I am looking to buy a new "standard" lense this week. I have been using a kit lense (28-135, F4). During wedding receptions I need a wider apeture. I want a lense that goes to at least F2.8. The problem I am having with my kit lense, is that a ton of photos are not focusedand dark. I am sure this lense is not giving me the quality I should have. I was looking at Sigma's and Tamrons since they are SOOO MUCH CHEAPER. What is the difference between an expensive canon lense and a Sigma or Tamron? I have only used canon before, so I don't understand all the details of these other lenses. What is a "Di" lense, or a "EX DG"? Has anyone had any experience with these lenses?