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?
Answer
In my experience, IF lenses frequently autofocus faster, because there is less mass to drive back and forth. For non-zoom lenses, internal focusing probably means that the bellows effect (in which air is sucked into the lens) is minimised since the outside of the lens probably won't move during focusing. That means the interior of your camera doesn't get humid or dusty (and so less crud adheres to the sensor).
According to The Manual of Photography (ISBN 0240515749; page 147), internal focusing mechanisms make it easier to have elements of the lens move nonlinearly with focus distance. This means that some kinds of aberration can be better corrected with such systems, or adequately corrected over a wider range of focus distances (this reminds me of Nikon's "Close Range Correction" feature; it looks to me like all their CRC lenses are also IF).
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