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I've seen this rule of thumb repeated recently here and in other forums, and I wonder if it's really true for binoculars. Is aperture king? Does aperture rule? Or is that a simplistic and misleading assertion, verging on a myth, in relation to binoculars (that's carried over unthinkingly from telescopes)?

Of course I believe that besides optical quality (obviously!), BOTH magnification and aperture must be considered. I also think that the user's observing location(s) is(are) relevant. If a person never goes to a dark-sky site or prefers to observe the Moon more than anything else, should he or she get an 8x56 or an 8x40 (or even an 8x32)?

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Ed Cannon - Austin, Texas, USA

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Ed, in answer, your statement "Of course I believe that besides optical quality (obviously!), BOTH magnification and aperture must be considered." is absolutely correct.


Same topic has come up 3-4 times in the last two weeks.

Various noted individuals over the years have adopted scales that attempt to represent relative performance of binoculars, based on some ratio of magnification and aperture. Roy Bishop created an index that is Mag x Aperture. Alan Adler created an index that is Mag x Sqrt Aperture. I created an index call the Binocular Performance Index (BPI) see How to Understand Binocular Performance (Feb. 2003) that adapted a scale based on the Adler Index (mag x sqrt ap), but then applied factors based on quality (coatings, contrast, polish, light cutoff, etc.). While I have always been a stong supporter of magnifiaction having greatest influence, that can be very misleading. In my opinion, it is of far greater importance to consider the intended targets to then determine on which you should place greater emphasis.

(For purposes of this discussion, I am assuming all instruments of eqiual quality. My BPI would give the same result as Adler Index.


Some examples:

Bishop Visiblity Index
20x60 = 1200
15x80 = 1200

Adler Index
20xsqrt60 = 155
15xsqrt80 = 135

The magnification gain from 15x to 20x will shows stars 0.3 to 0.4mag deeper.
However the aperture gain from 60mm to 80mm gives an almost exactly equal 0.3-0.4mag. gain in limiting magnitude of stars that can be seen. Stop there and you would be convinced that a 15x80 and a 20x60 are equal, and as far as seeing faint stars goes, that would be true, they are. But you can't stop there. Suppose you want to view extended objects such as nebula?

The 15x80 will produce images that are brighter. First consider the 80mm aperture vs the 60mm aperture. Light gathering increases by 6400/3600 = 77%. No matter how you look at it, the 80mm aperture gathers more light. It will be able to see fainter extended objects.

But also consider the apparent brightness due to the significantly larger exit pupil. The 15x80 has an exit pupil of 5.33mm, while the 20x60 has only a 3mm exit pupil. The 20x60 will have much deeper apparent contrast, but it doesn't gather as much light. So, even though the 80mm gathers more light, it gathers that light from ALL objects, including the sky background. The 15x80 will perform better on extended objects if used under darker skies.

So, it very much depends on what you are trying to see. Both will see equally faint stars (Bishop's index shows this equality); The 20x60 would be better for observing close doubles or dense open clusters (Adler's index shows this gain by magnification); However, due to the larger aperture in the 15x80, it would be superior for observing faint extended objects such as face-on galaxies (neither index correctly shows this).


Another example:

Bishop Visiblity Index
16x70 = 1120
10x70 = 700
12x50 = 600

Adler Index
16xsqrt70 = 134
10xsqrt70 = 84
12xsqrt50 = 85

In this instance, I compare two Fujinons to a Nikon SE12x50. The gain from 10x to 16x will shows stars 0.5 to 0.6mag deeper. The gain from 10x to 12x will shows stars at least 0.2 to 0.25mag deeper. However the gain from 50mm to 70mm aperture is almost equal about 0.3mag.

We could be convinced that a 10x70 and a 12x50 are nearly equal, and as far as seeing faint stars goes, that would be true, they are. However, the 10x70, assuming you can use all of the 7mm exit pupil, will gather 100% more light, a better choice for faint extended objects. It will produce brighter images. That may not really be a good thing if used most often under mag 5 or brighter skies, since images would be washed out by the bright sky background. But take that 10x70 out to a near mag 6 sky and it's almost like it's a different instrument.

Comparing the Fujinon 10x70 to the Fujinon 16x70: My studies show for a sky conditions difference of 1 mag (from about mag 4.8 up to mag 5.8) the Fujinon 16x70 gains in what can be seen from mag 10.7 up to mag 11.0. However, for the same sky conditions range the 10x70 was found to show an increased limit of stars from mag 10.0 up to about mag 10.7. The 10x70 falls off expectations when used under brighter skies. In fact, it doesn't do much better than a 10x50, and no where near what would be expected from 100% more aperture.

So, not only does it depend on your conditions, but also it very much depends on what you are trying to see. Both 10x70 and 12x50 will see equally faint stars. But the 16x or 12x would be better for observing close doubles or dense open clusters due to gains by magnification. However, due to the larger aperture and much larger exit pupil in the 10x70, it would be better than the 12x50 for observing faint extended objects such as face-on galaxies, but only under darker skies.

Magnification (Adler index) will have greater emphasis IF your targets are stars. Aperture and exit pupil will have greater emphasis if your targets are extended objects such as faint nebula and galaxies.

edz

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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21

Edited by EdZ (12/19/06 12:26 PM)

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