Condensation happens when camera gear moves between different temperature/humidity conditions too quickly. The main prevention is to let the camera and lens acclimatize gradually.

Helpful practices:

• Put the gear in the shooting environment several hours before use so it reaches the same temperature as the air.
• Avoid moving it directly from cold to warm, or warm to cold, conditions.
• When bringing gear back indoors, seal it in a plastic bag (ideally with silica gel) or keep it zipped in the camera bag first, so moisture forms on the outside of the bag instead of on the gear.

If condensation has already formed:

• Let the equipment sit and warm/cool gradually until it clears.
• If the gear is really wet, don’t turn it on until it is dry.

Is it harmful? Occasional condensation usually won’t cause damage, but it’s best avoided. Repeated moisture exposure is not ideal for cameras or lenses. The safest "quick fix" is prevention and gradual temperature equalization rather than trying to heat the lens aggressively.