Why Damp Air Causes So Much Trouble

Stand on a fell top in the Lake District at first light and the air can be carrying more moisture than you would believe, even when it isn't raining. That moisture does two things to a camera. It fogs glass — the front element, the rear element, and the elements inside a zoom barrel, which is where it hurts most — and it turns every speck of dust, peat grit and sea spray into a sticky film that dries onto the coating. Bring a cold camera into a warm car or a café and you get condensation inside the lens, and repeated condensation in a dark barrel is exactly how fungus gets started.

Sensor spots are the quieter problem. A damp, slightly sticky speck that lands on the sensor stack during a hurried lens change will not blow off later. You will find it weeks afterwards as a soft dark smudge in the sky of a long exposure, and by then you have a hundred frames to retouch.

What to Carry in Your Pack

• A bulb blower — the single most useful item you own. It shifts loose grit without touching glass.
• A retractable lens brush — for the grit the blower won't move. Always before any cloth or swab.
• Three microfibre cloths, each in its own zip bag. One for glass, one for the body and barrel, one spare. A damp, gritty cloth is a scourer.
• Lens tissue for the front element only, plus a small bottle of lens fluid.
• Two sealed sensor swabs and a sachet of cleaning fluid, kept for genuine emergencies.
• Silica gel sachets in the bag, recharged on a radiator once a month.
• A dry bag or heavy zip-lock sized for the camera alone, for when the weather turns properly.

Keep the glass cloth in a different pocket from the one you use on wet hands. Nothing makes a worse job of a lens than a cloth that has already wiped sea spray off a filter.

Blower First, Brush Second, Cloth Last

The order matters more than the technique. Blow off loose material with the front of the lens angled downwards so gravity carries debris away rather than into the barrel. Then brush very lightly, working from the centre outwards, and lift the brush away at the end of each stroke rather than dragging grit back across the coating. Only reach for a cloth when there is an actual smear — a fingerprint, a dried rain spot, salt spray — because every pass of fabric carries some risk of grinding something microscopic into the glass.

Don't breathe on the lens. Your breath is warm, wet and full of exactly what you are trying to keep out. If you need moisture to lift a smear, put a single drop of lens fluid on the cloth, never on the glass itself. Use a fresh part of the cloth for each wipe, and turn it as you go.

Changing Lenses Without Letting the Weather In

This is where most sensor contamination happens. If rain is falling, don't change a lens outdoors at all — find a porch, a bothy, a car with the heater off, or a stone wall with a decent lee. If you must do it in the open, get under something: an overhanging crag, a dense spruce, your own jacket pulled over your head.

Technique matters. Sit down if you can. Have the incoming lens uncapped and ready before you twist the old one off. Turn the camera body face-down throughout, so any falling moisture hits the back of the mount rather than the sensor. Work fast, but not so fast that you fumble. Keep the camera switched off, because a powered sensor attracts dust electrostatically. And never change lenses in direct rain, spray from a waterfall, or on a beach with an onshore wind — those three conditions will put salt and grit exactly where you don't want it.

One practical habit: carry two bodies if you can, or accept a single lens for the day. A 24–70 or a 16–35 covers most upland work, and never opening the mount is the best sensor protection there is.

Condensation, and the Warm-Room Problem

Going out into the cold is rarely the issue. Coming back in is. Cold glass in warm, humid indoor air fogs instantly, and the moisture can sit inside a zoom for hours. Leave the kit in its bag with the zip closed and let it reach room temperature slowly — two to three hours is not excessive. If you need to shoot quickly, keep the camera in the bag and bring only the battery inside to warm up.

The reverse applies in winter: leave the camera in a cold porch or an unheated boot room rather than beside a radiator, and never leave it in a car overnight where it will cycle through condensation with every dawn.

Sensor Swabs, and When to Use Them

Check for spots properly before you touch anything. Set the lens to its smallest aperture, defocus, and photograph a bright even sky or a white wall. Spots that show at f/16 are worth a swab; anything you can only see at f/2.8 can wait.

Do the wet clean at home, indoors, at a table, with the camera on a fresh surface away from draughts. Use a blower first to remove loose grit, then one pass per swab in one direction, discarding the swab immediately. Never reuse one, and never press hard. If a spot survives two attempts, stop and take it to a specialist — scratching the sensor stack costs far more than a professional clean.

Finally, after every damp day out, empty the bag, wipe the outside, dry the silica sachets and leave everything open overnight in a warm, airy room. Ten minutes of housekeeping after a wet walk saves an hour of retouching later.