thanks, I'll keep you updated. So far, I didn't encounter any focus problems in any light situation. In the museum shots, it's halogen and tungsten, mixed with daylight. I have two jobs coming, one Sunday, one Monday to Wednesday. Daylight, indoor tungsten, indoor fluorescent, energy-savers, all mixed with daylight. I'll try the Sigma 50A, have the 24-70/2.8 L II with me as backup.
After completing to upload the 3 JPEGS from RAW, I'll add six night-shots, 960px, EXIF intact.