Ok... I'm not going to refute much of what has been said here (from a technical viewpoint)... but here are my observations, so far, from a month's work with the a900 in paying situations (even if that makes no difference
).
I think exposure optimization has more to do with
photographic intent than technical means. I use ETTR to keep the amount of shadow noise to a minimum in my photographs
to a point. Blindly using it is just being ignorant, if you ask me, but using it smartly to control the dynamic range of a scene is essential at times if you need the most data to work with at the beginning of the post process.
In wedding work, I'm often in situations where the majority of the scene is in deep shadow. I'd rather ETTR to keep chroma noise at bay amap than to lose even more detail by having to deal with excessive noise (dictated by the scene)
I totally agree about the color hit you can take with ETTR... given that the scene isn't all highlights. There are high key scenes that beg for ettr and underexposing deeply and boosting in post (the opposite of ETTR) will (in my book) yield poorer results than an ETTR exposure.
Lastly... and I need to test this more... but 320 is a great ISO on the a900 but i see way to much texture in broad smooth areas (skies, for example) to warrant it's use in many situations. Even if the camera isn't optimized below 320, the camera does take a noise hit at 320... ettr or not.
For me... exposure is just as much a question of intent as it is about optimization... and I make that decision taking a BUNCH of factors into considerations.
Great topic!