It seems that I have a very stubborn particle on my sensor. I've wet cleaned it about 15 times (!) and the sensor is gleaming clean, except for this one spot. I've checked it out with a scope and it looks like something on the sensor.
It starts appearing (smudge) at F8, and show nicely at F22.
I think this might be a professional cleaning job (aka Nikon)?
Any advice? I've tried a blower, brush, wet cleaning...maybe a sensor pen?...