cpsTN
Carpal Tunnel
Reged: 04/26/07
Posts: 2387
Loc: Rutherford Co, TN
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You are observing and your eye is dialated to 4mm. What would be the difference if you are using binos of 8x32 or 25x100. I understand that the 100s gather about 6x more light than do the 32s, but if the exit pupil is 4mm for each, wouldn't they seem to be the same brightness? If this is the case, what accounts for the extra resolution given by the 25x100s?
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Charles Sands
Observing since 12/29/86
12" f/5 Apertura AD12 Dobsonian
90mm f/13.9 Sky Watcher Mak Cass
70mm f/5 refractor, Orion GoScope
10x50 binos (Bushnell Falcon)
7x35 binos (Sears Discoverer)
Orion StarShoot Solar System Imager IV
Sky & Telescope's POCKET SKY ATLAS
CyberSky 4.0 astronomy software
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Never underestimate the power of stupid people in large groups - Lewis Grizzard (1946-1994)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
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Quote:
You are observing and your eye is dialated to 4mm. What would be the difference if you are using binos of 8x32 or 25x100. I understand that the 100s gather about 6x more light than do the 32s, but if the exit pupil is 4mm for each, wouldn't they seem to be the same brightness? If this is the case, what accounts for the extra resolution given by the 25x100s?
the fact that they are the same brightness doesn't address the fact that 100mm has 4 times finer resolution and gathers almost 10x (not 6x) more light than 32mm. That's 2.5 magnitude fainter. Not to mention 25x allows you to see objects 3x smaller than 8x as the same apparent size. There is so much more light delivered through the 100mm lens that the contrast threshhold is dramatically lower.
Read the Best Of section on Surface 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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patter1
professor emeritus
Reged: 01/19/05
Posts: 597
Loc: Canada
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brightness in 8x32 vs. 25x100
Both binocs will show the same surface brightness on anything other than a point source (anything other than individual stars). But individual stars will be much brighter in the 25x100s because each star image is being formed with 100mm of aperture rather than 32mm. Even if you were comparing an 8x32 to a 40x100, the latter will still show stars much brighter, even though its exit pupil is much smaller.
And then there's the benefits of greater magnification that Ed mentions.
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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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cpsTN
Carpal Tunnel
Reged: 04/26/07
Posts: 2387
Loc: Rutherford Co, TN
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Thank you both. After I posted, I realized I figured wrong with the LPG. I added the two 3.125s (and got 6) rather than squaring them to get 9.xx. 
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Charles Sands
Observing since 12/29/86
12" f/5 Apertura AD12 Dobsonian
90mm f/13.9 Sky Watcher Mak Cass
70mm f/5 refractor, Orion GoScope
10x50 binos (Bushnell Falcon)
7x35 binos (Sears Discoverer)
Orion StarShoot Solar System Imager IV
Sky & Telescope's POCKET SKY ATLAS
CyberSky 4.0 astronomy software
--------------------
Never underestimate the power of stupid people in large groups - Lewis Grizzard (1946-1994)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
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Quote:
brightness in 8x32 vs. 25x100
Both binocs will show the same surface brightness on anything other than a point source (anything other than individual stars). But individual stars will be much brighter in the 25x100s because each star image is being formed with 100mm of aperture rather than 32mm. Even if you were comparing an 8x32 to a 40x100, the latter will still show stars much brighter, even though its exit pupil is much smaller.
And then there's the benefits of greater magnification that Ed mentions.
[Both binocs will show the same surface brightness on anything other than a point source ]
The 25x100, by nature of it's greater light gathering ability will be able to see potentially to about Surface brightness (Sb)= 15.0. The 8x32 at best might reach about Sb=12 (Both given moderate sky conditions). So for faint extended objects, that's almost light night and day. Thats' roughly the difference of seeing M101 versus M65/M66.
The image brightness of the Sb15.0 galaxy thru the eyepiece in the 25x100 will appear to have the same image brightness of the Sb12.0 galaxy thru the 8x32 binocular. Hence, while image brightness (exit pupil) in both will appear the same, the larger, higher powered binocular is showing much fainter Sb objects at that image brightness.
See Not only the Surface brightness Article, but also the Best Of thread on what can be seen with different sizes.
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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