Originally posted by Wheatfield
Yup. I do find it annoying that 25 years after AF was invented, Pentax still doesn't have one that works reliably.
......Having said that, and knowing full well that I should have known it, it is still really crappy AF performance. There is no reason in my view that the shutter shouldn't release either once the AF has locked, but it did exactly that, and that, my dear friend, is the camera's failure to perform.
I hope you take this positively, because I'm sure others are interested in what may have gone wrong, to avoid a recurrence.
I'm sorry, I'm still not satisfied with "the camera failed to perform" explanation.
From my experience, 3,000 photos on the K-7, some 30,000 shots on the K100D, easily 20% of them with P-TTL flash, once the AF-confirmation lights up, the AF - slow, crappy as it may be, is no longer holding you back from taking the shot. The "slow" portion happens before the AF-confirmation Light and Beep comes up, not after.
There may be some other setting, interlock or limitation holding you back. You may even have set it accidentally without realising it.
The rare occasions I've had where the object was in focus, but the shutter wouldn't trigger have all been Flash related.
With on-board flash, the default setting is shutter will only trigger once the flash is ready. With external flash, the shutter will still trigger regardless, but I've noticed a delay compared with No Flash or Flash Ready.
If the flash had powered down in power saving mode, half-pressing the shutter will "wake up" the flash, but there may be a delay in shutter trigger as the camera communicates with the flash. For important occasions, I make sure the power is back on first.
With P-TTL flash, I'm familiar with a few other built-in delays after the shutter button is pressed e.g. Red-Eye reduction, P-TTL pre-flash, but since you are using a Manual flash, that doesn't apply.
Another delay may be a flash-camera communication problem. There is some minimum interaction even with a Manual flash.
Just trying to walk through the possibilities to diagnose the problem. "Camera failed to perform" doesn't look the right explanation to me.