Ok. So in doing that you're making the exact same decision regarding exposure the camera would've made for you in an instant. I'll try and explain it a bit better... You're on Av mode f/8, ISO 100. You frame that shot and focus. In that instant the camera has decided the shutter speed needed to make that photo with 0 EV, so meter at 0. Do the same in manual and zero the meter and the photo will be exposed identically. The only difference is it took longer to arrive at that result. Shooting on manual doesn't add an extra element to the photo that is missing from the other modes.