Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

relative brightness or power of magnification

binoculars
  • Please log in to reply
12 replies to this topic

#1 kootenay49

kootenay49

    Lift Off

  • -----
  • topic starter
  • Posts: 1
  • Joined: 10 Nov 2019

Posted 10 November 2019 - 06:51 PM

I am looking to purchase a pair of binoculars for viewing the night sky.  I have been considering the Nikon Prostaff 10x42, the Celestron Skywatcher 8x56 and the Nikon Extreme 7x50. 

 

I remember something from an astronomy course many years ago about the importance of a large lense diameter to maximize the light transmission to the eye.  I already have a pair of 7 power binoculars (Bushnell Custom Compact 7x26), so thought 10 power binoculars might be a better choice.  I think my question to the forum is: what is more important for night sky viewing, relative brightness or magnification power?

 

Also, I read that as we age our pupil does not expand past 5mm.  How might this influence my choice?



#2 Yarddog

Yarddog

    Explorer 1

  • -----
  • Posts: 75
  • Joined: 20 Aug 2019

Posted 10 November 2019 - 07:23 PM

I have the Nikon Prostaff in both 10x42 and 8x42. They both are good for the price. Either one would be OK. The action extreme in 7X50 would be a bit brighter and maybe a hair sharper but you might not notice the difference.

 

I am not familiar with the Celestron.

 

I might add that I also have the Aculons in 8X42 and 10X42. They seem to be about as good for less money.



#3 DeanD

DeanD

    Explorer 1

  • -----
  • Posts: 98
  • Joined: 05 May 2018
  • Loc: South Australia

Posted 10 November 2019 - 08:20 PM

They will all give nice views, but I would suggest aiming at around a 5mm exit pupil. Say the Action Extreme 10x50's (or their Aculon 10x50's). You are right in that our pupils don't open as wide as we age, and 7mm exit pupils like in the 7x50's or 8x56's mean that some of the light is wasted.

 

Using 10x optics means that the view will shake a bit more than 7x, but they are still quite easily hand-held, especially if you lean against a support. 

 

As for your question: both aperture and power influence the view! Some experts say the easiest way to quantify a comparison of suitability for astronomy is to work out a "visibility factor" by multiplying the aperture by the magnification. So 10x50's = 500, which is better than 7x50's = 350 (Have a look at this S&T article for some useful info: https://www.skyandte...-in-binoculars/ ). There are lots of other ideas out there, but this one seems to work pretty well as a great place to start.

 

All the best,

 

Dean



#4 hallelujah

hallelujah

    Cosmos

  • *****
  • Posts: 7508
  • Joined: 14 Jul 2006
  • Loc: North Star over Colorado

Posted 10 November 2019 - 09:20 PM

https://www.cloudyni...ere/#entry44966

 

Stan



#5 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 79507
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 11 November 2019 - 08:29 PM

You are right in that our pupils don't open as wide as we age, and 7mm exit pupils like in the 7x50's or 8x56's mean that some of the light is wasted

 

I have read this many times but in my old age I have come to realize that we are individuals and it's best not to generalize too much.. One study of dark adapted pupil versus age included 30 people in their 60's, the average dark adapted pupil was 5.6 mm with a range from 3.5mm to 7.5mm.  That's quite a range.  I recommend measuring your dark adapted pupil diameter. 

 

In any event, even for someone with a 7mm plus dark adapted pupil, in most situations, 10x50s will show more than 7x50s.. 10x70s will show more than 10x70s but 15x70s will show more than 10x70s.  These are all based on personal experience with very similar binoculars..  (I am that person in the age group study with a large diameter pupil)

 

One rule of thumb is that binocular equivalency is determined by M x D1/2   Magnification is more important than aperture.

 

Jon


  • hallelujah likes this

#6 Swedpat

Swedpat

    Apollo

  • *****
  • Posts: 1452
  • Joined: 18 Feb 2005
  • Loc: Boden, Sweden, Scandinavia

Posted 13 November 2019 - 03:51 PM

One rule of thumb is that binocular equivalency is determined by M x D1/2   Magnification is more important than aperture.

 

Jon

If I recall correct this is also called "Adler's index".

I am interested in what you and other perceive in this matter. A 8x32 has ~45 while a 10x25 has 50. Do you consider 10x25 to be better than 8x32?

7x50 has 49,5, is it then similar in practice to the 10x25?

I have not done a careful comparison between these configurations. My thought still is that 8x32 at the most objects outperforms 10x25 and that 7x50 strongly outperforms 10x25(8 times the relative brightness is a very significant difference).

10x50 has 70,7. Is it only ~41% better than 10x25? My experience is that 4 times the brightness should result in more than 1,41x better performance.

Yes, I know this also depends on the kind of objects and atmospheric conditions, but my feeling is that magnification x aperture generally is a better way to compare the visual difference of performance at night sky. Based from that formula 10x50 is twice as good as 10x25 and I think it is at least.

Therefore I think it's more correct to state that magnification is more important than area of the aperture.


Edited by Swedpat, 13 November 2019 - 04:21 PM.


#7 KennyJ

KennyJ

    The British Flash

  • *****
  • Posts: 36424
  • Joined: 27 Apr 2003
  • Loc: Lancashire UK

Posted 13 November 2019 - 05:12 PM

Patric,

 

For what it's worth, I happen to agree with most points you made in your post above.

 

Using an even more extreme example, based upon practical observations, no one will ever convince me that a 21x 25mm  binocular image of any object ( yielding a factor of 105 on the Adler Index scale ) is "roughly equal" in any way to looking through a 15x50 ( also 105 on the Adler Index scale ).

 

Indeed, ( and however ) even the "Bishop Index" ( of 21 x 25 = 525)  versus (15 x 50 = 750) in no way comes close to being truly representative of the real world superiority of the 15x50 in comparison.

 

When this binocular group of the day were discussing these issues back in the period around 2004 - 2006  ( many extracts of which can still be read in the "Best Of" archives compiled by Ed Zarenski ) I gave the matter a lot of thought for a long time, and came close to providing an alternative formula that took EXIT PUPIL much more into account as an extra, significant ingredient factor, and cannot for the life of me remember now why I didn't go ahead with the suggestion.

 

I seem to recall being unsure whether to MULTIPLY either the Bishop or Adler figure by the exit - pupil, or multiply one or the other by the square root of the exit-pupil, or something along those lines, anyway. smile.gif

 

Kenny


Edited by KennyJ, 13 November 2019 - 05:18 PM.

  • Swedpat likes this

#8 Tony Flanders

Tony Flanders

    Hubble

  • *****
  • Posts: 17079
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 13 November 2019 - 08:46 PM

Using an even more extreme example, based upon practical observations, no one will ever convince me that a 21x 25mm  binocular image of any object ( yielding a factor of 105 on the Adler Index scale ) is "roughly equal" in any way to looking through a 15x50 ( also 105 on the Adler Index scale ).


They're certainly not equal -- roughly or otherwise. They're two very different instruments, and each can do things that the other can't. Assuming that they're both mounted, of course. Without a mount, most of the 21x of the 21x25 binoculars is going to go to waste.


  • Jon Isaacs and Swedpat like this

#9 Swedpat

Swedpat

    Apollo

  • *****
  • Posts: 1452
  • Joined: 18 Feb 2005
  • Loc: Boden, Sweden, Scandinavia

Posted Yesterday, 04:25 AM

Patric,

 

For what it's worth, I happen to agree with most points you made in your post above.

 

Using an even more extreme example, based upon practical observations, no one will ever convince me that a 21x 25mm  binocular image of any object ( yielding a factor of 105 on the Adler Index scale ) is "roughly equal" in any way to looking through a 15x50 ( also 105 on the Adler Index scale ).

 

Indeed, ( and however ) even the "Bishop Index" ( of 21 x 25 = 525)  versus (15 x 50 = 750) in no way comes close to being truly representative of the real world superiority of the 15x50 in comparison.

 

When this binocular group of the day were discussing these issues back in the period around 2004 - 2006  ( many extracts of which can still be read in the "Best Of" archives compiled by Ed Zarenski ) I gave the matter a lot of thought for a long time, and came close to providing an alternative formula that took EXIT PUPIL much more into account as an extra, significant ingredient factor, and cannot for the life of me remember now why I didn't go ahead with the suggestion.

 

I seem to recall being unsure whether to MULTIPLY either the Bishop or Adler figure by the exit - pupil, or multiply one or the other by the square root of the exit-pupil, or something along those lines, anyway. smile.gif

 

Kenny

Kenny,

 

I calculated a bit on it and find multiplying the Adler or Bishop idex numbers by the square rot of exit pupil very interresting.

Multiplying AI and BI by the exit pupil makes no sense though; all configurations of same aperture within each index get the same rating undependent of magnification.

 

With BI magnification and aperture have the same importance. Multiplying AI with square root of EP 10x50 gets twice the value as 10x25. Multiplying BI with square root of exit pupil 10x50 gets ~2,8x the value as 10x25. Multiplying AI with square root of exit pupil 20x50 gets ~2,8x the value as 10x25. And multiplying BI with square root of exit pupil 20x50 gets 4x the value as 10x25.

If we compare 21x25 with 15x50 multiplying square root of EP gives:

AI: 114,5/193,5 ~ 1,69times difference

BI: 573/1369 ~2,39 times difference

 

It's a known fact that the eyes perception of brightness differences isn't linear and a common belief is that 4 times brighter is perceived as twice. This matter is now and then discussed in the flashlight community I participate. While I can agree that 4 times brighter often does not feel like 4 times I still perceive it feels more than 2 times. Therefore the 2,8times difference between 10x50 and 10x25 by multiplying BI with square root of EP seems to be very interesting. I am leaning towards the belief that multiplying BI with square root of EP may be the best way to measure performance under pretty clear sky.

But the more polluted the sky is the more BI without square root of EP, or AI may be applicable.

 

Patric


Edited by Swedpat, Yesterday, 04:44 AM.


#10 Swedpat

Swedpat

    Apollo

  • *****
  • Posts: 1452
  • Joined: 18 Feb 2005
  • Loc: Boden, Sweden, Scandinavia

Posted Yesterday, 05:00 AM

They're certainly not equal -- roughly or otherwise. They're two very different instruments, and each can do things that the other can't. Assuming that they're both mounted, of course. Without a mount, most of the 21x of the 21x25 binoculars is going to go to waste.

There is one thing I think 21x25(providing they are of enough high quality(which I don't think any pocket binocular of ~20x magnification is) can do better than as well 7/10/15x50s: observing the moon and the planets with their moons. The small exit pupil on these bright objects dims the light and produces sharper images.



#11 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 79507
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted Yesterday, 06:42 AM

My own thinking is that the Adler index in meant to compare "reasonable" binoculars, that is binoculars with relatively large exit pupils so that they're not hurting for brightness. 

 

I'm not sure if one can buy 21x25s but I've never seen a pair. 

 

It might be useful in answering a question like:

 

"I have a pair of 10.5 x 70 binos. At what aperture would 12x binos provide roughly comparable views?"

 

The Adler index says 53.6 mm.

 

Or, "which shows more 7x50s or 8x42s?"

 

The Adler index says the 8x42s would show slightly more.

 

Jon



#12 Tony Flanders

Tony Flanders

    Hubble

  • *****
  • Posts: 17079
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted Yesterday, 07:15 AM

My own thinking is that the Adler index in meant to compare "reasonable" binoculars, that is binoculars with relatively large exit pupils so that they're not hurting for brightness.

I agree. And for all-purpose astronomical binoculars, I would define "reasonable" as an exit pupil of 3 mm or wider. The 15x50s are already skirting pretty close to the edge of reasonable by that definition.

 

Also, apertures smaller than 30 mm are really pretty marginal for astronomy. I recently bought a pair of Pentax Papilio 6.5x21s, and while they are stupendous for terrestrial use, I find them a little frustrating for astronomy -- despite having almost exactly the same exit pupil as aforesaid 15x50s, which are magnificent astro binoculars.

 

Some specifications that fall clearly within the range of "reasonable astronomy binoculars" to my mind are: 7x35 through 7x50, 8x32 through 8x56, 10x30 through 10x70, 15x45 through 15x80. For such binoculars, I find the Adler index useful. But it still doesn't say that two different pairs are equivalent. More like that they have similar limiting stellar magnitudes, which is quite a different statement.


  • Jon Isaacs likes this

#13 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 79507
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted Yesterday, 08:45 AM

I agree. And for all-purpose astronomical binoculars, I would define "reasonable" as an exit pupil of 3 mm or wider. The 15x50s are already skirting pretty close to the edge of reasonable by that definition.

 

Also, apertures smaller than 30 mm are really pretty marginal for astronomy. I recently bought a pair of Pentax Papilio 6.5x21s, and while they are stupendous for terrestrial use, I find them a little frustrating for astronomy -- despite having almost exactly the same exit pupil as aforesaid 15x50s, which are magnificent astro binoculars.

 

Some specifications that fall clearly within the range of "reasonable astronomy binoculars" to my mind are: 7x35 through 7x50, 8x32 through 8x56, 10x30 through 10x70, 15x45 through 15x80. For such binoculars, I find the Adler index useful. But it still doesn't say that two different pairs are equivalent. More like that they have similar limiting stellar magnitudes, which is quite a different statement.

:waytogo:

 

Tony:

 

Well said. You should be a writer... :)

 

You mention 10x30s, 15x45s and 15x50s, these are all popular models of the Canon IS binoculars and point to the value of a stable mount/image when using higher magnifications in a smaller aperture binocular. Seeing faint stars and details requires a steady image.

 

Binoculars are generally overly bright for eeking out fine details, greater magnification and smaller exit pupils most often is the most effective combination.

 

Jon




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: binoculars



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics