Monocular

Half a binocular, which is frequently the point

A monocular is a single small telescope built to be used with one eye and carried in a pocket. It gives up the depth and the comfort of a two eyed view and returns half the weight and half the bulk, which is why one ends up in a coat pocket on walks where a full sized pair would have been left at home. The optic listed here is a ten by forty two.

What do the two numbers mean?

The first is magnification and the second is the diameter of the front lens in millimetres. Ten by forty two therefore describes an image drawn ten times closer, seen through a forty two millimetre objective lens. That front lens is what gathers light, so a larger objective brightens the view at dusk and under trees while making the whole instrument heavier. Dividing the objective by the magnification gives the width of the beam leaving the eyepiece, the figure that predicts how any optic performs in poor light, and the same notation labels the 10x42 binoculars format.

Monocular or binoculars?

A monocular wins on pocketability and on price, and it leaves a hand free for a lead, a map or a handrail. Binoculars win on everything to do with looking: two eyes give depth, hold a target more steadily and cause far less strain across a long session. For a quick identification of a bird or a distant sign, one eye is enough. For an hour of watching, a pair from the compact binoculars range is much easier to live with.

What should be checked on an optic like this?

• Magnification: higher is not automatically better, since every increase magnifies hand shake along with the subject.
• Eye relief: how far back the eye can sit and still see the full image, which decides whether it works over spectacles.
• Focus action: a wheel or a rotating eyepiece, judged by whether it can be turned with one cold hand.
• Sealing: a sealed and gas filled body resists internal fogging when it comes from cold air into a warm room.

How is one held steady?

At ten times magnification a heartbeat is visible in the image. Brace the hand holding the tube against a cheekbone or the bridge of the nose, then rest a shoulder or an elbow on something solid, a fence post, a wall, or a raised knee when seated. Breathing out and holding briefly settles the picture further. Anything above ten times really wants a rest or a tripod, which is one reason this format stays popular for general use.

How are the lenses looked after?

Blow loose grit off first, then use a soft brush, and only then a microfibre cloth with a drop of lens fluid. Wiping a dusty lens straight away grinds that dust across the coating. Keep the caps on in a pocket, where keys and coins do the real damage. A wider and brighter view at dawn comes from lower magnification on the same size of objective, which is what the 8x42 binoculars format offers.