changing properties of a videoinput

Hello everybody, I created a GUI showing a webcam-live-video. Now i want to change the properties like brightness and exposure while running the cam. Therefor I used the command "getselectedsource" and changed the values with a slider. This is working very well. My problem is the opening fcn. I want to start with some specific values for the camera. I tried to set them the same way like in the callbacks (just without slider) but it does only work if I type in every command two times! Why?