Originally posted by Kendrick ". . . why my Pentax Km will not synchronise with an external slave-flash triggered by the built-in flash even at exposure settings as long as 1/30 second".
You need to read up on the Pentax p-TTL flash system. In short, the reason why your K-m (or any recent Pentax dslr, for that matter) won't sync to an optically triggered external slave flash, is that the camera's built-in flash goes off
twice. The first flash is done before the mirror flips up. This allows the light sensors in the viewfinder to "read" the flash and calculate the necessary aperture.
After the first flash, the mirror flips up, the camera stops the lens down to the calculated aperture and the shutter opens to take the picture.
Unfortunately, your optical trigger saw the pre-flash and fired the slave while the mirror was still down. I've heard that there are some optical triggers that can ignore the pre-flash, but I've heard that they are not very reliable.
Try this. Set the shutter speed at 1 second. Set the camera for force it to use the flash, regardless of ambient light. If your K-m has the option of firing the flash on the trailing shutter, select that option. Trip the shutter. You should see the preflash, then, a little less than a second later, the main flash will go off.
Setting the camera in manual mode MAY turn off the preflash and fire the onboard flash at full power. However, the downside is that you will have to calculate the aperture yourself. Of course, if you're using off-camera flashes and they're not p-ttl flashes, you'd have to calculate it yourself.
BTW, Nikon and Canon dslrs do essentially the same thing. They have different names for the system. One is i-ttl and one is e-ttl. I can never keep straight which is which.
If you're interested in off-camera flash, check out The Strobist website at
Strobist