I have to ask - are you certain the dust is in the lens? Generally speaking, a piece of dust in the lens will have no effect on the image, because of the repeated refraction of the light as it passes through the lens.


Sorry for the basic question, but have you tried shooting with a different lens (stopped down shooting a clean, white surface or a clear blue sky)? I suspect you're looking at dust on the sensor (there may be dust in the lens, but that's not causing the spot on the image).