Equipment Care: Controlling Relative Humidity

Humidity, sometimes also referred to as ‘absolute humidity’, refers to the amount of moisture in a given volume of air. Relative Humidity (RH) is the amount of moisture in the given volume of air compared to the maximum amount of moisture the air can hold at that particular temperature. When we talk of humidity, temperature of the air is not taken into consideration; when we talk of relative humidity, we take into account the amount of water vapour and the temperature of the air.

Optical equipment (lenses, camera bodies etc), if stored above 50% RH, can develop ‘fungus’ (plural, ‘fungi’), which is a bacterial growth that eventually eats up the lens coatings, which in turn could cause flare and eventual loss of image quality.

Fungus looks somewhat like a spider’s web. You need to point the lens at an open sky or some other diffused light source and carefully see through it (with the diaphragm blades wide open). Sometimes it may be necessary to turn the focussing ring from one end to the other; with a zoom lens, you may even have to change the zoom setting before you may notice the fungus.

A camera technician can open the lens and clean away the fungus, but if the fungus has been there for a long time, scar-marks may remain where the fungus has eaten up the lens coatings. There is also a chance that the lens elements, after the cleaning, do not fit or align exactly in the original position, causing focusing problems. Use of wrong cleaning chemicals or vigorous cleaning can wipe away/partially wipe away the lens coatings, essentially lowering the lens quality. If fungus penetrates between two joined elements (a doublet), it becomes impossible to remove it.

Lens cleaning can be a costly affair. Besides the problems mentioned in the above paragraph, fungus can grow again within a very short span of time, especially if the relative humidity is high.

The best solution to avoid fungus in the first place is to use a specially designed drying cabinet, which allows the user to control the RH. Some years ago, when drying cabinets were not available, photographers used indicating type (blue colored) silica gel to control the RH but for various reasons, the method was nowhere as good or convenient as using a drying cabinet.

Some users store the equipment in a box using a zero Watt bulb, but this isn’t a perfect method of protection – the user does not know how much RH is in the storage space. Furthermore, he or she has to keep up with frequent changes or re-heating of the silica gel.

Note: Photo equipment should not be stored in boxes made from plywood. The various ply are bonded together using adhesive which give off gases that harm the lens. For the same reason, adhesives that carpenters use should also be avoided.

Modern storage cabinets are controlled electronically, consume hardly any power and can be actively monitored for RH. They provide 20~50% RH control depending on loading (how much stuff you have in the cabinet) and environmental conditions.

Photographic equipment is best stored between 38-45% RH and, on the higher side, should never exceed 50% for prolonged period. Very low RH can dry the lubricants within the lens/camera/ other optical equipment, besides causing micro-creases in multiple coatings of the lens.

Lenses should preferably be kept on the upper shelf and camera and other equipment in the lower area. Although these cabinets require hardly any attention or no maintenance, it is worthwhile to keep an eye on the RH once in a while. The cabinet needs to run 24 x 7. Please do not jam-pack your equipment in the cabinet; there should be adequate empty space for air movement.

Once you have completed your photography for the day, care should be taken to clean the lenses / camera of dust and other physical dirt. Take special care to clean the lens mount and camera body mount. Ensure that both lens caps as well as lens surfaces are clean. Micro-fibre cloth that may have been used to externally wipe the camera / lens, should not be used to clean the front / rear lens element; for that you must use another piece of micro-fibre cloth, which after use, should be placed in a clean zip-locked plastic pouch. After the necessary cleaning, place the items in the drying cabinet. The lens and camera can be stored with the cover caps on.

Do note that once fungus has started, keeping the equipment in a dying cabinet will not take away the fungus; at the most, it may prevent it from spreading further. It is also advisable that such lenses should be isolated from other equipment and ideally, the fungus should be removed by an expert technician.

Look after your equipment and it will give you many years of faithful service.