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Binocular aperture question

Ursa Major

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Why is it that with a pair of binoculars, the aperture is the diameter of one of the objective lenses? Surely double the amount of light is hitting your eyes and being processed in your brain.

If the aperture IS truly just the diameter of one objective lens, does this therefore mean that when using binoviewers, the aperture is effectivly halved? :)

I think this is quite an interesting question and one that hopefully someone will be able to help with.


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Binoviewers reduce the effective aperture by 0.7. A simple test to show the light entering both eyes is not 'integrated': look at some stars with bins and then close one eye: the stars do not become dimmer.

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It is indeed interesting and the answer seems to lie in neurology, not optics. Bins clearly do gather twice the light but, equally clearly, when you close one eye things do not get to be half as bright. In fact they don't seem to get any less bright! So bins are not equivalent to a single lens of equivalent optical area in terms of brightness but they do have other advantages like 3D (for real at terrestrial distances but only psychological at astronomical ones.)


Edited by ollypenrice
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I have asked this question quite a few times (once here) and have received an interesting variety of answers........

Consensus seems to be that there is some light advantage using binos compared with a single telescope of the same nominal aperture as 1/2 of the binos. One optimist thought it was the direct sum (ie 1.414 x nominal aperture of the binos), others less so.

With my 15x70s, I would say that the perceived image is somewhat brighter than from my ED80 with a 40mm EP (ie x15), but not a lot....

If I close one eye while looking through the binos, I am not sure if the image is fainter or not - I suppose the eye is not very good at assessing actual brightness, and most folks' eyes are not exactly matched either!


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Try looking at a very faint star, then close one eye. Contrast threshold for monocular vision is 1.414 times the threshold for binocular vision, so objects near threshold will disappear when you view with only one eye. We don't notice the effect with bright objects because they are well above threshold.

The binoviewer on a telescope splits the available light between two eyes, as has been said, whereas true binoculars are effectively two telescopes, delivering twice as much light as a single one.

As to the reason why the stated aperture is that of a single objective, I'd say it's a far simpler convention than multiplying by root-2. If somebody offered me 70.7mm binos and they turned out to have 50mm objectives I'd be pretty cheesed off.

Some other threads relating to this:




The root-2 correction for monocular versus binocular contrast threshold has been held to be valid for a long time though it has been questioned by a recent study suggesting two eyes might be 1.7 times better than 1.


Edited by acey
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it's worth noting that for many people the two eyes "see" quite different levels of brightness and colour - personally, I see colour much better with my right eye but I see things brighter with my left. binoviewers or bins seem to give you the best of both (or maybe that's just my imagination having spent ages finding sets of matching e/ps...)

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I tested out some of the points here two nights ago with my 15x70s. It was an exceptionally clear night here (South Oxfordshire) with a stunning view of the Milky Way through Cygnus down to the southern horizon. I chose a point 200m from my house where I can get a 360 degree horizon view, and looked at a range of DSOs of varying brightness, both with two eyes, and then with each eye on its own in turn.

First point - a surprise to me - was that, before my eyes had adapted to the dark, I had better monocular vision (leading eye, right) than binocular (tested by looking at the apparent size of M31)! One eye adapted within seconds, the other took 10 mins.

As my eyes adapted, binocular vision became clearly superior to monocular.

For example, M31 occupied about 2/3 of the vertical FoV with binocular vision, 1/2 or so with either eye on its own.

With the Eagle and Omega nebs, the nebulosity was not easily visible with one eye, but clear with two. Little difference in the stars.

M51 was clearly visible with two eyes, with some idea of shape, at the absolute limit with only one.

M101 was just visible with two eyes, but not at all with one.

Certainly for my aged eyes, when dark adapted, I get a very significant gain in using both eyes rather than just one. The views of eg M51 are roughly intermediate between that of my ED80 and my 127 Mak, in other words about equivalent to a 4" scope, which is the same total area of glass of the 15x70s combined.


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