Telescope Reviews: Re: What does the 'Visibility Index' tell you?

I would agree you will see plenty with a 20x90 combination. But how they will rank compared to others is not only dependant on the hard numbers of magnification and aperture, but also quality. More on that in a minute.

How true it is, we can't have it all ways. You must always consider what you will be using the instrument for. When I'm out viewing one of several indexed open clusters that I use for recording BLM (limiting magnitude), I often have out with me an 80mm scope. I can use that scope at 40x or 50x and see far deeper than even a 100mm binocular. So, yes, magnification gives a tremendous added benefit for seeing point sources. But as stated above, it would be somewhat impracticle to have a fixed power binocular with real high magnification. A 40x80 binocular would be great for seeing deep into clusters, but it would fall down to a 25x100 on faint extended objects.

I do believe we have Roy Bishop ?RASC? to thank for the term Visibility Index. We do have both Bishop and Alan Adler to thank for presenting indices in different ways, and as you know, I prefer the Adler Index.

Once again, let me include that link here
Is aperture king? Does aperture rule?
where you will find a good basic comparison of these two indices.

Following in the footsteps of Bishop and Adler, the acronym that I coined was BPI, Binocular Performance Index, which is Adler index adjusted for quality.

What the BPI attempts to do is rank binoculars dependant on their performance characteristics. A normal binocular gets no adjustment. The binoculars that exhibit the best attributes, contrast, transmission, lack of aberration, get adjusted up. Binoculars that exhibit some aberrations or are not well corrected for contrast and transmission, get adjusted down.

Premium coatings and full exit pupil light distribution contribute to greater total light transmission. Well corrected lenses, high polished finish, premium baffels and premium coatings and premium eyepieces contribute to lower scatter and high contrast and better resolution. Transmission, Contrast and Resolution (these are related in various ways) are considered attributes that will have an overall additive or deductive contribution to performance. I have arbitrarily assumed to use a factor for two of these that would either contribute or deduct 10% from the overall index. I used Adler Index (mag x sqrt aperture) as the base.

I would rank the BPI of two different binoculars as such:

Nikon SE 12x50
premium coatings, premium baffles, low scatter, high contrast
lack of aberrations, high resolution, high transmission
Bishop = 12 x 50 = 600
Adler = 12 x sqrt 50 = 85
EdZ BPI = 85 +10% for contrast +10% for transmission = 103

Barska 15x70 - Celestron 15x70 - 2002(1st) version Oberwerk 15x70
less than normal coatings, more scatter, lower contrast,
higher than normal aberrations, lower resolution, lower transmission
Bishop = 15 x 70 = 1050
Adler = 15 x sqrt 70 = 125
EdZ BPI = 125 -(minus)10% for contrast -10% for transmission = 100

In a series of performance tests in which I compared all the results of many binoculars (and in continued tests since the original article), the Nikon SE 12x50 performed nearly the equal of my original Oberwerk 15x70. That would seem to agree with the value indicated by the BPI adjustments to the Adler index. Had the binoculars not been adjusted for quality, Bishop's VI shows the 15x70 with a 75% advantage, the Adler Index would show the lower quality 15x70s as the clear top performer with a 50% advantage. Frankly, nothing could be more misleading. The BPI based on quality, the great equalizer, shows the superior quality of the 12x50SE equalizes the performance.

Neither Bishop's Visibility Index nor Adler's Index would have ever shown that quality performance relationship. Hence, the BPI is a better model for ranking the real performance characteristics of not only the magnification and aperture, but also now including the quality. Unfortunately, it is not simple.

You can find the complete article here
How to Understand Binocular Performance
http://www.cloudynights.com/documents/performance.pdf

edz

Edited by EdZ (01/03/07 09:02 PM)

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