The best all-terrain binocular: 8x42

If you’re looking for a pair of binoculars you can use everywhere, an 8x42 should be your choice, since they are the ones you can use for virtually anything you might be interested looking at through your binoculars and in any circumstances – all-purpose, all-terrain binoculars.

As far as size is concerned, they are right between the two extremes: pocket binoculars and giant binoculars. However, the most distinguishing features of this all-purpose class of binoculars has to do with how powerful they are as well as how much light they can gather through their big lenses.



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 function of the objective lenses and the weight of the binocular.

Light enters the binocular through the apertures at the front of the instrument, without which a binocular cannot operate. Bigger lenses gather more light, but bigger lenses weigh more and this means the instrument will be heavier and more bulky. Smaller lenses are not that heavy, but unfortunately do not gather the same amount of light as the bigger lenses. The aim is to strike a good balance between these two factors for an all-terrain binocular and objective lens diameter of 42 mm. is just that: They are perfect for viewing when lots of light is available and are effective enough for reasonable viewing at dusk or dawn, when it’s overcast or in the forest, but are not too big to make the instrument too heavy and bulky.



How powerful should a versatile binocular be?

The power available in ordinary binoculars vary from as low as 6x (some cases even 4x) to 12x; beyond that you move into the realm of giant binoculars. Magnification of 8.5 and 10.5 are also available. Power of 9x is associated only with big lenses of 63 mm.



Magnification of 6x is not powerful enough and 7x is better, but still marginally below par for an all-purpose binocular. On the other hand, more magnification power like 10x and 12x has certain implications: You may see more detail with a more powerful binocular, but the first implication of more power is a less bright image. Furthermore, more power is also associated with sensitivity to any kind of movement. The slightest motion, like normal hand tremors is amplified making focusing on an object (in particular when it’s moving) very tough. With more power of magnification comes a restricted FOV, 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, so the viewer has to keep his finger on the focus wheel in particular if the objects moves towards him or further away. So observing birds up close is not easy at all with high magnification instruments.



Clearly the golden mean as far as magnification is concerned, is 8x. You’ll have enough magnifying power at your disposal to see observe objects far away 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 passing through the binocular at the eye pieces. The exit pupil is determined by dividing the objective lens diameter by the magnification: 42/8 = 5.25 mm. With the same apertures (42 mm.) a 10x magnification gives you an exit pupil of 4.2 and a 12x magnification 3.5 mm. No problem using binoculars with these exit pupils during the day when the sun is shining, but when the sun is not shining and conditions are dim the smaller exit pupils will not be able to let enough light through for you to see a clear image.



Optical glass and coatings

A magnification of 8x and apertures of 42 mm. will not ensure good images; the final say will go to the glass used for the lenses and prisms, as well as the coatings put on these.



Summary

The 8x42 binocular is clearly the best all-terrain binocular. The power of 8x is good enough to give the viewer a good binocular experience, but not too powerful bring narrower FOV and "binocular shakes" into the frame. Apertures of 42 mm. are also a golden mean between too small and rather big, capable of letting enough light in at the front even in dim conditions, simultaneously avoiding unnecessary addition of weight and bulkiness. To complete the picture, the glass used for the lenses and prisms have to be of high quality.