I`ve used the 505 / 707 and now the 717 for a long time now. The advice you have been given is correct. If you need to recompose then you must release the half pressed shutter button and then half press again after recomposing. The AE lock does this but it is some times easier to use the shutter button. An important tip which applies to all digi cameras concerning shutter lag. If you need to take sport or action shots, the way to minimise and almost eliminate shutter lag is to set the camera to totally manual including focus. You will almost certainly be using infinity and if you set the camera to manual mode and meter for the general situation you should be OK. Track you subject with the shutter button half pressed and fire when required. The lag should be milliseconds. You can make any adjusments in Photoshop or similar.