Originally posted by Eruditass This makes sense, but what effect does dust / particles on lens front element as well as rear element do? loss of contrast in an area?
Loss of contrast or loss of sharpness if there is an extreme amount of dust on the lens.
Originally posted by emalvick As an aside to the other answers, the one thing that you can do to confirm a lens vs. sensor question is to see what happens once you swap lens. If it is the sensor, you should get the same dust spots no matter what lens (although their sharpness and size will vary with aperture).
Unless the plane of focus is actually inside the lens on one of the lens elements, you will not see dust on the lens as a visible artifact on the image. Changing lenses will tell you nothing, since if the spot is visible, it is not on the lens.
There will be the very rare circumstance where the plane of focus is so close to the front element that dust might be visible if one is doing extreme macro work with the wrong lens, but in normal picture taking situations, lens dust will cause little or no visible image degradation, and what it does cause will not be visibly apparent (spots on the image).