How to clean binocular lenses without scratching them

Most scratches on binocular lenses do not come from use. They come from cleaning: wiping grit across the glass with a shirt, or scrubbing at a smudge that did not need scrubbing. The safe method is slower than you expect, and mostly consists of not touching the lens.

This is general optics care as described in manufacturer manuals and camera and optics guides, not a test of any product. If your manual says something different for your model, follow the manual.

First, decide whether it needs cleaning at all

A little dust on the outer lens does not noticeably change what you see. A fingerprint or smear does, because it scatters light and makes bright scenes look hazy. Clean when the view is visibly affected, not on a schedule. Every wipe risks a micro-scratch, and the thin anti-reflection coating on the glass is softer than the glass itself.

What you need

• A soft lens brush or a rocket-style air blower (the squeeze-bulb kind used for camera sensors).
• A clean microfibre cloth that you only use for optics. Wash it without fabric softener and keep it in a zip bag.
• Lens cleaning fluid made for coated optics, or a lens pen. Plain breath fog on the lens is a fine fallback for a light smudge.

Skip household glass cleaner, anything with ammonia or alcohol-heavy formulas not labelled for coated lenses, paper towels, tissues and your shirt. Paper fibres and cloth that has been in a pocket can carry grit.

The method

1. Take the dust off first. Hold the binoculars with the lenses facing down, so loose grit falls away, and puff the blower across each lens. Then flick lightly with the brush, from the centre outwards. Never wipe a gritty lens, because the grit is what scratches it.
2. Breathe or add one drop. For a fingerprint, either fog the lens with a short breath or put one or two drops of lens fluid on the cloth, not on the glass. Fluid can run to the edge of the lens and seep behind it.
3. Wipe gently in a spiral. Start in the centre and move out in a slow circle, with almost no pressure. Fold the cloth to a clean section for each pass.
4. Dry with a clean area of the cloth and check against a bright background. Repeat only if a smear remains.

Eyepieces versus objective lenses

The eyepieces (the lenses by your eyes) pick up oil from eyelashes and skin and are the ones that most often need attention. The big front objective lenses collect dust and raindrops. Cap the objectives when you are not using them; this prevents most of the cleaning problems in the first place. The rainguard that fits over the eyepieces does the same job at the other end.

After rain, salt or sand

• Rain: shake off droplets, blot (do not rub) the lenses and body with a cloth, and leave the binoculars out of their case overnight with the caps off so any moisture can evaporate.
• Sand or beach salt: blow it off thoroughly first. Salt crystals dissolve in a damp cloth; the final wipe should be a lightly damp one, then dry. Do not wipe dry sand or salt.
• Fogging inside: if the inside of the lenses fogs when you move from a warm car to the cold, it is usually condensation that clears as the temperatures match. Persistent internal fog means a seal has failed, and the maker's repair service is the answer, not opening the barrels yourself.

What not to do

• Do not unscrew or take the binoculars apart. This usually voids the warranty and misaligns the prisms.
• Do not use compressed air cans held near the lens: they can spray liquid propellant.
• Do not put the binoculars in a hot car or a bag with the caps off. Heat and loose grit are the main enemies.

Keeping them clean longer

Keep the objective caps and rainguard on between uses, hang them from the strap rather than putting the lenses down on a surface, and store them in a case or a clean drawer. If you are still choosing a pair, the binocular chooser and spec decoder is free, and once you have them, set them up for your eyes before your first outing.