JCB
sage
Reged: 10/04/04
Posts: 278
Loc: France
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This post is intended for those who believe that image brightness in a 10x50 binocular is 4 times the image brightness in a 10x25 binocular.
You have to make a little experiment : take a binocular, and make an aperture mask of half the diameter of the objective lens, only on one side of the binocular. For example, if your binocular is a 10x50, then you will have a 10x50 on one side and a 10x25 on the other side. Now aim the binocular at terrestrial target at dusk, and try to estimate the brightness difference by using the same eye, alternatively looking through each barrel.
If the brightness is set at 1 for the full aperture, do you estimate the brightness of the masked aperture at 0.25, as theory tells ?
The following is even better :
Now, try to look through you odd binocular with both eyes, still with the aperture mask on one side only. Wait.
Wait...
Wait.............
And................... what ?????????   
Jean-Charles
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Phil Wheeler
Carpal Tunnel
Reged: 12/31/05
Posts: 2018
Loc: 3 miles WNW of Celestron
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Quote:
Wait.
Wait...
Wait.............
And................... what ?????????
Too much waiting (and I have no such bino) .. so "What"?
Phil
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
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JC,
You bring up some good points in the hunter's thread and also here. But people will always have great difficulty with these concepts.
To start, the physiology of the eye/brain function is notoriously poor at differentiating things such as contrast and brightness between two eyes when both eyes are used at the same time.
Not only that but people have no clear conception of what twice as bright is. Just as an example ask someone to point out two stars, one which is twice as bright as the other. My guess is people would be pointing to two stars that differ in brightness by as much as 5x.
Changing the parameter of aperture not only changes brightness, but also changes light gathering. This makes the comparison difficult to perform.
If you want to compare brightness of exit pupil, maybe you should keep aperture constant. BUT ah, now magnification is changed and I can see more. Another difficulty.
No matter what you do this is an extremly difficult comparison to isolate. This has been a topic of discussion for years now.
A 20x70 would give an exit pupil half as bright as a 14x70.
A 20x70 would give an exit pupil half as bright as a 20x100.
Obviously, even though both the 14x70 and the 20x100 have an exit pupil twice as bright as a 20x70, one will show far more than the other because it has significantly greater light gathering. But the exit pupils in these two will be equally bright.
These examples were chosen to illustrate the difficulty in extracting light gathering or magnification from this equation, both of which have an affect on what can be seen. Hence your title, Perceived Brightness.
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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Steve Napier
Carpal Tunnel
Reged: 05/10/04
Posts: 1559
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Ive experimented a lot with exit pupils and as Edz says its almost impossible to discern what is twice,four times brighter etc.
But,we know what you mean and the answer must be no,the difference is not as great as the figures would have us believe.
If like me you suffer from astigmatism you will probably feel the benefit of a smaller exit pupil for certain types of obseving,I know I do.
Steve.
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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Jean Charles ,
It's great to see you starting a new thread here :-)
Please don't get TOO dismayed by " opposing opinions " .
I'm probably FAR more used to it than YOU are :-)
I hope to be back here in a few days ! :-)
Have a nice weekend !
Kenny
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Milton Wilcox R.I.P
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Mark9473
Postmaster
Reged: 07/21/05
Posts: 6459
Loc: 51°N 4°E
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I think I'm missing the point. My 8x20 are a lot less bright than my 8,5x44, both during twilight and at night.
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Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Orion 15x63; Docter 15x60
WO Megrez II 80 FD / APM 107mm f/6.5 / Mewlon 210 on DM-6 + Berlebach Planet
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JCB
sage
Reged: 10/04/04
Posts: 278
Loc: France
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Thank you, Kenny
As you certainly imagine, my presence here is the only consequence of your encouragements, so I hope you will be back here soon.
Phil,
My hope is that someone will try this experiment, and tell us what he has noticed after, say, 1 minute. Any binocular should be suitable for that, only a piece of cardboard is needed to make the aperture mask. If nobody answer, I will post what I have discovered in a couple of days.
EdZ,
>>>Not only that but people have no clear conception of what twice as bright is. Just as an example ask someone to point out two stars, one which is twice as bright as the other. My guess is people would be pointing to two stars that differ in brightness by as much as 5x.>>>
This is the most important part your answer. People have no clear conception of what twice as bright is, for the excellent reason that it is impossible. The example you give is very good. This is because the response of the eye to brightness is not linear, but rather logarithmic. So, even if the GEOMETRICAL brightness is divided by 4 in my little experiment, the SUBJECTIVE brightness is much less reduced, according to the logarithmic sensitivity of the eye.
What I mean is this :
I always read advices, from numerous people, saying for example "a 10x50 binocular gathers 56% more light than a 10x40".
Mathematically, it's true. But does it matter ?
If the receptor behind the optical device has not a linear response, then the behaviour of this receptor must be taken into account. In such a case, the statement above provides no useful information, and can be misleading.
I prefer statements like:
" a 10x50 binocular shows stars 0.2 magnitudes fainter than a 10x40" (I don’t remember the exact value)
" a 10x50 is moderately brighter than a 10x40"
By the way, my point was not about the amount of visible details, but only the apparent brightness of the image.
Jean-Charles
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Rich N
Post Laureate
Reged: 09/22/04
Posts: 5618
Loc: San Francisco Bay Area, Calif...
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When I want to know which binocular to take birding that will give me the best views in deep shade, I go in my back yard with a few different binoculars and see which gives me the best view of objects in a shaded area.
I've done something similar in a camera shop that sells binoculars. They have an entry way into a back room that is quite a bit darker than the showroom. Standing in the showroom, near the dark entry way to the backroom, I try a few different binoculars to see which lets me see objects in that shaded entry way the best.
This isn't scientific. Nothing quantified. But, it is practical for me. I can see if the difference between different binoculars is great enough to be worthwhile.
Rich
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DJB
Carpal Tunnel
Reged: 02/23/05
Posts: 1742
Loc: Lisle NY
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Hello,
I was just about to make the point that the eye-brain response is a log response in our eyes, our ears, and our taste buds, as has been reported.
But the perception is what matters to us, i.e., what are we happy with according to perception.
As a corollary, using a test tone under controlled test conditions, a 50W amp and a 25W amp will produce a 3dB difference in "loudness," a log response of the ear-brain interface. But the perception is not dramatic at all.
So this discussion has two answers, at least. Very interesting.
Regards all,
Dave.
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Steve Napier
Carpal Tunnel
Reged: 05/10/04
Posts: 1559
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Mark,the point is that the percieved brightness is not as different as one would expect.
Example....Looking through my 7x42 binoculars in the daytime with a 6mm exit pupil then putting a 21mm stop over the objective to get a 3mm exit pupil,the difference in percieved brightness was so tiny as to be not there at all.
Ive also tried a 14mm stop over these binoculars to get a 2mm exit pupil and although the image was more noticeably dim the image was not as much of a difference as what you would THINK a 6mm to a 2mm exit pupil wopuld be.
I do realize a LOT depends on the optical quality of the binocular when conducting these experiments.
I"ve tried stopping down my 20x60 tentos but,because they"re a poor quality instrument I could barely see anything at all.
Steve.
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Swedpat
Pooh-Bah
Reged: 02/18/05
Posts: 1144
Loc: Boden, Sweden, Scandinavia
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The question about perceived brightness is interesting. The difference in perceived brightness is very difficult and sometimes impossible for us to estimate. Do we estimate a twice in brightness as twice? I am not sure, but the bigger the difference in brightness the more difficult to estimate the difference.
What is the difference in brightness between a sun-illuminated and a shadowed wall/street? I understand the difference depends on the color and surface, not all surfaces reflect the same percentual amount of light. When I have thought about this and tried to do an estimation I am very unsure. Is it 10 times? Or 50 times? Or may it be so big as 100 times?
If you would make an investigation of peoples estimation of the difference in brightness between the sun and the fullmoon you would get many different answers. Before I knew this I could never have been sure about it. Is it 100 times? Or 1000 times? I suppose that very few persons will without the knowledge about this question have answered that the difference in brightness between the sun and the fullmoon actually is more than a half million times.
It's an indisputable fact that a 10x50 provides a 4 times brighter image than a 10x25 (providing our eye-pupil is dilated to at least 5mm and the light transmission is equal), but if we perceive a difference of 4 times is another thing.
Regards, Patric
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
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Quote:
It's an indisputable fact that a 10x50 provides a 4 times brighter image than a 10x25 (providing our eye-pupil is dilated to at least 5mm and the light transmission is equal), but if we perceive a difference of 4 times is another thing.
Regards, Patric
That's right. Examples given above by people who can see little difference in a smaller aperature at same power actually do nothing to support such claims that this difference in brightness cannot possibly be true. In fact it probably says more about the inability of that observer to perceive small differences in light than anything else.
I did a little test while I was out observing last night. I picked out stars in Leo that I thought were twice as bright as some others. I picked
Regulus to be twice as bright as Algieba
Denebola to be twice as bright as Zosma
I was wrong on both counts.
Denebola is only 45% brighter than Zosma.
Regulus is only 65% brighter than Algieba.
It is well known that when observing doubles, if they do not vary by more than 20% few people can pick which one is the brighter of the two.
edz
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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Mark9473
Postmaster
Reged: 07/21/05
Posts: 6459
Loc: 51°N 4°E
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Steve, I think you're doing a different experiment. In broad daylight your eye pupils are just too small.
At dusk, as Jean-Charles proposed, for me there is just the brightness difference I would expect from the difference between 8x20 and 8,5x44.
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Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Orion 15x63; Docter 15x60
WO Megrez II 80 FD / APM 107mm f/6.5 / Mewlon 210 on DM-6 + Berlebach Planet
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Rich N
Post Laureate
Reged: 09/22/04
Posts: 5618
Loc: San Francisco Bay Area, Calif...
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It depends, if the shaded area covers virtually the entire field, the view will be relatively dark and your pupils will adjust to the reduced light.
Rich
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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I've been suffering from such ACUTE blurred vision over this past weekend that this has not been the best time to conduct any kind of experiments involving my eyes .
I'm currently having SERIOUS trouble trying to read ANY of these posts , even though , as always , the font is set to LARGE and THE BLUES , which I've always found the easiest to read .
I think a PM to Peter ( pcad ) is in order , forthwith !
Regards , Kenny
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Milton Wilcox R.I.P
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 2447
Loc: Connecticut
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Kenny,
PM sent.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 12 - 100 mm
Microscope 50x - 1000x
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Steve Napier
Carpal Tunnel
Reged: 05/10/04
Posts: 1559
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What has brought this on Kenny and,can you see alright now?
Im pleased I don"t have my own PC as Im sure it would forward the deteriation of my eyes.
I think you"re on the PC FAR too much for your own good Kenny. I know I must sound like your mother when you were young but,Im doing it for the good of your eyes.
You will thank me one day.
Steve.
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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Peter ,
Thanks for your VERY informative PM !
Thanks also to you , Steve , for your concern ( a REAL dark horse you are :-) ) ,
I'm SURE too many hours looking at a PC screen is no good for ANYONE'S eyesight --
-- but in THIS case , I'd been AWAY from a PC screen for about 74 hours !
I woke up this morning and my eyesight was restored to it's " normal service " .
Last night , when I wrote the above post , I could ONLY JUST raed the PC screen , could NOT read the TV teletext , even with glasses on , and even with my usually " super clear " TWIN pair of prescription glasses on , I was seeing TWO crescent moons in the north west sky , one slightly to the lower right of the other .
This was the same with EITHER or BOTH eyes .
I can only put this down to having read TWO 300 plus page books over the weekend , with poor quality print , then driving home in very bright sunlight ( with tinted prescription glasses worn ) .
The journey home was unusually stressful and tedious due to very heavy bank holiday motorway traffic , and took 3 hours , as opposed to the usual 90 minutes .
How RELIEVED I felt this morning though .
SEVERE PERMANENT eyesight problems must an ailment from hades for people like ourselves !
P.S -- advanced apologies to Phil W. for THIS post being unrelated to PERCEIVED BRIGHTNESS ! :-)
Regards , Kenny
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Milton Wilcox R.I.P
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Rich N
Post Laureate
Reged: 09/22/04
Posts: 5618
Loc: San Francisco Bay Area, Calif...
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Hi Kenny,
Sounds like it's time to see the eye doctor. I hope it is nothing serious and easily corrected.
All the best,
Rich
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Rich N
Post Laureate
Reged: 09/22/04
Posts: 5618
Loc: San Francisco Bay Area, Calif...
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Hi Kenny,
Out posts went out about the same time. I'm glad your eyes are better today.
One reason I perfer using a binoviewer when doing planetary observing for extended periods, I found that when using only one eye the eye would get "tired" and not want to focus.
All the best,
Rich
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