If your focus lock is not engaging or is too loose, the fix is extremely simple. As I couldn't find instructions with photos, I decided to post my own. Most people probably won't need the detailed instructions with photos, but I believe it's still good to have.
1. Remove the leather cover (A) around the tripod socket o the bottom of the camera. I used some very fine tweezers for this. The cover is glued down and will probably be tougher to remove on some cameras than others.
2. Remove the 3 screws hidden underneath.
3. Pry open the Plastic bottom cover. I started in the top right corner of the camera, as there is a little cavity in the chassis that allows for easier access (B). The cover is held down by a flexible glue. If you have ever repaired a phone screen, the procedure is very similar. Carefully pry open the cover in one place, use a shim (e.g. guitar pick) to wedge into the gap, and work your way around the camera.
4. Once open, locate the mechanism for the focus lock as shown in the photo. Slacken the screw (C) and turn the dial (D) clockwise to tighten the focus lock. My lock was not engaging at all, but a small rotation of around 10° was enough to fix it.
5. Test to make sure that the focus lock is not engaging when the lever is in the open position, but locking when the lever is pushed back. If the focus is stiff even in the open position, repeat step 4 and turn the dial (D) anti-clockwise again to loosen the mechanism.
1. Remove the leather cover (A) around the tripod socket o the bottom of the camera. I used some very fine tweezers for this. The cover is glued down and will probably be tougher to remove on some cameras than others.
2. Remove the 3 screws hidden underneath.
3. Pry open the Plastic bottom cover. I started in the top right corner of the camera, as there is a little cavity in the chassis that allows for easier access (B). The cover is held down by a flexible glue. If you have ever repaired a phone screen, the procedure is very similar. Carefully pry open the cover in one place, use a shim (e.g. guitar pick) to wedge into the gap, and work your way around the camera.
4. Once open, locate the mechanism for the focus lock as shown in the photo. Slacken the screw (C) and turn the dial (D) clockwise to tighten the focus lock. My lock was not engaging at all, but a small rotation of around 10° was enough to fix it.
5. Test to make sure that the focus lock is not engaging when the lever is in the open position, but locking when the lever is pushed back. If the focus is stiff even in the open position, repeat step 4 and turn the dial (D) anti-clockwise again to loosen the mechanism.