Are regular binoculars are the same or as good as astronomical binoculars?

Dean Asked:

Are regular binoculars are the same or as good as astronomical binoculars? I have been reading about them and I have been getting conflicting opinions. I want to buy a pair for night time sky gazing and I'm not informed enough about what I need.

Reply:

Astronomical Binoculars are described in terms of 7x50, 10x50, 15x70 and 20x100 for example. The first figure gives the magnification, the second figure gives the diameter of the main lenses in mm.

So for example a pair of 7x50 binoculars have a magnification of 7x and 50mm (2in) object lenses.

Now - if we divide the second figure by the first (e.g. 50/7) we get the diameter of the exit pupil and this is important when considering binoculars for astronomy.

The pupil of the human eye when it is properly dark adapted is between 5 and 7mm across. For optimum use in astronomy, our binoculars need to have an exit pupil diameter which matches as near as possible the diameter of the pupil of your eye. From this 50/7 = 7.1mm which means that 7x50s are excellent for astronomy. They wil give bright views and good contrast which makes them ideal for scanning the Milky Way and searching for stars/planets in the early twilight sky.

A pair of 20x100 binoculars on the other hand will only give an exit pupil of 5mm which means slightly dimmer views by proportion even though the objective lens diameter is twice that of the 7x50s.

The smaller exit pupil diameter combined with higher magnification will mean views show less contrast because you are not using the whole of you pupil to receive light. They will also be heavier and so should be used with a tripod.

I personally have a pair of 15x70s for quick scanning and searching, and also a pair of 20/30/37x100s which come with interchangeable eyepieces to provide the different magnifying powers. Having said that I virtually never use them at anthing other than 20x due to optical performance. These binoculars are also right angled to make them more convenient to use at high altitudes.

The exit pupil size and weight are probably the most important considerations when choosing for astronomical binoculars combined with your budget that should be enough to allow you to make a choice.