EdZ
Professor EdZ
Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
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Binoculars are like hammers.
You couldn't use a 12 oz. finishing hammer to drive a 20 penny spike. Nor would you use a 20 oz. framing hammer to set a 6 penny finish nail on your trim moldings. A carpenter does not own just one hammer.
I wanted to test out the difference between two perfectly matched binoculars, but with one variable, exit pupil. I've owned the 16x70 Fujinon for a while, so recently I bought it's twin, the Fujinon 10x70. Not identical twins, but sure from the same family. The 10x70 has bigger eyes but speaks a little lower. These two binoculars share exactly the same everything except the eyepieces. So, only power, and hence exit pupil, are different (and fov, but not important here).
I am able to test the difference between these two since I have eye pupils that dilate to accept the full incoming light from the 10x70s (or very nearly so, anyway). The 16x70 has a 4.38mm exit pupil, the 10x70 has a 7mm exit pupil. If your eyes don't dilate any larger than 4.38mm, you would not even be able to test for the difference between these two binoculars.
On most nights I compared them in mag 4.5 to mag 5.6 skies. The 16x70s are my guidepost. Every binocular gets compared to them. Both binocs "gather the same amount of light, but the 10x70s deliver a lot brighter image to the eye. I was real curious as to what affect all the extra brightness delivered to the eye by the 10x70s 7mm exit pupil would have.
Think of the sky as the backdrop in a portrait. Your subject must stand out from the backdrop to be seen. Consider it to be a grey scale. You all understand the affect of dark sky background. Higher magnifications in a telescope reduce the extended light of the background until it approaches a completely dark sky, to the point you can no longer see the field stop. Lower magnification with a larger exit pupil delivers a brighter image, not only from your subject, but also from the background. This can be beneficial or detrimental, depends on the condition. Generally, this makes it much more difficult to see most subjects, but in dark conditions may allow you to see nebulous objects.
In moderate 4.5-5.5 skies, the 10x70s show a background about three or 4 shades brighter on the grey scale than what is delivered by the 16x70s. With a 7mm exit pupil, there is so much light coming into the eyes from the extended sky background, it makes it more difficult to see anything and everything. The 16x70s provided such a contrasty view against the darker backdrop, the contrast gain from the higher magnification of the 16x70s was far reaching and unmistakable. These are exactly the same binocular, so the only improvement in grey scale background most definitely comes only from the increase in magnification, and hence a smaller exit pupil.
But on one night, I did get both binocs out under mag6+ skies. Since the sky was already so dark, the backdrop in the 10x70s did not appear so much brighter than the 16x70s. Much more could be seen with the 10x70s in a darker sky than in the bright sky. Faint nebula stood out now that they were being used in their productive element. More faint stars came into view. On this night the difference between the two was not nearly as great. The 16x70s still see even more stars, but on the faintest nebula, the 10x70s may have done a little better.
My point is not to say one is better than the other. For me, in most conditions, the 16x70s will perform better. But those 10x70s, under the darkest skies I can get to, would be just the tool to observe extended objects and deliver that "finishing" light to my eyes. They have their place. But used in the wrong circumstances, they are no match for the right tool.
Every tool has it's purpose.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Anonymous
Unregistered
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I've been waiting for your comparison ever since you said you were getting a 10x70mm Fuji. This is a very informative post. I guess that's why higher magnification binoculars have steadily become more popular. As light polution becomes worse and worse so do large exit pupil binoculars making room for high magnifications.
What kind of improvement did the 16x70mm's get from the darker sky?
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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Ed,
I agree that SOME binoculars are like hammers.
I've owned one like that for almost 39 years now.
It is still useful for knocking in flat-heads but persistent use with oval brads has caused a series of rather unsightly indentations in the objective rims and one particular job during which I had to use it with a cold chisel has caused a permanent dent in the same rim.
On the other hand I find the Swarovski 8 x 20 useful only for small tacking work , such as when tacking the masks to the Zeiss 7 x 42.
On my third hand , I find hammers to be notably poor performers , particularly for astro -use , but for daylight terrestrial I would put my Spear and Jackson cross -pane on a par with the Sunagor 20 -120 x 30 megazoom, which I returned for a full refund when it snapped whilst using it to knock down a redudant partition wall.
On a less serious note,Exit -Pupils are ,as you know ,one of my favourite subjects, and I have recently taken to studying them through hand -held microscopes , which is not as silly a pastime as one might imagine.
P.S does anyone know if the Rothenberger 2 pound claw -hammer is nitrogen purged ?
Regards -- Kenny.
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Milton Wilcox R.I.P
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
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Quote:
P.S does anyone know if the Rothenberger 2 pound claw -hammer is nitrogen purged ?
It is not. But I bet if you hit your thumb with it you would be oxygen purged!
edz
Dan,
Only about a 30-40% gain in stars in the 16x70, while about a 50-60% gain in stars in the 10x70. The 16x70 still see far many more stars than the 10x70, in any type of sky.
In mag 6 skies, the North America nebula stood out all across America, and a portion of the Veil was readily visible in the 10x70s. I've seen both of those in the 16x70s but I think not to the same extent. On another night with the 10x70s in mag5.8, I thought I might have actually seen the Rosette for the first time. I've never seen that in the 16x70s, or any instrument for that matter, as far as I can remember.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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STRICTLY and SERIOUSLY on EXIT -PUPILS now !
I do not wish to "spoil the fun" anyone may derive from reading the most interesting "paper" , written by one of my more notable acquaintances, which I have forewarded to the site (should it in fact ever get published) but one aspect related to exit -pupil that I have never even considered before , nor read about anywhere , is the most fascinating assertion ( with scientific evidence - as will be attached in the form of relevent graphs and charts) that what a larger exit pupil DOES provide ,in daylight , regardless of magnification ( e.g 8 x 40 , 10 x 50 , 12 x 60 ,20 x 100) as opposed to ( e.g 8 x 32 , 10 x 40 , 12 x 50 ,20 x 80)
and regardless of entrance pupil dilation of the observer or natural lighting circumstances ( bright sunny day v dull cloudy twighlight ) is an increased CENTRAL AREA of "sharpness" or "increased area of central resolution".
Although admittedly this is , strictly speaking ,off the topic of "bino -astronomy" I think it is well worth a mention.
I am convinced this is the reason why , even in bright daylight, my Zeiss 7 x 42 provides a "superior" image to when it is masked down to 25mm.
This may not seem surprising to the casual reader , but believe me there are enough "experts" out there who will still deny this truth even when it is spelled out for them.
The reasons for this dogmatism will probably be a combination of personal pride and a prejudice borne from complex psychological reasons they will also be unlikely to admit to.
Regards -- Kenny.
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Milton Wilcox R.I.P
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
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This may be the beginning of our having information leading to potential understanding of one of the single most complex issues I have yet encountered in all aspects of binocular astronomy. I will read it with a open mind.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Diego
super member
Reged: 07/29/03
Posts: 179
Loc: Argentina South America
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One of the reasons I got the 20x90 was because of the moderate 4.5mm exit pupil (as compared to 25x100 binos) and the higher magnification to improve contrast. A friend of mine owns a 14x100. While I've never used it, he says the sky turns almost light blue when viewing. Too large an exit pupil and too bright an image.
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Diego
Celestron 80 mm f11.4
Oberwerk 20x90
6" f7.8 Reflector
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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The empirical evidence makes it clear...larger exit pupil (up to 7mm) means brighter. But, I don't understand the physics of this. Wouldn't a smaller exit pupil just mean that you've got the same amount of light (determined by objective diameter) packed into a smaller area? And since a 4mm exit pupil is smaller than your eye's pupil, your eye would be able to take in all of this light, just as your eye could take in the 6-7mm exit pupil image.
Obviously I don't have this right, but I'd appreciate if someone could explain where I'm wrong.
jeff
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Hardin 12"
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Anonymous
Unregistered
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For low magnifications the light from the sky is still used for a small part of what you see, but as you keep increasing the magnification the image is spread out more and more and with less light to work with it becomes dimmer. I'm not sure if that answers your question, but I think if you were to simply measure the amount of light comming out you would get more from the lesser magnification here's why I think this:
The low mag. shows a bigger piece of the sky, while the high mag shows a smaller piece of the sky. In both binoculars the part of the sky always seen gives the same light, but the low mag also gets light from everything else it shows which isn't in the high mag field of view. An eyepice is like a microscope magnifying the light gathered by the main lens, but for higher magnifications it only pics up a small piece of it.
I might be wrong, but I think this makes sence.
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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The way I understand it is this.
IF one's entrance -pupil / pupil dilation IS actually 7mm at a given point in time ( which I happen to think is much less likely and less often than some people seem to think --and after all it is not exactly easy to measure )AND the location is a very dark one with minimal light pollution AND the objects being viewed do not include anything as bright as the moon or planets, but are faint fuzzy objects, THEN ,and probably only then,will a 7mm exit -pupil from a binocular actually be of sufficient benefit to maximise resolution and contrast as seen by the user.
I'm convinced that if the pupil needs around half an hour in next to complete darkness to dilate to it's maximum diameter , then no way is it going to retain that maximum diameter if the next thing you do is look at the moon with 20x magnification and no filters !
Surely the brain / eye is going to react to this huge and sudden ingress of bright light and close down the pupil
accordingly ?
In which case I doubt that a 7mm exit -pupil can ever be the best through which to view the moon ,regardless of magnification or objective diameter ,viewing location ,or age of observer.
Conversely in the "perfect" conditions and scenario described in the first paragraph ,a binocular with a 7mm exit -pupil would probably be as close to "perfect" as one could wish for , bearing in mind that odd as it may sound , when pupil dilation PERFECTLY matches the exit -pupil from a binocular , this is when the problem of vignetting becomes most difficult to avoid than when the exit -pupil is EITHER larger or smaller than th pupil opening.
The more I try to learn about this subject ,it seems the more inclined I am to conclude that there is no such thing as the "perfect exit -pupil" any more than there is such a thing as the "perfect binocular" or "perfect telescope"
Regards -- Kenny.
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Milton Wilcox R.I.P
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Anonymous
Unregistered
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Too true. I myself think 5mm is a good all around size; good in light pollution and good in dark skies. Also, divide the main objective lens by 5 and you have the magnification. Doing this for several sizes of binoculars shows it to give a good magnification for the size. The smaller and lighter a binocular is the less magnification it should have in my oppinion- because it's harder to hold a light pair still. But as the objective lens keeps getting heavier and heavier it becomes easier and easier to hold it steady.
8x for 40mm
10x for 50mm
12x for 60mm
14x for 70mm
One of the reasons these sizes are so popular is because of the good ratio between size, weight, and magnification. The fact that all of them have a 5mm exit pupil, and that so many binoculars have this size exit pupil is merely a result of this. I think most people will agree that the previous argument is valid even if it turns out to not be true.
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Erik D
Post Laureate
Reged: 04/28/03
Posts: 4066
Loc: Central New Jersey, USA
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Diego,
I do not own any 14X100 binos but I have done sufficient testing with 7X50, 10X40, 18X50 IS 12X60, 20X80 and 25X100mm binos to know exactly what your friend is talking about. 7.1mm exit pupil is just too large for optimum astronomy from locations with any light pollution at all. I have come to prefer ~4mm exit pupil for astro binos with 50mm or larger objectives.
Even with my over-40 eyes I can still see the brightest image with my 7X50 binos for night time terrestrial viewing. However, what's desireable for spotting wildlife at night is not the same for astronomy. I find it more difficult to see dim stars/nebulas/galaxies with low power binos and 7mm exit pupil.
My preferred binos for astronomy are 12.5X50, 15X60, 20X80, 25X100, 30X120 or something close. I'd certainly like to have the option of higher power EPs for the Nikon 20X120 or the Fuji 25X150s. I think the Vixen 25-75X125 45 deg binoscope offers the most versatile combination of maganification/objective size in Jumbo astro binoculars today. The only down side is the narrow 1.6 deg FOV at 25X.
Erik D
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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Hi Daniel ,
From memory you a young man and as such your preference for 5mm exit -pupils as a general rule of thumb is well advised and will probably continue for many years yet.
For me , in my fifties , I would knock around 1mm off that figure and go with 4mm as probably the most useful all round for astronomy and bright daylight use.
So for me : 8 x 32 ,9 x 36 ,10 x 40 ,12 x 50 ,14 x 56,
15 x 60 ,20 x 80 and 25 x 100 would each have great appeal to me for different intended purposes.
In fact for everyday use, a really high quality 9 x 36 would be very nice for me ,all things considered.
Regards to you , Kenny.
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Milton Wilcox R.I.P
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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Dan and Kenny, both of your first responses to my question make sense, but I admit, I'm looking for a more mathematical explanation. [ok...i see there have been about 3 more posts in time i was writing this, so maybe this is an outdated message already].
Dan...sure...more magnification means more diffusion of the extended object's light, but I'm asking about the explicit relationship between exit pupil and brightness, not magnification and brightness per se. yes, these are related, but they're not the same thing (keep in mind that two different scopes/binos can give you a different exit pupil for the same magnification...or the same exit pupil with different magnifications).
Kenny...I agree...even if you dilated to 7mm in near-total darkness...how often do we observe in these conditions? More often than not, I've got some unwanted source of light nearby, so I'm sure I'm not usually wider than 5mm (just guessing). To generalize my question then...why is larger exit pupil (5mm if you prefer) brighter than a smaller exit pupil (e.g., 3mm)?
I've actually thought about this a bit since my last message, so i think I might have a sense of the answer...
Intuitively, smaller exit pupil might just seem to suggest a smaller visual field (smaller true FOV). However, I can see how it also means less light throughput. Shorter eyepieces have smaller-diameter lenses. Thus, the edge of the light cone coming from the objective misses the EP lens, just as an image >7mm diameter can't fit in our pupil. The smaller the EP (higher power), the more of the light cone that falls outside of the lens. Thus, the exit pupil will be related directly to the diameter of the EP lens.
The higher curvature of short EP-lenses makes for a steeper angle of light, which is of course where you get the higher magnification, but which is also why eye relief is short.
Anyway, I don't think the relative dimness of higher magnifications is because of the magnifcation per se (light spread over larger area), but because the amount of light making it through the higher-power EPs is actually less. Is this correct? I'm still eager to get someone else's scientific explanation.
And changing the subject slightly, why does our eye allegedly resolve detail better with a small exit pupil(e.g., 1-2 mm)?
Is there a good user-friendly text that deals with these concepts in detail?
cheers all,
jeff
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Hardin 12"
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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Quote:
The smaller and lighter a binocular is the less magnification it should have in my oppinion- because it's harder to hold a light pair still. But as the objective lens keeps getting heavier and heavier it becomes easier and easier to hold it steady.
I've heard this before, but I disagree. Heavier binos are harder to hold still for me because....they're heavier. I get the shakes as my muscles get tired. Also, for a given level of shakiness, the image suffers less if it's at lower magnification.
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Hardin 12"
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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Hi Jeff,
I think you are already on the right track.
Larger Exit Pupil EQUALS BRIGHTER IMAGE -period.
The thing is ,important and desireable as "brightness" is
-- and as I've said in earlier posts , without a certain level of brightness everything else is hypothetical , there are certain situations ( many in binocular astronomy applications) where less than maximum brightness and higher contrast is more beneficial , regardless of actual magnification and objective diameters , but more so in these instances as a result of the higher magnification
that automatically accompanies the smaller exit -pupil with a given -sized objective.
There are some very interesting web sites that cover tis topic in greater detail , one of which I think would be worth your while looking at is available via the main page of the following site , which was originally written in French language , but many relevant features are available in English when applying a modicum of determination.
http://www.astrosurf.com/lombry/index.htm
Regards -- Kenny.
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Milton Wilcox R.I.P
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Anonymous
Unregistered
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jmoore, I suppose it's not surprising that this works differently for different people. For myself, my hands are extreamly shaky; people comment on this all the time. I'm always asked if I'm nervus or if somethings wrong because I *CANNOT* hold my hands still out in front of me. But when I add several pounds it's completely eleminated. You and Kenny are right that my list is really only true for myself- oh well.
I think you're right when you say that the high mag eyepiece is only gathering the light from a small portion of the sky while the lower mag gets that light and more. Add it up and the low mag gets more light. I don't know how you can get any more mathamatical than that.
Edit: I missunderstood and said something stupid  sorry jmoore.
Edited by Daniel_Schwartz (02/01/04 06:00 PM)
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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Hey Daniel,
First, I didn't say that exit pupil and magnification don't relate. They very much do. For a GIVEN objective diameter, the same magnification will always give the same exit pupil. Change one, and you change the other. I'm just saying that strictly speaking, exit pupil and magnification aren't the same thing. They have their own definitions. Magnification refers to ratio of light path angles coming from the objective vs. the eyepiece, while exit pupil refers to the diameter of the image when all the parallel rays of light coming from the EP converge. So, they're directly related, but they describe different things.
Just to clarify...I'm not saying that a higher power EP gathers light from a narrow portion of the sky than a lower power EP. I mean, yes, this IS true, but what I was talking about specifically is that a higher power EP will collect fewer rays of light...even from a given object.
Consider this figure I put together. At top, 5 on-axis rays of light coming from a single object, e.g., the center of Saturn. All 5 rays pass through the EP lens. At bottom, the lens is high-power (shorter focal length, smaller lens diameter). Not all of the rays pass through. So, with the higher magnification lens, the object is dimmer. So, I'm talking about less light passing through...not just in terms of less-sky, but in terms of fewer light rays from the same piece of sky.
Is this right? Ed?
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Hardin 12"
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edcannon
professor emeritus
Reged: 11/19/03
Posts: 693
Loc: Austin, Texas
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I think I may have learned enough to try to explain why larger
exit pupils are brighter. Take a specific magnification, say
10. Then in relation to that, the larger objectives (gathering
more light) yield larger exit pupils:
10x30 = 3
10x50 = 5
10x70 = 7
So the brightness is directly expressed by the exit pupil: the
ratio of light-gathering to magnification.
In terms of the brightness, the largest exit pupils can only be
fully used by dark-adapted younger folks on moonless nights
under dark skies, because the sky background is very dark in
that situation. But under moonlight and light pollution, then
having a lower ratio of gathered light in relation to
magnification yields more contrast, which makes fainter stars
easier to see, since the background is darker. For faint
fuzzies, only really dark skies will give the best views,
because they tend to be low-contrast objects that really need
a lot of light-gathering.
Kenny -- What takes 20 to 30 minutes to adapt at night is the
dark adaptation chemical situation in the eye. The dilation of
the pupils is a nearly instantaneous reflex. (I don't know if
it slows down with age.)
Ed Cannon - Austin, Texas, USA
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Ed Cannon - Austin, Texas, USA
Bushnell H2O 8x42, Celestron Skymaster 12x60
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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Quote:
Hey Daniel,
First, I didn't say that exit pupil and magnification don't relate. They very much do. For a GIVEN objective diameter, the same magnification will always give the same exit pupil. Change one, and you change the other. I'm just saying that strictly speaking, exit pupil and magnification aren't the same thing. They have their own definitions. Magnification refers to ratio of light path angles coming from the objective vs. the eyepiece, while exit pupil refers to the diameter of the image when all the parallel rays of light coming from the EP converge. So, they're directly related, but they describe different things.
Just to clarify...I'm not saying that a higher power EP gathers light from a narrow portion of the sky than a lower power EP. I mean, yes, this IS true, but what I was talking about specifically is that a higher power EP will collect fewer rays of light...even from a given object.
Consider this figure I put together. At top, 5 on-axis rays of light coming from a single object, e.g., the center of Saturn. All 5 rays pass through the EP lens. At bottom, the lens is high-power (shorter focal length, smaller lens diameter). Not all of the rays pass through. So, with the higher magnification lens, the object is dimmer. So, I'm talking about less light passing through...not just in terms of less-sky, but in terms of fewer light rays from the same piece of sky.
Is this right? Ed?
The ray diagrams are wrong. The exit pupil is an image of the entire aperture. If you will, it is the effective aperture of the system. The image gets dimmer with magnification because the effective focal length of the system is increasing, but the aperture is not, hence, larger effective focal ratio and dark image.
What is "image brightness"? Can anyone give a clear definition?
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