I wanted to see if there're any stains on the sensor of my Canon 5D MK3 and I took this shot of a white surface shaking the camera with a slow shutter speed. I keep getting the same image with such a weird pattern.
Is this just dirt?
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It's something on the surface of the cover stack over your sensor. I doubt it's "just dirt" though. Some of it is probably dust that can be blown off easily with an air blower. (Do NOT use "canned air"! It will leave residue behind.) But some of it is probably stuck to the sensor a little bit. It's time to do a wet cleaning of your sensor if you need to take photos at narrow apertures. Wide open with a fast lens those spots probably won't even show up.
The subject of cleaning sensors has been well covered here. For more, please see:
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