If you’re looking for a pair of binoculars you can use everywhere, an 8x42 should be your choice, as they are the true all-purpose binoculars suited for any application. As far as size is concerned, they are right between the two extremes: pocket binoculars and giant binoculars. The compromise also involves the features of magnification and objective lens diameter. These matters will now be discussed.

What is the ideal size of the objective lenses for an all-purpose pair of binoculars?

Two factors determine what the apertures will be: The purpose of the big lenses at the front of a pair of binoculars, as well how heavy the binocular is going to be.

The big lenses at the front are for gathering light, without which a binocular cannot operate. Bigger lenses gather more light, but bigger lenses are heavier and this leads to a bigger instrument. Smaller lenses are not that heavy, but on the downside, gather less light as well. The challenge has always been to make a binocular which gathers enough light, but does not weigh too much for an all-purpose pair of binoculars and they came up with apertures of 42 mm.: They feed in lots of light in bright daylight and let enough light through for reasonable viewing in dim conditions, but have only a moderate effect on the overall weight of the instrument.

What is the ideal magnification of an all-purpose binocular?

The magnification of ordinary size binoculars vary from 6x to 12x; giant binoculars go as high as 30x. Some binoculars with 8.5x and 10.5x magnification are also on the market. Power of 9x is associated only with big lenses of 63 mm.

Magnification of 6x is a bit too weak and 7x is better, but still not powerful enough for a versatile binocular. On the other hand, more magnification power like 10x and 12x comes at a price: You will have the joy of more magnifying power, but the image will not be as bright.

Furthermore, more power is also associated with movement sensitivity. The slightest motion, like "binoculars shakes" is amplified making finding an object (in particular when it’s moving) very tough. More power inevitably means a smaller field of vision, which in combination with the problem of movement sensitivity, makes focusing on an object even more difficult. The depth of vision of the instrument is also impaired, which means that the focus will have to be adjusted more often. Imagine how difficult it will be watching active birds at close range with binoculars with flat depth of vision.

Clearly the golden mean as far as magnification is concerned, is 8x. You won’t have any problems getting a view of distant objects and to see enough detail at any range. However, 8x is not too powerful to have to contend with things like narrower field of view and movement sensitivity which come with more magnifying power.

Exit pupil:

The exit pupil is the stream of light at the eye pieces entering the viewer’s eyes. An 8x42 gives you an exit pupil of 42/8 = 5.25 mm. So it follows that the exit pupil of a 10x42 is 4.2 mm. and 3.5 mm. in the case of a 12x42. Exit pupils this small will be quite sufficient when the sun is shining, but when the sun is fading, in overcast conditions or in a darker environment like a forest the smaller exit pupils will not be able to let enough light through to deliver a relatively clear image for the viewer to observe.

Optical glass and coatings:

A magnification of 8x and apertures of 42 mm. will not be your ticket to perfect viewing; the final say will go to the glass used for the lenses and prisms, as well as the coatings put on these.