zoom_zoom wrote in post #8002256
The potentiometer? I don't have a problem with my mkI, but what exactly is the potentiometer? It sounds like you had to physically take apart the lens to fix the problem?
To be honest, I wasn't clear either, at first. Here's the wikipedia link: potentiometer. Essentially it's a variable resistor. In this case, it controls the AF in the lens. Too much power, and we get Mk I lenses overshooting the focal point and oscillating wildly trying to find it.
To make the repair on the 50mm 1.8 mk I, remove the three screws in the metal mount plate and carefully remove to expose the inside of the lens (it will remain connected by a set of circuitry wires). There's a dark, anti-static plastic sheet that needs to be removed next (it's flimsy, unconnected to anything and unattached, but make note of its proper orientation). The potentiometer is not obvious at all. It looks like a tiny screw head, 2-3mm in diameter, and it's hidden beneath a folded over flap of circuitry wire. You'll need a small jeweler's screwdriver to turn it (you'll need the same to remove the metal mount as well). Once the adjustment is made, put everyting back together the way you found it. Simple and straightforward.
I was nervous as all get-out when I first started taking the lens apart--I didn't want to do anything to ruin it, after all. But once I got it apart and recognized the various pieces, I was able to relax a lot. This particular repair isn't brain surgery, and unless you slip with the screwdriver and damage some circuits or scratch the glass, there's not much opportunity to do any real damage.
Note that this repair only applies to Canon's oldest, first-generation EF autofocus motors. I don't even know if later lenses like the Mk II even have potentiometers. But it's a good fix to know if you've got a Mk I. Saved me the time and expense of shipping it off somewhere.
