Originally posted by sledger I have no problems triggering my Yongnuo slave optically even with the K-7. Manual flash exposure compensation is available on the K-7 (even in wireless mode), is this what you're talking about, if so then it's been there a while hasn't it?.
No, the Yongnuos feature a "smart" optical slave modes, and a "dumb" optical slave mode. I don't own one, but I think they call it S1 and S2 mode, not sure which is which though. Traditional optical slaves were just a simple photo cell that would trigger the flash when it sensed a burst of light, but because modern flash systems use a preflash to determine exposure, a "dumb" optical slave will be fooled into firing the flash prior to the exposure.
If Pentax has added manual control to the pop-up flash, that means there is no need for the preflash to determine the exposure, hence, it will be possible to trigger studio strobes, or other dumb slaves with the popup flash.
To answer your other question flash compensation is not the same thing as manual control. Flash compensation tells the camera "add one stop to whatever you think is the correct exposure" (or subtract), but it still uses the pTTL preflash reading to determine it's base exposure. Manual control simply tells the camera "Fire the flash at quarter power", or half, or full etc... Manual control does not rely on the camera to make the exposure decision.