... But if I go smaller than say f/13, you can see them. ...

Then they're on the lens, not the sensor.

I assume you have use a Giotto blower as well? I had one spec visible in my small aperture shots, hit it few times with a Goitto blower, and it went bye bye.

I am sure you found this video, but if not lensrentals.com has a nice one - http://www.lensrentals.com/blog/2010/04/how-to-clean-a-camera-sensor

Its on the sensor.  At least that one large one.  I get it on other lenses too.

I did some experiments, you're right: the surface of the sensor (where the dirt is) must be much further away from the actual silicon than I thought 

Quote from: pineconetreehouse on February 19, 2012, 05:11:48 PM

Its on the sensor.  At least that one large one.  I get it on other lenses too.

I did some experiments, you're right: the surface of the sensor (where the dirt is) must be much further away from the actual silicon than I thought 

The surface you can clean is not actually the sensor, but the "optical low pass filter" which is located just in front of the actual sensor.  This is one reason you don't want to use "canned air" or cleaning fluid in excess of what the sensor swab instructions say. 

If you manage to get anything down in between the sensor and the OLPF (canned air propellant, cleaning fluid, or dust driven between) there is no way you will ever be able to remove it yourself.  The camera then has to go in to Canon for what I assume will be a very expensive repair/service.