Not too many years ago, everyone assumed that the maximum exit pupil (second binocular number divided by the first number) commonly offered in a binocular - 7mm -was the best choice for an astronomy binocular, because on paper, a 7mm exit pupil will give you the maximum image brightness and therefore allow you to see fainter objects. That's why a 7x50 was traditionally the classic astronomy binocular for many, many years and a good one, such as the Nikon 7x50 Sports and Marine binocular still makes an excellent choice.

Then came the giants with larger 70mm and 80mm objectives. This made it possible to offer even more magnification and still maintain a large 7mm exit pupil. 10x70s and 11x80s were the hot products. I have owned many of these and currently use a Nikon 10x70 Astroluxe binocular, not so much for the larger exit pupil as the excellent optics and construction. It's a true astronomy classic.

The market then shifted to giants with more magnification and therefore smaller exit pupils. The 20x80 with its 4mm exit pupil became the best selling giant (and still is, today). There are so many good 20x80s, I can't list them all, but I have always liked the Oberwerks, such as the Oberwerk 20x90mm Binocular with hard case. It's one of the best giant binoculars I have used for the price.

The 10x50 with its smaller 5mm exit pupil has largely replaced the 7x50 as the handheld astronomy binocular of choice and you don't have to spend a fortune to get a good one. Under $200, I like the Nikon 10x50 Action Extreme Waterproof Binoculars 7245 and the Pentax 10x50 PCF WP II Binoculars - 65808.

The bottom line is that we astronomers have learned over the years that an exit pupil (second binocular number divided by the first number) of 4mm or 5mm is plenty for nearly any type of binocular astronomy and that most users are seldom in the position to take advantage of larger exit pupil models. This is especially true if you observe under light polluted skies and have to deal with nearby lights (since the pupils of your eyes never completely open to their widest). It is also a fact of life for most of us past the age of forty or so, since our eyes can no longer open wide enough to use larger exit pupils.

Joanie