What is the benefit of an internal focus lens?

Question

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 1

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).

Answer 2

In addition to what James posted, internal focus lenses more-or-less eliminate focus breathing, the change in magnification and angle of view that happens when you focus a unit-focus (traditional) lens.†

With a traditional lens, framing can change substantially as you focus, to the point that you actually need to recompose (which means a change in focus, which leads to recomposition, ...). Depending on the criticality of framing, that can be a real pain in the patook. By changing the infinity focal length of the lens, IF lenses maintain the focus length of the lens system, which means that the apparent magnification of the elements in the scene, as well as the overall angle of view, is maintained throughout the focal range.

It also means that you don't need to compensate for "bellows draw" in the exposure, since the ratio of the physical size of the aperture/entrance pupil to the length of the lens system remains the same throughout the focal range.‡

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^{† I say "more-or-less" because floating one or more elements in an IF lens to optimize corrections throughout the focal range often has the side effect of re-introducing a slight amount of breathing which, though it may be noticeable, is not nearly of the same degree that it would be in a unit-focus lens.}

^{‡ If you use shutter priority exposure and a controlled close-focus scene, and the camera reports f/8 at infinity and when the image is properly in focus with an IF lens, it's really a bit of a fib. The system f-number is still f/8, but the actual f-number relative to the current focal length of the lens at the close focus distance will be wider than f/8. I've tried to explain in more detail here.}