Zoom binocular exit pupil

[ed9428 (Kris)]

Hi there.

Just wondering if anyone knows how a zoom binocular can maintain its exit pupil even at high magnification?
For instance the Celestron 20-100x80 has an exit pupil of 3.5mm, which would be fair enough at about 23x magnification. But at 100x would you end up with an exit pupil of 0.8mm? Or is the 3.5mm maintained across the entire zoom range, and if so, how?

Cheers.

[SkyWatch (Dean)]

You are correct: the exit pupil has to reduce with increasing magnification. 3.5mm would only be at 23x, assuming the full 80mm objective. (It should be 4mm at 20x, unless the objective is stopped down to an effective 70mm.)

All the best,

Dean

[ed9428 (Kris)]

Thanks for the answer.
So isn't an exit pupil of 0.8mm rather tiny at100x80?
I have no interest in buying a zoom binocular but would like to try some high magnification eventually for a project I am working on.
How small an exit pupil do people find comfortable?

[Wavytone]

Kris,

An exit pupil around 0.8-1.0mm is obtained at the optimal useful magnification of a set of good optics for a given aperture. The magnification is approximately X1 ... X1.3 per mm of aperture.

Binocular objectives however are essential fast doublets, similar to finder scopes, optimised for LOW power wide field views. The images from binocular objectives don't usually stand up well at high power - typically you'll find some aberrations.

For an 80 mm lens to work well at 100x there are only three solutions - either it must be about f/9 or longer (resulting in a long tube), use ED glass in one element (expensive) or resort to a triplet lens (even more expensive).

There's ultimately only one conclusion - if you want high power buy a telescope with good optics. Not binoculars.

[SkyWatch (Dean)]

Wavytone is right: if you want high power you need a quality telescope. Zoom binoculars are compromises at best, and I suspect it is highly unlikely you would get a sharp view with these at 100x! There would also be a fair bit of false colour; which is why the suggestion of a triplet or ED lens.
Regarding the exit pupil size: a lot of people would say around 1mm is the smallest you should go for sharp and relatively bright high power views.
However, you can go smaller if you have a top quality objective lens: my Tak TSA 102 for instance can very easily cope with >200x, which means an exit pupil of 0.5mm or less.
In fact the old rule of up to 50x per inch of aperture (or 2x per mm) for a top quality scope gives an exit pupil of 0.5mm...