My thumb controls the focus button and my trigger finger controls the shutter release button. In AF mode, you must activate the focus button before the shutter release button. Many people said you have to letup on the shutter release button before the focus button or the camera will take a 30 second exposure. In the limited tries I attempted, I could not get a 30 second exposure.

In MF mode, you still have to press both buttons as above, but the focus button does no activate focus as you already set the camera to manual focus.