Focus adjustment. If you use replacement glass with the wrong specifications it may not fit in the hot mirror stage, and you will probably have to adjust the focus. The focus adjustment screw (requires special tool) is below the blue tape. I recommend not to tamper with this, because there is no way you can adjust the focus while the camera is disassembled. You need to power on the camera to check focus, and the camera cannot power on when partly disassembled. The only option in this case is to assemble the camera and if the focus is wrong repeat the whole procedure and adjust the focus adjustment screw.
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