Wilt wrote in post #13645320
The point is that in unchanging light
, when your framing changes or someone walks into or out of the frame, the exposure for the lighting theoretically is identical. Yet the change of brightness caused by someone walking in or out of the frame will cause automated modes like Av to set a different exposure--when any exposure change is totally unnecessary...you have unchanging illumination, right?!
Once again, Wilt nailed it here - especially in the context of a wedding.
Let's say you're using any of the automatic modes, and set up your EC so that it's metered perfectly for an empty scene. As soon as the bride walks into the frame in her big poofy white dress, the camera will think "WOW, look at all that white, I better dial it down a stop," and everything is underexposed compared to your original setup. Then, when the bride walks away and the groomsmen come in, the camera sees a row of black tuxedos and says "OH GEEZ, look at all that black, I better bump up the exposure by a couple stops," and it comes out overexposed. The camera doesn't know that tuxedos should be black and wedding dresses should be white. It just wants to make everything gray.
"M" mode isn't about chasing the needle; it's about getting a consistent exposure from one shot to the next, in situations where the lighting isn't changing.
In situations where the lighting is changing but the subject is not, the automatic modes are appropriate. In situations where the lighting is changing and the subject is changing - well, you've got your work cut out for you, I guess