Disclaimer: Disassembling your lens will void the manufacturer warranty and may cause your lens to become inoperable. Do so at your own risk. I will not be held responsible for your actions.
The Sigma 105mm f/2.8 EX macro has been my all-time favorite lens. It has the capability for me to explore tiny worlds around our environment, which we generally overlook. It allowed me to see little thing from a totally different perspective.
However, due to my busy schedule in the past year, the lens had sat in my closet most of the time. When I took it out to enjoy the small wonders again, I was shocked to find that it no longer auto-focused all that well any more. Being a macro lens, I use it in manual focus mode most of the time anyway, especially for 1:1 (one-to-one) magnification. I managed to continue using it for quite some time. But not being able to use auto-focus on an AF-capable lens bothered me.
Finally, I decided to disassemble it and try to fix it. I have always wanted to see the inside of a Canon EF lens anyway.
Step 1: Unscrew the three tiny screws securing the inner plastic ring at the rear of the lens.
Step 2: Once the inner plastic ring has been unscrewed, carefully pry it off.
Step 3: Unscrew the two screws securing the lens contact.
Step 4: Unscrew the four silver screws that secure the metal lens mount to the lens. Note, at this step, the plastic inner ring should have already been removed and the lens contact unsecured. The picture does not reflect the disassembly, because this picture was shot after the lens was put back together.
Step 5: Remove the metal lens mount.
Step 6: Remove the metal ring inside the lens.
Step 7: Remove the rear lens body ring carefully. This piece contains the AF/MF switch, which is mechanically connected to the mechanical gears and an electrical switch. Be careful not to break or bend any parts.
Step 8: After remove the AF/MF switch ring, the mechanical gears, the AF motor, and the electrical switch are exposed.
I figured that the lens do not AF all that well any more because the lubrication has dried out and the gears have too much friction that the motor can not overcome.
I had some multi-purpose oil at home, so I lubricated all of the mechanical gears with it. After the oil soaked into the gears (I sped up that process by exercising the manual focus ring), the lens auto-focused much faster, smoother, and better! I was surprised how well the lubrication worked.
To reassemble the lens, follow the steps of disassembly in reverse.
I told my co-worker, who happens to be a professional (certified) mechanical engineer, about my success. He said I was quite "gutsy" to use general-purpose oil on the gears. He claims that there are lots of different types of lubrication out there for different purpose, such as graphite and a few names I could not remember. There are wet and dry lubrications. He said it is quite risky to use pick a type of oil without knowing what the manufacturer uses for those gears.
If you are a lubrication expert, please feel free to educate us on lubrication and the type that should be used in the AF mechanism.
So far, I have lubricated it twice. Every time I lubricate it, it would work fine for a week. Then it would return to its previous non-focusing state.
Recently, I had obtained a Digital Rebel. It worked with the lens fine at wide-open aperture. When stopped down, the camera goes into error state. After contacting Sigma, they mentioned that the chip inside the lens has to be upgraded. The upgrade was free of charge.
When I sent the lens back to Sigma, I also mentioned the AF problem to them. They were very nice to send the lens back upgraded and fixed with a diagnostic report. It turned out that the AF belt has weakened and they had replaced it. Now the AF works just fine on this lens. With 20/20 hindsight, it is apparent the AF belt is the real problem and the lubrication just eased the labor on the weakened belt.