See this experiment done by a K200D guy for the physical and performance differences found and measured when using different battery types:-
(in Traditional Chinese, Google translated English page here)
So, his comparisons and results are tabulated:-
|Battery Type:||NiMH (Sanyo 2500 mAh)||AA Lithium (Energizer)||NiZn (PowerGenix)|
(4 x AAs):
|115g||98g (85%)||58g (50%)|
|AF Motor Speed|
(from 00 to nearest w/ DA 16-45)
|0.316s||0.254 (80%)||0.211s (67%)|
|Voltage (Per Cell):||1.38V (-8% Lower than the Designed 1.50V Nominal Voltage)||1.72V (+15% Higher)||1.78V (+19% Higher)|
So, it is clear that the NiZn wins all, it is lighter and yet more powerful. But this may also be too powerful for the Pentax AA DSLRs to tolerate, as the voltage might be too high and more importantly, the internal resistance of the NiZn is just too low, this may put some electronics and components inside our cameras at risk, in particular the built-in flash, I bet - Read my this previous article for more details:-
Better K-x Rechargable Solution? Is the NiZn Safe?
Anyway, if you hate much the NiMH battery performance and reliability in your Pentax AA battery DSLR(s) (I DO), and if you might want to overclock your DSLR(s) for better performance, you may give the NiZn a trial. But remember, do this at your own risk and you might get your camera burnt and damaged in the end!
P.S. Recently when I did some prolonged movie recording (which was just for a few minutes continuously for each clip and then clip after clip, air-conditioned indoor) with my K-x with fully-charged NiMHs in it, it quitted and left the "LiveView" mode a few times prematurely without any warning. The battery indicator changed from green to yellow (half-depleted) and then to red (fully depleted) and back to green after enough rest! This is really annoying as I found that when I did the same with the Canon 550D, even under a hot sun and high temperature outdoor, and for even longer recording time, no problem ever happened! That AA battery rechargeable "solution" used by Pentax in any Pentax AA DSLR does really SUCK, I have to say! >:-( It's just now an unique Pentax problem anyway, for the current lines and models of DSLRs that are in the market.
AA Battery Chargers Fully Compared (4 Models)