Originally posted by angerdan What do you want to protect your lens against?
If it's physical impact, hard glass would be the choice.
I always find it interesting how you can make almost anything look like "proof" in lab tests. Mechanical/physical impact in real life almost never takes the form of a perfect elastic collision at a 90-degree angle exactly centered at the middle of a glass surface by either a perfectly round sphere or a flat metal surface (and even less when the glass surface is supported from below with a ring about the same size as the metal ball). Let's also assume that such a situation happens 1 out of every 1000 impacts and that an impact on a front element sunken behind a protruding lenshood (because of course, we all use those 100% of the times) would only happen extremely rarely over the lifetime of a lens unless you are often using it under extreme circumstances, say 1 out of every 100000 uses of the lens.
You are more likely to win a couple of million in a lottery than have the front element of your lens nearly destroyed by "physical" impact only to have the filter save you in the nick of time.
Using a protection filter
always is
never a good idea. Using it
sometimes to protect against sand or spray may or may not be a good idea (chances are if the sand or spray are serious enough, shooting through a piece of glass is not going to help you get a good image anyway). So simple logic would confirm that
never using a protection filter may just be a very decent policy which may very
rarely lead you to have to repair or replace a lens..