clop
member
Reged: 07/15/06
Posts: 11
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Hi EdZ,
Like seemingly half the population of this forum, I am trying to decide between the Canon 10x42L IS and the Canon 15x50 IS binoculars. Unfortunately I'm unable to test the models together under the night sky, so I'm trying to make my decision based on other people's advice and opinions.
I read your article about the relationship between BLM, magnification and aperture (that is a fantastic piece of writing by the way), and I understand that a) the brightness of point sources is unaffected by magnification and b) the contrast of point sources increases with magnification. This makes the 15x50's an obvious choice for viewing star clusters and double stars.
What I would like to know is how the magnification and aperture affect the brightness (or contrast) of non-point source objects. If I were to look a smeared out target, such as M31, would the image be brighter and easier to see in 10x42 or in 15x50? If magnification dilutes the lightness of the background sky, surely it will also dilute the lightness of the non-point source? And make it harder to see?
Thanks and regards,
clop
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camvan
Carpal Tunnel
Reged: 05/02/05
Posts: 2142
Loc: British Columbia
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this is an interesting question and I wish I understood the mechanics (read math, read laws of physics) behind it better. there has been some interesting discussion about this in the case of 7x50's that are stable (due to larger exit pupil) vs. 10x50's that aren't stable (due to high magnification and unsteady hands) and which would let you 'see' better.
mathematically the 10x50's win, but in real world practice (I still hold fast to this, being it's not something you can figure mathematically) something that's not shaking and jittering would/should let you see better, just not necessarily farther/deeper.
your situation is different (two different powers, but also two different objective sizes) but I'm thinking the 10x32's could let you see more because of the larger exit pupil. 4.2mm is still small enough that most eyes should dilate to that size and 4.2mm lets more light in then 3.3mm, even if the objective is 50mm.
of course, I admit my understandings behind these matters are still pretty rudimentary, if even non-existent (to my shame) and I could be wrong (won't be surprised if I am)...so I look forward to EdZ response! maybe I will clue in finally 
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Cameron
"Aperture can only be replaced by even more aperture. Dark transparent skies cannot be replaced by anything else." - Stathis Kafalis
Intes MN66
Meade SN8
handfull of cheap ep's
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clop
member
Reged: 07/15/06
Posts: 11
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Thanks Cameron, but does a 4.2mm exit pupil always let more light in than a 3.3mm exit pupil? Maybe the 3.3mm exit pupil has a higher concentration of light per unit area? I also wonder how big the fovea is. Since (colour) cones are less sensitive to light than (black and white) rods, and the cones are concentrated in the fovea, it might be beneficial to make the image on the retina bigger than the fovea (like averted viewing). I would have thought that a retinal image smaller than the fovea would be inefficient. But then, I don't know how big the fovea is, or how the exit pupil relates to the size of the image in the eye.
clop
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camvan
Carpal Tunnel
Reged: 05/02/05
Posts: 2142
Loc: British Columbia
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as far as I know, a larger exit pupil lets more light in then a smaller. what I don't know is how that may relate exactly to having more aperture with higher magnification.
you WILL see deeper with the 15x50's, even with the smaller exit pupil, your 15x magnification guarantees that.
it does get confusing, eh? despite the smaller exit pupil, you'll see deeper because of the higher magnification. hard to correlate that with the fact the image in the 10x42's being brighter. 
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Cameron
"Aperture can only be replaced by even more aperture. Dark transparent skies cannot be replaced by anything else." - Stathis Kafalis
Intes MN66
Meade SN8
handfull of cheap ep's
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KennyJ
The British Flash
Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
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clop ,
Yours is indeed an interesting question , which I'm sure EdZ will answer for you .
However , for now , there is something about your choice of wording in " dilutes the lightness " which I think could probably be improved upon , not just for your own , but for clearer understanding by any other readers .
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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to understand this question read all the materials posted or linked in the Best Of that refer to Surface Brightness.
Surface Brightness of Faint Extended Objects
People often confuse brighter image and more light. These are very far from the same thing. Let me give you an example, going to the extremes, but sometimes going to the extremes helps others to understand some concepts.
We often hear some people say the larger exit pupil lets in more light. Well, how about if I handed you a 8x32 and a 33x100. The 8x32 has the larger exit pupil at 4mm vs the big binocular with only a 3mm exit pupil. That means the 8x32 has a brighter image. Do you think that also means the 8x32 lets in more light and will see more?
Well, of course, the answer is no, it will not. Bigger aperture gathers more light. The 100mm binocular gathers 10 times more light than the 32mm. Even if bigger aperture is used at smaller exit pupil, it may not be a brighter image, but it sure as heck will have gathered a lot more light and show much deeper or fainter objects. The 8x32 has a larger exit pupil and it has a brighter image, but it can only show the light it has gathered, and that is only 1/10th the total amount of light gathered by the 100mm binocular. So which one shows a brighter image and which one shows more light? I think you know now.
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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patter1
professor emeritus
Reged: 01/19/05
Posts: 597
Loc: Canada
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Quote:
What I would like to know is how the magnification and aperture affect the brightness (or contrast) of non-point source objects. If I were to look a smeared out target, such as M31, would the image be brighter and easier to see in 10x42 or in 15x50? If magnification dilutes the lightness of the background sky, surely it will also dilute the lightness of the non-point source? And make it harder to see?
clop, you're correct. Larger exit pupil = higher surface brightness for extended objects and equally so for the background sky (assuming your eye pupils are large enough). Higher magnification will often help on extended objects simply due to the larger image scale. But if the object is very large, like M31 or M33, there's such thing as too much magnification (not enough FOV) - you usually need to have enough surrounding sky to properly 'see' the object. Unless, perhaps, if you're looking for detail within the object. The right magnification becomes largely a matter of personal preference of how you want to view that extended object.
So for M31, It's very possible you'll find the 10x42 gives more visibility...I probably would. For most other, smaller objects, the 15x50. The best thing would be to see/compare for yourself...words are no substitute for actually seeing the differences for yourself, and seeing what you prefer. But since you say you can't, maybe you could compare two mounted, regular (non-IS) binocs to get a 'feel' of the differences in the image. Perhaps a 10x42 or 10x40 vs. a 16x50.
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Patrick
8" f/6 NewStar dobsonian
Orion Starblast 4.5" f/4 mini dobsonian
42mm SuperView, 17mm Nagler T4, some other cheapies
Omcon 7x50, Oberwerk 11x56, Olympus DPS-R 7x35, Olympus Magellan 8x25
homemade 50mm right-angle bino-scope prototype
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Quote:
What I would like to know is how the magnification and aperture affect the brightness (or contrast) of non-point source objects. If I were to look a smeared out target, such as M31, would the image be brighter and easier to see in 10x42 or in 15x50? If magnification dilutes the lightness of the background sky, surely it will also dilute the lightness of the non-point source? And make it harder to see?
clop, you're correct. Larger exit pupil = higher surface brightness for extended objects and equally so for the background sky (assuming your eye pupils are large enough). Higher magnification will often help on extended objects simply due to the larger image scale. But if the object is very large, like M31 or M33, there's such thing as too much magnification (not enough FOV) - you usually need to have enough surrounding sky to properly 'see' the object. Unless, perhaps, if you're looking for detail within the object. The right magnification becomes largely a matter of personal preference of how you want to view that extended object.
So for M31, It's very possible you'll find the 10x42 gives more visibility...I probably would. For most other, smaller objects, the 15x50. The best thing would be to see/compare for yourself...words are no substitute for actually seeing the differences for yourself, and seeing what you prefer. But since you say you can't, maybe you could compare two mounted, regular (non-IS) binocs to get a 'feel' of the differences in the image. Perhaps a 10x42 or 10x40 vs. a 16x50.
Patrick, this answer fairly well addresses how magnification affects the view. But this is how you should think of magnification in terms of the affect of magnification when you have a constant aperture. This does not address how the view changes as you increase aperture. And obviously, when you increase aperture, you gather more light. that is explained in the Best Of link I pointed to on the subject of Surface brightness.
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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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clop
member
Reged: 07/15/06
Posts: 11
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Thank you everyone.
clop
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