Hmm, so... does IS/VR somehow require USM?
That depends I think. The reason is that both the mechanical aperture linkage & AF shaft require specific spaces inside the Pentax lenses. They are straight phyically, and cannot be bended out of the way and still expected to work. For this reason, the optical design for IS/VR would be difficult because they have to be in front of both the AF & aperture mechanism. FA*200/2.8 for example, the front and rear lens group are fixed, the moving lenses belong to the middle lens group, and the aperture blades are between the front and middle lens groups. If IS had to be done in this lens, that means it has to be done with the front lens group which is huge in size. Now imagine, if both AF & aperture have their own motors, all 3 devices (USM, aperture & IS) can be placed anywhere the engineers wanted.
Regards, Alan Chan http://www.pbase.com/wlachan
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