Instruction: Generate an answer to the question by utilizing the information contained in the document.

Input:
Question: Is this an internal focusing lens?  Or does the lens barrel extend as you focus?
Document: To gain proper manual focus, you have to change the setting on the camera body, flip the switch on the side of the lens, and shift the focussing collar if you don't want it turning under your hand. Now why couldn't Sigma do like Tamron and Tokina and make it so you only have to shift the focusing collar? It would make switching a lot easier and quicker.The biggest performance disappointment is with the auto-focus itself.
Being such, the lens comes nicely finished in its matte black and gold decor with a very well made, padded, zip-up lens pouch. The lens itself has a nice heft, but is not overburdening and should fit quite comfortably on most cameras. One of the best, and most often used with macro, features is the mechanical manual focusing. This is much nicer to use than the focus-by-wire on other Four-Thirds lens brands and provides a solid and tactile feel when focussing.
By the time I open the aperture, focus and stop down they have often moved. And when a manual lens is stopped down to f/8 or f/11 it's too dark to see what's going on.
But the auto-focus is not very sensitive, which I know is in part due to my camera. But my LEICA zoom can focus faster than this lens in low light. And even in bright light, the lens tends to jerk back and forth multiple times before the focus is confirmed. This is also much worse than any Zuiko lenses I've tried.
The Sigma 105mm has received praises for many systems, and in some instances these rave remarks are rightly due. The lens is an excellent value with a good build and super sharp optics. However there are many quirks with the lens that often get overlooked. The auto-focus performance is worse than lackluster and switching from auto to manual focus is a bit tiresome.
One might view this as a money saver as well as a convenience but that's a very subjective opinion I do admit.Finally, the lens features a fully manual focus mode that works very well but unlike the 'fly by wire' Olympus lenses that many are familiar with this lens requires that you throw a switch on the side of the lens housing in order to disengage the auto-focus motors.
To make it worse, there is not even a provision that allows the provided lens cap to snap in place while the lens hood is in use. This could have been easily allowed by protruding the threads used to mount the hood to the lens on the inside of the hood to give the cap something to attach to. A pretty big oversight in my book.To switch between auto and manual focus on other lenses is straight forward. You flip a switch on the camera body or lens or shift the focussing collar.
And of course being a Four-Thirds mount, that 105mm focal length acts like a 210mm making for a very nice telephoto prime and gives excellent magnification for use as a macro lens.But all of these bonuses come at a cost. To be such a nicely finished lens, I found the build could have been better in two areas; the lens hood design, and the auto-focus switching procedure. It is nice that the hood has a provision to use 77mm filters, especially polarizers.
Of all my Four-Thirds compatible lenses this has become a favorite. The Sigma is sharp, reasonably compact and features an actual manual focus mode.
I found that I can switch from auto to manual without looking, and love the large focus ring.I use this coupled with the Olympus 25mm macro extension ring, and it produces amazing close shots.
Answer: It is not an internal focusing lens - the barrel extends as you focus.  That being said, there is a convenient macro ratio scale on the lens barrel, and it's very handy when setting up a macro shot.  This lens is NOT a fast-focuser; it does not have the Sigma HSM (hyper-sonic motor) technology.  This being said - the 105mm f2.8 EX DG is probably the sharpest lens in my bag, including some lenses that were 3x as expensive.  Bokeh is spectacular, and if you're willing to slow down and accommodate the older mechanical design, you'll be rewarded with exquisite sharpness and beautiful bokeh.