I can obtain predictable p-ttl exposure with my K-5 on firmware v1.12 and my AF-540FGZ flash
on camera in p-ttl bounce mode. But I must set the flash to manual zoom, otherwise exposure will vary with the zoom lens focal length, as the OP noticed.
I use the 18-55 kit lens, manual exposure mode 1/125, f/5.6, ISO 200, flash zoom set to 16mm, flash exposure compensation -1.3 set in camera.
Originally posted by audiobomber Try dropping the wide diffuser for bounce shots.
This will work because it fixes the flash zoom setting at 13mm.
I suspect there's a bug in the p-ttl calculations that erroneously factor in the flash head zoom setting even in bounce mode.
Regards,
--Anders.