Quote Originally Posted by balders
...All test photos were taken using same shutter speed!
Perhaps you're mistaking a shallow depth-of-field for "out of focus"? To be using the same shutter speed, you're obviously using a wider aperture when shooting with the CP - perhaps even at the maximum aperture. This alone will "degrade" the image slightly and the shallow DOF will require more care on your part (depending on what and how you are shooting).