The Sigma 15-30mm EX IF is a fantastic lens..... with horrible flare/ghosting. The skylight in this beautiful church is my nemesis! All shots taken handheld @ ISO800-1250. Most wide-open.
Not bad at all, in what are obviously rather tricky conditions! Apart, that is, from the last one, which to me looks like a very bright window without much else. Could you try bracketing and blending? Should be possible, even hand-held, although I realise that the flare will be a challenge.