20D Sensor Dust Question

Hello, I am new to the forums. I have a question. Why would sensor dust show up at the higher apertures but not the lower ones. I have tried this over 10 times with 10 different backgrounds (mostly different shots of the sky) and get the same result each time. 100mm, f4.5, no specs on my image. 100mm, f20, the same 5 specs on my image, in the same place every time!! I have tried this multiple times with both my Canon 100mm 2.8 macro and Canon 28-135 is usm (at 100mm) and the spots are in the same place so I know it is not the lens...

Also, how can I clean the sensor? I have done exactly what the manual says at least 25 times. The dust went from about 12 good specs down to 5 but those 5 will not go away no matter how hard I blow. I am a newb so sorry for the simplistic question...

thanks, andy

At smaller aperture the light strikes the sensor at more of a direct angle causing the dust on the sensor to be more well defined (it's essentially the shadow of the dust). The smaller aperture make the light hitting the sensor (and the dust) more like a point source (harder light) than a plane source (soft light) which causes the dust to cast a harsher, more defined shadow on the sensor.

If you have stubborn particles after blowing off the sensor you may need to use a static brush (search Visible Sensor Brush) or a wet cleaning method (search Copperhill Cleaning Method) to remove the dust. The Static Sensor Brush uses an extremely clean brush which is statically charged by compressed air to remove the dust by wiping it across the sensor filter, while the Copperhill Method uses ultra clean Pec Pads, a sensor swab, and Eclipse Cleaning Solution (methanol) to wet clean the sensor filter.

Mark