What causes this effect?

[xluben]

there was a paintball vs. camera discussion on pentaxforums a while ago. UV filters were the most common suggestion...but some thought of like a sheet of plexiglass mounted in front of the lens...sort of like the ones used on TV cameras so people can paint on them/throw stuff etc.

It's about 50/50 if a filter will stop a paintball hit. If it breaks the filter it
could actually be worse, because then you are cleaning glass shards of the
front element of the lens. Plexiglass on the other hand wouldn't scratch the
glass, but is much harder to come by with good optical performance, and it
most likely will still break under a direct hit.

But when I'm on the field the dust/dirt that builds up on the filter and has to
be wiped off during the day is a good enough reason to have the UV filter on there. I'd rather scratch a $25 filter than a $1700 lens