If you live in a place live Hong Kong where at least half the year is humid, then you will need to protect your cameras and lenses from humidity which may promote the growth of mold. Once you get mold on your lenses, even if you clean the mold off, it may permanently damage the coating or the glass. The lens will usually be too expensive or impossible to repair and the only resort will be to buy a new one, which will be quite costly.
The ideal relative humidity to keep camera and lenses is somewhere between 30 to 50%, 40% being the middle of that range and is probably best. Above 60% is where most mold will grow. Too dry may not be good too, as some type of mold actually grow in very dry humidity (less than 20%).
There are a few ways to keep the humidity down. You can keep the air-conditioner on. Despite what some people claim that air-conditioner cannot keep relative humidity below 60%, in my experience, air-conditioner can easily maintain a relative humidity of between 40-50% in a room so long as the air-conditioner is not under-power for the room. The disadvantage of using an air-conditioner to keep a room dry is that it uses a lot of electricity and not cost effective and environmentally friendly as a method to keep humidity down. It is also not a good method to use when the temperate is low, but the humidity is high.
You can also use a dehumidifier. The problem with these is that you will need to constant keep emptying the water tank. When the tank is full and you don’t empty the tank, it would stop working.
The best way to store camera and lenses at the ideal humidity is to store them in a dry cabinet. There a few brands of dry cabinets, although all brands claim they are the best or have some proprietary method for keep a constant humidity inside the cabinet, they actually all use very similar mechanism. Most brands are actually just OEM models and they share exactly the same design. Only the brand names are diferent. So it probably does not matter which brand you get. Any brand is probably just as good as the next. The size of the dry cabinet is usually measured by volume, the smallest one around 40L and with sizes going up to as large as 1000L or more.
I recently invested in a 200L model and I am very happy with it so far.