K20D and bounce flash

So far, every time I tried bounce flash with my Sigma EF500DG Super and the K20D, I get serious under exposure and flash compensation or exposure compensation don't do anything to cure the problem. Some people I know having K200D and AF540FGZ have the same problem. Some body can give me a solution or a work around? Any help or suggestion will be highly appreciated. Thanks.

Normally, when you have serious underexposure with bounce, but not with direct flash, then the bounce distance is too far.

The flash-to-subject distance is not the camera-to-subject distance, but the route from flash-to-ceiling-to-subject.

Check the flash confirmation light in the viewfinder as a start...
(Or you can roughly guess if the flash is fully discharging with a "pop" and takes a full recycle time for the ready light)
Matt

And if the distance is too far, time to bump up the ISO!

Would 2 meters with a 2 1/2 meter high (white) ceiling be too far? No matter what I try, bounce flash ALWAYS underexpose.

It depends...
That's a total distance of about 5.4 meters.
But, it depends on (a) your ISO, (b) your f-stop, (c) the ambient light, as to how much light is actually needed.

Additionally, the Pentax metering tends to expose to maintain the highlights. This is generally a good thing - except where there are highly reflective surfaces. On the K10D at least, the "TAv" mode tends to expose the flash for a general exposure, rather than maintaining the highlights.

Personally, I find the better indoor approach is as follows:
* Use "M" on the camera
* With the flash off, press the Green button to get an exposure
* Adjust the ISO, shutter speed, and aperture so you are around 1-stop to 2-stops underexposed on the meter graph. Obviously, you're aiming for a sensible shutter speed primarily
* Now turn on the flash with TTL and shoot.

You should get perfect shots, with some of the ambient light shining through (rather than overly harsh flash). This method works a treat when using a large bounce card on the flash, with the flash pointing straight up. (See abetterbouncecard.com and make your own!)

Matt