My friends very often ask me this question. In fact, just a week or so ago one of my friends bought Canon 500D and Canon EF-S 17-85/4-5.6 IS USM IF lens. Hopefully my memory did not fail me and I mixed the letters in the proper soup with that lens.
I've stumbled upon this blog entry, which I find sensible, reasonable and logical. Methinks I ought to put it here so that I can reference it later.
7 hours ago