Sorry you guys, busy to pack some of my stuffs for my trip thus cannot follow the thread.
@Adelfin: I've tried quite a lot, not in low light condition but in sunny day with the subject in the shade. Yes, pumping up the brightness can have a better look but as I said the pix look ok in full screen mode but not in 100% crop. In 100% crop can see blur, in situation 2 & 3, 100$ crop the pix is very sharp.
@ cantaresg: Im using dSLR with 50 1.4. Taking 3/4 body with Av = 3.5 -> 4 for the test. This makes me irritated coz in situation 1, with a PnS the pix seems sharp in 100% crop (maybe because the pix size a bit smaller and PnS makes the pix more sharpen then dSLR does in post processing)