Attached is what happens when you work in an area with water spray. This is an example of ocean spray and can happen in a few minutes (even seconds). The problem with these scenarios include: the water, whether salty ocean air, or clean water from a waterfall reacts with any dust spec on the filter. That reaction can weld the dust to the coating making it hard to remove. The reactions destroy the coatings. This will also happen on a lens surface. The attached image is from ocean spray on the Oregon coast and happened in seconds.
Contrary to popular belief, the front element of all lenses I have seen/examined are optical elements ;). So it is important to keep the surface clean or you will suffer reduced contrast and increased flair. But it matters not if the lens is curved or not; as anywhere light is passing through must be high optical quality. Filters too should be high optical quality.
If you work in dirty situations, the lens will get dirty. Coatings will degrade. So you can choose to send your lens in to have the front element replaced periodically, or use filters, or for big telephoto lenses, always use the hood.
Hoods do not help much on wide angle lenses.
Roger
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