Number one reason to use a filter on your lens.

This is why you might want to have a filter on your lens! One day, at the end of a beginners workshop, one of the students had a Tamron 70-200 f2.8 zoom that was making some scary rattling noises. I gently pulled off the lens cap to reveal a shattered filter. The lens had taken a hard hit on its front edge. I carefully unscrewed the filter and the slightly bent mounting ring, pulled out the shards of glass and blew off the remaining glass powder. All was well. If the filter hadn’t been there would have likely been serious damage to the lens itself.

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