The focusing screen is one of the most important aspects of a single lens reflex (SLR) camera's function, as it serves to focus the image seen through the viewfinder. Short of zone focusing (based on the distance ring attached to a lens), it is the only way to tell whether the resulting image will be properly focused. The milky-white screen is located just beneath an SLR's top prism, and is met by the retractable mirror when it is flipped upward. A foam damper is glued to one end of the screen to cushion it from the mirror.
Ensure that your work desk is completely free of small debris, dust, moisture and other materials that could fall into your camera's body.
Remove the lens from your Canon AE-1 and turn the body upside down. This will allow for easier access to the focus screen.
Depress with your tweezers the small metal tabs that secure the focus screen. Some focus screens are secured using more cumbersome mechanics, but the AE-1's tabs make removal comparatively easy. Pay attention to how the focus screen is positioned during this step, as the orientation of the new one must match.
Gently lift the edge of the focus screen with your fingertip. Grasp the edge with your tweezers and lift it out of the camera. Set it aside.
Grasp the edge of the new focus screen with your tweezers, ensuring that it is positioned in the same way as the old screen.
Lower the screen into the body of the camera. Gently move it into position, ensuring that it is placed exactly like the old one.
Nudge the screen back into the camera body until the two tabs engage it and click into place.
Blow a few short bursts of compressed air into the camera body from about a foot away to clear any dust that may have settled.
Replace the lens and test out your new focus screen.
Acquaint yourself with the anatomy of the Canon AE-1 before replacing the focus screen. The more you know about the inside of your camera's body, the better prepared you'll be.
Replacing a focus screen on a Canon AE-1, or any SLR camera, is an advanced procedure and can result in costly damages if not performed correctly. If you are not confident with your technical abilities, consult a professional repair service.