Pentax's P-TTL flash metering system tends to be unreliable and may underexpose at times. When and why does the system underexpose and what can I do? I want to know the situations that cause flash underexposure so that I can compensate for it accordingly, with as little trial-and-error as possible.
Note that I often manually set the ISO lower than what the Auto ISO would choose in order to reduce noise (but not so low that the flash doesn't have enough output), even though this means that the flash will need to discharge at or near full power. This often, but not always, causes underexposure unless exposure compensation is applied. In addition, direct flash never seems to cause underexposure, but the appearance of direct flash is generally poor, with heavy shadowing and red-eye detracting from the image.
The flash unit I'm using is a Metz mecablitz 50 AF-1 digital (guide number 50 m / 164 ft at ISO 100, zoom 105mm), Pentax P-TTL dedicated, running the latest firmware (3.0). The flash is primarily being used with my Pentax K-5, but both my K-5 and K-r suffer from this problem, and apparently to the same degree.
I'm now suspecting that the metering pre-flash is too weak, causing the camera to incorrectly assess the amount of flash exposure required. The camera's flash metering algorithms are also a possible suspect. Would a more powerful flash unit produce more accurate flash exposures, even if the 50 AF-1 does have enough power to correctly expose the scene with a given set of exposure parameters?
Upon further testing, it seems part of the problem is distance to the subject--the camera often underestimates the amount of flash power needed to properly light the subject at longer distances. This could be caused by the camera or the flash unit, and it's possible that the flash's pre-flash timing or power is inconsistent with what the camera body expects (keep in mind this is a third-party flash unit). Any workarounds? (I do know that the lens communicates focus distance information to the camera, but it does not appear to be taking advantage of this information, other than adjusting the aperture when not set manuallly.)