivery (exit pupil). Changing the field of view cannot result in the optical elements increasing or decreasing brightness of the image. I once wrote that it seemed a narrower field of view seemed to provide for higher contrast. But I do believe the perception of change in apparent brightness due to a change in field of view is an illusion. It cannot be supported by anything else we know about optics.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (01/25/05 04:58 PM)
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Anonymous
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Quote:
While a better coating will provide for greater light transmission, the construction of the binocular provides for the illumination of the exit pupil. I think the percent illumination far outweighs the gain or loss in transmission due to coatings.
If you mean baffling regarding construction, then absolutely this has a far greater contribution to the brightness.
Quote:
I think the perception of increased apparent brightness due to a larger field of view is an optical illusion.
I have never known an increased FOV to seemingly provide an increased apparent brightness. Does this really happen? If anything...for astronomy anyway...a larger FOV could provide a greater collection of light pollution thereby reducing the apparent brightness. Intrinsically, I don't see how FOV and brightness are related.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12517
Loc: Cumberland, R I , USA42N71.4W
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Quote:
If you mean baffling regarding construction, then absolutely this has a far greater contribution to the brightness.
No, I don't mean baffles (which would only improve contrast), I mean vignetted light path, which contributes to the total light reaching the exit pupil. Proper baffles only stop stray light, improving contrast. Any baffle that stops anything more than stray light is either an aperture stop or a stop that causes vignette. (But I really don't want to get into that conversation in this thread).
Quote:
I have never known an increased FOV to seemingly provide an increased apparent brightness. Does this really happen? If anything...for astronomy anyway...a larger FOV could provide a greater collection of light pollution thereby reducing the apparent brightness. Intrinsically, I don't see how FOV and brightness are related.
I believe it does not happen. A larger fov does not reduce apparent brightness. My point is that the larger field of view has no affect on the brightness, neither increasing or decreasing it.
edz
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btschumy
Think Astronomy
Reged: 04/13/04
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Quote:
I would say the biggest (or at least one of the biggest) factors in determining the brightness of the image is determined by the illumination of the exit pupil. If you remember a while back it was found we could determine at what point light entering on the objective no longer produced illumination of a full exit pupil. In some binoculars, the full diameter of the exit pupil might be illuminated from 40-50% of the aperture while in others it is fully illuminated by only 20-30% of the aperture. Further out, the light entering from the edges in some models illuminates maybe 40% of the exit pupil, while in some models light from the edges illuminates only 10-20% of the exit pupil. In some of the more premium models light does indeed illuminate a greater percentage of the exit pupil.
edz
Ed,
My Nikon 10x42 SE binos have a very bright image according to me and others that have looked through this model. However, if you judge exit pupil illumination via looking down the objective end of the barrel, the SE's have almost 0% illumination right at the edge (i.e. the exit pupil isn't visible at all right at the very edge). This is in contrast to the (not as bright) Takahashi 22x60 binos the have almost a fully illuminated exit pupil at the edge off the objective. Admittedly we are comparing different powers and apertures, so the comparison may not be totally valid. So either I'm misunderstanding what you are saying or I think there must be other factors at work other than "exit pupil illumination".
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Bill Tschumy
Where is M13? Freeware -- Add a new dimension to your observing.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12517
Loc: Cumberland, R I , USA42N71.4W
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Well,
you're right about one thing, you can't compare these two binos. To start with one has a 2.7mm exit pupil and the other a 4.2mm exit pupil. And one gathers twice as much light as the other. These are oppostie ends of the spectrum.
I don't measure by looking down the objective, I measure by sliding a target laser across a (glass covered) objetive and taking measurements of the exit pupil image with the incoming light at various locations. I'm not guesstimating the outcome, I'm measuring the actual output.
My 12x50 SE measures 40%+ illumination of the exit pupil from light at the very edge of the objective. I doubt the other SEs would be much different, although that how I get in trouble, saying something about a binocular I haven't tested.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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Henry Link
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Reged: 03/31/04
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Loc: Greensboro, NC
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Quote:
Quote:
I have never known an increased FOV to seemingly provide an increased apparent brightness. Does this really happen? If anything...for astronomy anyway...a larger FOV could provide a greater collection of light pollution thereby reducing the apparent brightness. Intrinsically, I don't see how FOV and brightness are related.
I believe it does not happen. A larger fov does not reduce apparent brightness. My point is that the larger field of view has no affect on the brightness, neither increasing or decreasing it.
edz
I've also never been able to detect any effect on brightness from AFOV and I've tried. But one question nags at me about this. A 70 degree evenly illuminated AFOV should transmit twice as much light into the eye as a 50 degree AFOV looking at the same area. Shouldn't that extra light sometimes trigger constriction of the eye'e pupil causing a reduction in apparent brightness for the wider field?
It's easy to think of an extreme example where this would certainly happen. Imagine you are looking at an area of sky adjacent to the full moon using a binocular with a 50 degree field. Then you substitute a binocular with a 70 degree field. Now the moon is visible at the edge of the field and your pupil constricts substantially. Shouldn't there be other less extreme examples of the extra field width adding enough total light to cause some pupil constriction?
Henry
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12517
Loc: Cumberland, R I , USA42N71.4W
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Quote:
I've also never been able to detect any effect on brightness from AFOV and I've tried. But one question nags at me about this. A 70 degree evenly illuminated AFOV should transmit twice as much light into the eye as a 50 degree AFOV looking at the same area. Shouldn't that extra light sometimes trigger constriction of the eye'e pupil causing a reduction in apparent brightness for the wider field?
A 70 ° Afov DOES NOT transmit any more light to the eye than a 50° Afov. The size of the exit pupil is what determines how much light has been transmitted to the eye. If it remains constant, then so does the amount of light transmitted, regardless of Afov. A larger Afov would not trigger any constriction of the eye pupil.
Quote:
It's easy to think of an extreme example where this would certainly happen. Imagine you are looking at an area of sky adjacent to the full moon using a binocular with a 50 degree field. Then you substitute a binocular with a 70 degree field. Now the moon is visible at the edge of the field and your pupil constricts substantially. Shouldn't there be other less extreme examples of the extra field width adding enough total light to cause some pupil constriction? Henry
In this example, you have modified the input by increasing gathering light from ?mag 6? stars to gathering light form the mag -10? moon. You've changed the input. Invalid example. That is not measuring an equivalent source by modifying the field of view. That is changing the source.
A more appropriate example might be observing a group of mag six field stars on a mag 9 sky background. Now just enlarge the field to 70° and observe the same mag 6 field of stars with the same mag 9 sky background, only over a larger area. If you haven't changed the size of your exit pupil, then the image is equally bright.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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Henry Link
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Ed,
I think I chose an example that was a little too extreme. How about an example in which the light source is an evenly illuminated gray wall that completely fills the binocular field. Aperture and magnification remain the same. Only the apparent field changes. The 70 degree field doesn't actually "modify the input" since I am assuming identical optics except for the eyepiece field stop diameter, but the extra field width does modify the light through put by exposing a larger surface area of the light source compared to a 50 degree field, even though the exit pupil remains the same. The amount of light entering the eye has doubled just from increasing the apparent field because the visible surface area of the light source has doubled. The question this raises for me is why couldn't that increase in the total amount of light energy entering the eye cause the pupil to constrict and reduce the apparent brightness of the wall in the 70 degree eyepiece compared to the 50 . I have looked for such an effect but haven't seen it.
Henry
Edited by Henry Link (01/24/05 06:00 PM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12517
Loc: Cumberland, R I , USA42N71.4W
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Quote:
I think I chose an example that was a little too extreme. How about an example in which the light source is an evenly illuminated gray wall that completely fills the binocular field. Aperture and magnification remain the same. Only the apparent field changes. A 70 degree field in such an example does indeed "modify the input" by including a larger surface area of the light source compared to a 50 degree field, even though the exit pupil remains the same. The amount of light entering the eye has doubled just from increasing the apparent field because the visible surface area of the light source has doubled.
No the 70° field does not modify the input. The 70° field is seen by the eyepiece, not by the objective. It is the objective that gathers the light. It has not changed. The aperture is not gathering light from a larger surface area. Light gathering is not dependant on filed of view. It is fixed by aperture.
The amount of light entering the eye has not and can not change unless you modify the exit pupil. I repeat, changing the field of view does not change either aperture or exit pupil. Light gathering does not change and brightness does not change.
Quote:
The question this raises for me is why couldn't that increase in the total amount of light entering the eye cause the pupil to constrict and reduce the apparent brightness of the wall in the 70 degree field compared to the 50 . I have looked for such an effect but haven't seen it.
There is no increase in the total amount of light entering the eye. That is probably why you have looked for it but haven't seen it.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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btschumy
Think Astronomy
Reged: 04/13/04
Posts: 1107
Loc: Austin, TX, USA
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Quote:
Well,
you're right about one thing, you can't compare these two binos. To start with one has a 2.7mm exit pupil and the other a 4.2mm exit pupil. And one gathers twice as much light as the other. These are oppostie ends of the spectrum.
Then it would seem your statement that "The biggest (or at least one of the biggest) factors in determining the brightness of the image is determined by the illumination of the exit pupil" is not true. Here you are saying that the exit pupil size is more important.
Quote:
I don't measure by looking down the objective, I measure by sliding a target laser across a (glass covered) objetive and taking measurements of the exit pupil image with the incoming light at various locations. I'm not guesstimating the outcome, I'm measuring the actual output.
My 12x50 SE measures 40%+ illumination of the exit pupil from light at the very edge of the objective. I doubt the other SEs would be much different, although that how I get in trouble, saying something about a binocular I haven't tested.
edz
Any idea how your scheme compares with looking down the barrel? Seems like If the exit pupil totally closes off when looking down the barrel at the edge (like the 10x 42 SE) then it can't be contributing much at that location. However, maybe I'm thinking about it wrong.
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Bill Tschumy
Where is M13? Freeware -- Add a new dimension to your observing.
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KennyJ
Reged: 04/27/03
Posts: 10082
Loc: Lancashire UK
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I'm following this thread with great interest.
The way recent posts are reading , it's almost as if there is a hint that EXIT PUPIL and AREA OF OBJECTIVE LENSES are the main factors most influencing BRIGHTNESS in a binocular.
Perhaps they are.
Of course , those of us who have compared the "brightness" of image through a 8 x 32 Nikon SE to the "brightness" of image through yer average "car boot sale" 10 x 50, might well continue to question this.
Regards, Kenny
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Two eyes and a preference to use both
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Quote:
I don't measure by looking down the objective, I measure by sliding a target laser across a (glass covered) objetive and taking measurements of the exit pupil image with the incoming light at various locations. I'm not guesstimating the outcome, I'm measuring the actual output.
EdZ:
You're the man! 
Anyway, I didn't mean that a larger AFOV actually increases the "apparent brightness" only that it might make a binocular seem brighter.
Another point is that many of the brightness comparisons being discussed are based on daylight conditions. The 8x32 SE may seem brighter than an 8x42 LX in daylight, but at night it's no contest--the 8x42 LX performs significantly better on fainter objects.
To be frank, even in daylight testing I can't say that the 8x32 SE seems brighter than the 8x42 LX to me. I've made this check before, but the next time I get by the Backyard Bird Center I'll do some additional comparisons.
The brightness claims seem similar to claims regarding CA performance. In making a large number of side by side comparisons between the SE and LX binoculars, I haven't seen much of a difference in CA between them.
In off-axis sharpness, too, both lines are similar in performance, with the LX possibly having a slight edge (pardon pun).
The SE binoculars are terrific instruments, no question about that, but I think they've developed a cult status that tends to exagerate their performance.
It's similar to claims one hears about the Celestron C9.25 SCT. It's good for the price, but doesn't begin to compare with Celestron's C11 despite claims you hear to the contrary. I owned both instruments for several years and had plenty of opportunity to compare them. I finally traded off the C9.25 because I just wasn't using it very much.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
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Anonymous
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With regards to apparent brightness, I believe it is not enough to speak of the exit pupil size without mentioning the pupil size too.
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Henry Link
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Reged: 03/31/04
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Quote:
edz
No the 70° field does not modify the input. The 70° field is seen by the eyepiece, not by the objective. It is the objective that gathers the light. It has not changed. The aperture is not gathering light from a larger surface area. Light gathering is not dependant on filed of view. It is fixed by aperture.
Sorry, I missread your term "input" to mean eye input rather than binocular input. I realized my mistake and edited my post but not before your reply. I know the diameter of the eyepiece fieldstop doesn't effect how much light enters the binocular but it definitely effects the total amount of light energy that exits. Given an evenly illuminated real field like the gray wall and identical optics except for the eyepiece fieldstop diameter, a 50 degree eyepiece fieldstop compared to a 70 degree field stop will simply block the ring of light between 50 and 70 degrees, preventing that light from exiting the eyepiece and that will reduce the total amount of light energy that exits the binocular by about 50%.
Quote:
edz
The amount of light entering the eye has not and can not change unless you modify the exit pupil. I repeat, changing the field of view does not change either aperture or exit pupil. Light gathering does not change and brightness does not change.
Yes, I know that changing the field of view does not change aperture or exit pupil, but it does change the number of photons that exit the binocular and enter the eye. Once again, given an evenly illuminated real field , the total amount of light that enters the eye from a 70 degree apparent field is about twice the amount that enters from a 50 degree field. Exit pupil and aperture are irrelevant, as long as they remain constant. However, the catch is that the light from the 70 degree field is spread across twice as large an area of retina, so even though there is twice as much light entering the eye the light intensity per unit of retinal area is exactly the same for the 70 and the 50 degree fields. That is why the perceived brightness is the same regardless of apparent field width. We are in complete agreement about that. My original question was only about why the mechanism of pupil dilation is apparently not triggered by the larger amount of light energy entering the eye from a larger apparent field (even if that energy is going to be spred across a larger area of retina).
Quote:
There is no increase in the total amount of light entering the eye. That is probably why you have looked for it but haven't seen it.
edz
Rather than an increase my expectation was actually for a possible decrease in perceived brightness from a larger apparent field.
Henry
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EdZ
Professor EdZ
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Loc: Cumberland, R I , USA42N71.4W
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Quote:
Then it would seem your statement that "The biggest (or at least one of the biggest) factors in determining the brightness of the image is determined by the illumination of the exit pupil" is not true. Here you are saying that the exit pupil size is more important.
Well'
it's beyond me how you could possibly reach that conclusion. All I said here was if you want to compare two binoculars for brightness you need to use two similar binoculars. The discussion was about coatings. I pointed to illumination. You threw out two widely varying systems. I rejected it.
Obviously the very large exit pupil would give a brighter image. But I would never use such an example to attempt to comare factors that affect the brightness of image. I would start with two similar system, and then determine what is different between them.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12517
Loc: Cumberland, R I , USA42N71.4W
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Henry,
I'm sorry that I must beg to differ with this. I could not find a single text on optics that could support what you are saying here.
Amateur Astronomer's Handbook
Basic Optics and optical Instruments
Startesting Astronomical Instruments
Telescope Optics
and several others of lesser importance.
If the exit pupil does not change, the brightness of the image does not change. You are saying that by inserting a wide angle eyepiece versus a narrow angle eyepiece that you can change the brightness of the image entering the eye. Please, point me to at least one technical resource on optics that can support that theory.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12517
Loc: Cumberland, R I , USA42N71.4W
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Quote:
The way recent posts are reading , it's almost as if there is a hint that EXIT PUPIL and AREA OF OBJECTIVE LENSES are the main factors most influencing BRIGHTNESS in a binocular.
Perhaps they are.
Of course , those of us who have compared the "brightness" of image through a 8 x 32 Nikon SE to the "brightness" of image through yer average "car boot sale" 10 x 50, might well continue to question this.
Regards, Kenny
This is a discussion we have had before. Yes they are. But don't lose sight of the several other important factors brought up here, transmission due to coatings and illumination due to construction, even the purity of the glass will have a factor. Even these we have discussed before at length. These are all factors that cannot go overlooked.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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Anonymous
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Dear Henry,
Here are my 2 cents. Let us first assume there is no vignetting, perfect anti reflection coatings, sufficiently large pupil of the human eye and linear response of the eye to number of photons. ( doubling the photons to a point on retina is perceived as a doubling in brightness)
Then every point in a distant object produces a parrallel beam of light when it enters the objective of the binoculars. The diameter of this beam is the diameter of the objective and the number of photons in this beam is proportional to the diameter.
Then this entire beam ends up as a focused point on the retina. so the brightness of this point depends only on the diameter of the objective. It does not depend on magnification (or exit pupil diameter), or apparent field of view. These are all related of course. for example higher magnification makes a smaller exit pupil but all the light from the objective still passes through the exit pupil so the image point is the same brightness.
In the real world it is all downhill from here.
vignetting reduces the amount of light reaching the point on the retina (pointed out by Ed). a constricted pupil of the eye will reduce the amount of light reaching the retina,(pointed out by Nightwatch). the response of the eye is not linear, but logarithmic (more or less) so doubling the number of photons is not perceived as doubling of brightness( maybe 50% instead?).
so it is possible to have a highly vignetted 10x50 binocular that is not as bright as a 10x40 unvignetted bino.
Hope this helps,
anton
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Anonymous
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Henry, I don't believe that the apparent brightness has much to do, if anything, with the AFOV. When you look at a star whether it's at 50 or 70 degrees, everything else being the same, the star brightness is still the same. No change. Now change the magnification, keeping everything else the same. The exit pupil has now changed, and let's assume that all the light enters the eye. Did the apparent brightness, or the apparent magnitude, change? You betch ya! The same thing occurs, albeit with a lesser contribution I think, when changing the aperture area. This is how I think of my binoculars--instruments that increase the apparent magnitude.
This may be an overly simplified example...pin point light sources at infinity against a black background. I hope this helps too.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12517
Loc: Cumberland, R I , USA42N71.4W
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There is so much mis-information going on here it makes me wonder what astronomy texts everyone has been reading!
First of all, point sources. the apparent increase in brightness of a point source is directly related to the light grasp of the instrument. Light grasp is aperture.
Now extended objects, every thing else that is not a point source. The brightest image you can get through your telescope (or binocular) is that which is at the largest exit pupil that does not exceed the eye pupil. An optical system can never provide image brightness brighter than what the naked eye can see. Image brightness can be measured as the ratio of image brightness in the instrument compared to brightness in the naked eye. It is determined by Diameter of exit pupil squared / diameter of eye pupil squared. When exit pupil equals eye pupil, brightness is at unity.
Brightness of image changes with exit pupil. If you keep aperture constant and change exit pupil (by changing magnification), then image brightness will change. As magnification increases, image brightness decreases. this can be tested with the above formula. 10 million astronomers have not been getting this wrong for decades. Larger exit pupil provides a brighter image.
The angle of view provided by the eyepiece does not change either the aperture or the magnification. It does not change the exit pupil. It does not change the brightness of the image. A narrower Afov eyepiece (ortho) does not miss some of the rays. You cannot make the image brighter by using a wider field (at equal magnification) eyepiece. If this were true no astronomer would ever pick up an ortho eyepiece over a wide field eyepiece.
Increasing magnification (in the same aperture) will decrease the brightness of the image delivered to the eye. It will however increase the apparent size of the object in the image. At some point it becomes easier for the eye to detect the image, not because it is brighter, but because it crosses more detectors in the eye. As magnification increases, it indeed becomes less bright.
At some further point of magnification, it will no longer be easier to see, but more difficult. The concepts of Optimum Magnified Visual Angle and Optimum Detection Magnification put forth by Roger Clark and Mel Bartels are accurate after all.
just my 2 cents.
edz
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