Focus Lock SX 50
I too am amazed at your Jupiter photo with its moons! Well done.
I just tried AF Lock with my camera set to Tv and it seemed to work as explained in the manual. I held the shutter button down half way and then pressed the nine o'clock outer edge of the large control wheel on the back of the camera. In the viewfinder/LCD it showed MF and the little distance gauge and stayed at that focus point under I repeated the same steps to turn AF Lock off. It does not work in AUTO but you can see on pages 260-261 which shooting modes it does work. Hope this helps?