SX50 Framing Assist Lock test...

:-)Seems your steady hand is better than the IS. (when the target is big, perhaps?)

Could you try with a smaller target (object so small that would jump around in the EVF and you've to struggle for getting it within the focus box) and see if there is any difference?

One thing I wonder is: whether the lock on actually has effect on the final output or just for comfort preview on the EVF during the composing of picture?

cn