Originally posted by Hornet My understanding of focus stacking is that one wants the magnification (=focus distance) to remain constant hence the reason focus rails are used to move the whole camera relative to the stationary subject. Another example is a microscope - the whole optical system is moved up or down relative to the subject to move the focal plane.
The stacking software automatically scales each image to fit. It also corrects for any misalignment. The auto alignment is good enough even to make good for handheld macro shots and you are careful enough. What it cannot correct for is perspective changes if there is too much lateral movement between each frame.
The focus bracketing should be quite easy to implement in firmware. If they were to make it shoot at 8 FPS and preferably without lowering the mirror between each shot it would make handheld macro photography much more practicable.
I've been thinking for some time of making something myself with a stepper motor coupled to the focus ring, probably much like what wombat2go describes except that it would adjust the focus ring of the lens rather than the bellows extension. Probably the day after I get round to actually making it Pentax will release a firmware that does it in camera.