[sbbtim]

I found the shutter speed determined in Av with Matrix metering mode are not quite consistent. When I half press the shutter release button, the camera gives me a shutter speed number. If I release the button, it tells me another number. No recomposition, no big source light changes. Sometimes the difference can be 1.5 stop.
Anyone has experienced similar thing with his/her K-x? Thanks.

[enoeske]

Is the camera on a tripod or are you handholding it? My guess would be you're handholding and thus there is camera movement. The camera calculates a different shutter speed based off of what is in the frame and when you move the camera, it changes. Try it indoors on a tripod without florescent lights or a TV/monitor in the room

[KungPOW]

also, depending on the light conditions, you can get 1 to 2 stops under exposed if you do not have the viewfinder covered.

So if you are not looking through the view finder, cover it.

[pbo]

Also, about settings changing as soon as you let the shutter button go.
It's probably because: 1) you're using shutter half-press as AF button and have camera set to AF.S and 2) you have AE-lock on AF-lock enabled (in layman's terms, camera locks exposure settings as soon as it obtains focus).

So, when you half-press to focus, and camera obtains proper focus, it immediately locks the exposure settings at that moment. You can try shifting camera around - the values will not change until you release the trigger.
Also, if I'm not mistaken, matrix metering does have bias - so center is the most important part, while the other 15 points are less so. So, if you had a dark something in the center, and then during composing shifted off of it (while the scene remains generally same), matrix results will be different.