Originally posted by surfcr8zy IMGP2917.JPG - IMGP2920.JPG were taken with the 50mm at F/16.
F/16 -- I doubt it! The shutter speed in the EXIF data is 10 times SHORTER than in your zoom frames, under, I presume, similar lighting conditions. Check whether the 50mm lens is really stopping down. If it is (nearly) wide open, any dust spot effects are less obvious.
As others say, this is the classical appearance of dust on your sensor. If you don't have a rocket blower handy, a turkey baster works, too (just make sure it's not greasy from recent use!!) And, DON"T touch the sensor with whatever blower you use.