Put my 580EXII Canon Speedlite onto the hotshoe of my K-5, the Pathetic TTL issue of the Pentax and K-5 has been resolved immediately and forever!
The trick is to set the Canon flash to External Auto mode (Custom Function No. 5 Selection 3) and the flash will remember every setting including this and the Av and ISO as well as the manual zoom angle for the flash coverage, even after it is powered off. This is different from Pentax' own dedicated flashguns of the current models of 360 and 540 which will never remember the External Auto mode, i.e., they will be reset to Pathetic-TTL mode once the unit is turned off or power saved! :-(
Nonetheless, there is one thing really pathetic about this Speedlite and Canon. That is its External Auto metering is *always* about 2EV darker and I really don't know why. A quick web search reveals that it is just a known infamous problem. But no problem, after a +2EV compensation (either deliberately creating a +2 stop deviation between the body and the flash, either with the ISO or Av, an example set of working parameters as shown in the above), the flash exposures are usually dead accurate, direct, diffused or bounced. So, that never-ending Pathetic TTL flash problem with an external flash on Pentax DSLRs has been solved completely, simply with the use of a Canon flash! And there is also no need for the irritative pre-flash anymore! ;-D
Last but not least, there is yet another advantage for using the 580EXII on K-5, for what some Pentax fans have been emphasizing about(!), that is, the 580 flash is weather-sealed, whilst no original Pentax flashgun of any is and has such capability! :-)
Related:-
P-TTL Vs TTL: Is Newer Better? (How Auto Flashes Work?)