Binoculars for birdwatching, walks and stargazing

What the two numbers mean

The first number is the magnification, the second is the diameter of each objective lens in millimetres. An 8x42 makes things look eight times closer through 42 mm lenses. Divide the second number by the first and you get the exit pupil, the width of the beam of light that leaves each eyepiece: 42 / 8 = 5.25 mm for an 8x42, 50 / 10 = 5 mm for a 10x50. A wider exit pupil keeps the view brighter in poor light. What the numbers on binoculars mean goes through every figure on a spec sheet.

Which magnification?

• 8x gives a wider and steadier view. It is the easier choice in woodland and gardens, for moving birds and for anyone new to binoculars.
• 10x shows more detail over estuaries, reservoirs and open hills, at the cost of a narrower field and more visible hand shake.
• 12x pulls in distant detail, but you will want to brace your arms against something or use a tripod for long looks.

Best binocular magnification for bird watching compares them in real birding situations.

Roof or porro prisms?

Roof prism binoculars have straight barrels. They are slimmer, and easier to seal against water. Porro prism binoculars have the classic offset shape, with the objective lenses set wider apart than the eyepieces. Both designs make good binoculars. The quality of the glass, the coatings and the build matter more than the shape.

Binoculars for the night sky

A 10x50 is the classic first instrument for astronomy: a 5 mm exit pupil and enough magnification to show craters on the Moon and, held steady, the four largest moons of Jupiter. Very low power binoculars such as our 3x50 show a much wider patch of sky. Read Binoculars for stargazing before your first clear night.