This is not 100% the story with opera shots since our business expects something more formal than they do for screen and acting, but I think that both of these articles are marvellous and make some of the key points (links to the sample shots at the bottom of the article)
http://more.showfax.com/columns/avoice/archives/000397.html
http://more.showfax.com/columns/avoice/archives/2006_05_22.html
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"If you only look at what you left behind youll never be able to focus on what lies ahead." - Gusteau