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Exit Pupil Size and Binocular Performance

Started by John Finnan, 04/12/2003 03:56PM
Posted 04/12/2003 03:56PM Opening Post
Ed Zarenski wrote: "My own experience with binoculars in more than one side by side, all 80mm and less, is that a higher quality binocular at higher magnification but a smaller or equal aperture, will outperform the larger aperture that has a lower magnification".

Ed - This has been my experience as well. About 10 years ago I bought a pair of 10 x 70 Fujinons because they were highly recommended in The Backyard Astronomer's Guide. They were very sharp and clear and overall an excellent binocular. However, a few months later I got a chance to try a pair of 12 x 50 Aus Jena Nobilems and was amazed to find that on 19 out of 20 objects that I compared them on, the 12 x 50s gave me better views than the 10 x 70s.

Obviously, magnification plays a role in this. But, what about exit pupil size? The Adler Index for a pair of 12 x 50s is 85 and it is 84 for the 10 x 70s. But I found the difference between those two to be much more significant. If I was told the 12 x 50s had a Adler Index of 85 and was then asked to guess what it was for the 10 x 70s were I would have estimated around a 75.

But the other factor that plays a role in this is exit pupil size. The 10 x 70s exit pupil is 7.00mm, the 12 x 50s is 4.17mm. And my own personal entrance pupil at that time was around 5.00 - 5.50mm. Now it has further shrunk to about 4.25 - 4.50mm (I use Sky & Telescope's Pupil Guide to estimate this). Anyway, for purpose of making the example clearer, let's say that my entrance pupil was 5.00. That means when I'm using the 10 x 70s (with their 7.00mm exit pupil) half of the light that is being collected by the binocular objectives is not reaching my eye. So my smaller entrance pupil is functioning like an aperture stop on a telescope. And, when you cut the amount of delivered light by half, those 10 x 70s should be performing (for someone like me) like a pair of 10 x 50s. And, if that is the case, then it is clear why the 12 x 50s are delivering 100% of the light that they are collecting (and at a higher magnification) should be outperforming the 10 x 70s.

Anyway, what I just wrote was "conventional wisdom" 10 years ago. Do you know of any reason why that type of user entrance pupil/binocular exit pupil type of analysis should not be taken into consideration when trying to numerically estimate how well one binocular should perform in comparison to another when used by a particular observer?

Posted 04/12/2003 05:17PM #1
Hi John,

I'm not positive what my pupils measure. I'd like to find out.

Each of these issues you raised needs to be addressed.
magnification plays a role in this
what about exit pupil size?
smaller entrance pupil is functioning like an aperture stop
user entrance pupil/binocular exit pupil analysis should be taken into consideration when trying to numerically estimate how well one binocular should perform in comparison to another

In addition to the notes above from your post, any index should take into consideration the qualities of the binocular. A binocular index needs to include a factor for other quality attributes in order to accurately give an indication of relative performance rank. it is my opinion that resolution and contrast are the two qualities that need to be addressed.

I will not be able to address all the issues here, so I will refer you to two articles, a binocular comparison article and a binocular performance article.

In the binocular comparison, note the sections on masking the binocular objectives and the effects on comparitive performance. It provides a clear indication of the benefits of magnification and other info on the change in exit pupil.

In both the comparison and the performance articles note the sections on attributes affecting the overall performance of binoculars.

User entrance pupil compared to binocular system exit pupil produces a PERSONAL index, as you refered to when you stated "by a particular observer". But remember this, aperture is probably the largest single component of the binocular system that could vary for an individual observer with the least perceivable change in performance. Aperture is the most under-utilized component of the whole system.

Also, the larger the aperture the better the resolution. Your entrance pupil will reduce the overall light entering the eye, but the resolution will still be dependant on the larger aperture.

Excuse me if you have already been to these articles and you are continuing your discussion as a result. If not, I hope you find reading them worthwhile.

"How To Understand Binocular Performance"

"Comprehensive Comparison of 4 Binoculars"