Never had this problem before. Not even with Fisheye unless the blob of dirt is so huge it can be easily seen with naked eye on visual inspection of the lens, why even bother to use an image to evaluate?
Unless you're mistaken and talking about dust on the sensor?
No, definitely not dusts on the sensor cos other pics i've done with the same config, no signs of it.
Cos last time i went to TCW and wanted to buy the 17-35 f/2.8L but i saw some dirt inside the lens and are rather visible. So i took a pic and went back to see. I saw abt 2 tiny spots on the pic. Fortunately i didn't buy the lens, so i settled for the 17-40.