The inner structure of the Pentax *ist D(L) and the *ist Ds(R). Note the larger size of the shutter window on the *ist D. That camera uses a full-sized, 35mm shutter assembly. Part of the reaosn the *ist Ds is smaller and lighter is that it uses a shutter designed specifically for the APS-sized sensor. The *ist Ds also has a mirror instead of the the pentaprism that the *ist D uses in the viewfinder.