Quote Originally Posted by Vince
Well~ thats difference between Full frame and 1.6x body...... Full frame sensor getting more lights into, am I right?

In low light, the FF only gets more light if you shoot it at a thinner DOF. If you use the same DOF, then the light, too, is the same. FF is no more or less difficult to get sharp photos in low light. In the case of the 7D vs 5D2, I would say the 7D has the advantage due to better autofocus.