The sigma APO is a sharp lens, however, it has its weakness.
The photo below showed a strange color of the sun. No PS color was applied in the sunset pic.
Indeed, same occurence with my friend's sunset pic whilst he was shooting with a Canon. A few more pics for your reference.
My other friend , a K10d owner, also had a tamron, the lens is as sharp as sigma but didn't encounter any strange color for the sunset.
