Actually just to clarify that - when you look through the viewfinder and focus, you're actually looking at the focusing screen and the meter measures the exposure at the focusing screen. You can see the focusing screen if you remove the lens and look upwards into the camera lens opening. On the half -press of the shutter button, the meter reads the exposure. When you press the shutter all the way, the mirror flips up, the shutter opens, and the light through the lens being no longer diverted by the mirror, projects the image onto the sensor.