Nate,


First of all, congrats on the lens. []


As to your question, I believe it acts like a filter. I watched this link some time ago, and it shows the manufacturing process of the 500/4L. At one point in the assembly the worker screws what appears to be a large filter on the lens. I believe, but cannot prove, that that is the protective front element. Also, I know you don't use UV filters, but here are the reasons I believe that Canon likes UV filters:
  • Canon sells them (but B+Ws are better)
  • Canon makes lenses (i.e. 16-35) that require a filter to complete the sealing
  • A Canon rep told me that he recommends using them
  • If the paragraph above is correct, Canon uses a protective filter on the supertelephotos.