The "IF" in Pentax DA★ 200mm f/2.8 ED (IF) SDM stands for "Internal Focus". I know what this means: the lens doesn't change in size as I focus. (And it's true; it doesn't.) What's the point of this, and why is it important enough to rate a few letters in the product name alphabet-soup?
I know that a non-rotating filter ring makes it more pleasant to work with orientation-sensitive filters (like polarized and graduated ND filters), but as I saw in this (unrelated) lens review, lenses can have non-rotating filter threads without being IF.
For a macro lens, I see how IF might be important, since you might be at actual risk of bumping your subjects. But this lens has a close-focusing distance of about four feet (1.2 meters), so that can't be a concern.
So what's the big deal? Is there an advantage I can't see? Wouldn't a non-IF lens be more compact for storage (when set to its minimum extension)? Are there any optical benefits? Are there any drawbacks — compromises in other areas which must be made to enable this feature?