I had an educational experience the other week with a camera repair guy who was showing me some examples of sensors he has cleaned. He shoots a frame of a lightbox at f/22 and then processes the file in some way that shows all the marks. I'm not sure what the processing was, I'm guessing it was something like contrast & sharpening, but it really shows up all the marks including some that were not visible on the initial frame. Some of these more subtle marks were rather soft & grey in appearance and according to the tech these were caused by people using rocket type air blowers to clean the sensor. He reckons they also blow some humid air in & this causes the extra marks. Might not be a problem if you are making small prints of for the Internet, but would be bad if you are printing big for an exhibition. Anyway it really opened my eyes to the complexity of this matter. funny thing is, i was planning to buy a rocket blower after I'd left his premises, but I think from now on I'll just give the digital camera to him for cleaning. I'm not sure how he cleans them but the after shots were spotless.