You should replace your focusing screen, it cannot be repaired. Replacement focusing screens can be found from a variety of sources, ebay lists several, at around $20 for the original Canon part.
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There are several different replacement screens available. The original standard screen fitted to the 70D is the Ef-A. A popular non standard screen is the Ef-D which has grid lines added. If you like to use vintage lenses without autofocus, you might prefer the Ef-S screen.
Replacing the screen involves popping out a retainer clip while holding the camera upside down, you can then lift out the screen with a pair of tweezers and put the new one in place. The replacement screen comes with instructions and a pair of plastic tweezers for handling. Wear latex gloves and avoid touching the surface of the screen, ''it must be handled only with the tweezers supplied''.
You should replace your focusing screen, it cannot be repaired. Replacement focusing screens can be found from a variety of sources, ebay lists several, at around $20 for the original Canon part.
Replacing the screen involves popping out a retainer clip while holding the camera upside down, you can then lift out the screen with a pair of tweezers and put the new one in place. The replacement screen comes with instructions and a pair of plastic tweezers for handling. Wear latex gloves and avoid touching the surface of the screen, ''it must be handled only with the tweezers supplied''.