Issue with Sigma Lens

Experiencing an issue with my trusty Sigma 24-70 f2.8 Macro lens. I think it may have been my fault in the long run, but it looks like there is an issue with the focus ring. The few times I have shot video with this lens, I forget to engage (or disengage as the case may be) the clutch mechanism of the focus ring, in conjunction with the AF/M switch. Those times I did feel a bit of extra resistance, as if the AF gears are not disengaged, and then I am reminded to slide the clutch focus ring. I have done that maybe 3-4 times since I got the camera about 2 weeks ago. Video was never a consideration so I have not played with it much.

As of this evening, I was playing with video a bit at a friend's house and when I switched back to AF, there seems to be a "rough spot" right about 2ft to MFD (coming back from INFINITY) on the focus ring. When I try to AF its smooth one way, but its struggles in opposite direction - you can actually hear the motor stressing to overcome it. It eventually will lock focus, but not without a slight delay, and what seems like the lens barrel jerking a bit. I have not dropped the lens, nor do I treat my gear poorly. When you switch to MF you can feel this "rough spot" of travel in the focus range. EDIT: note - I was playing with it a bit more and I just missed focus on a close distance shot. It appears the lens barrel is very loose and there is 1/8th inch of play. Enough so that when that "jerk" I mentioned above takes place, it throws the focus off. I just wiggled the barrel by hand while looking through the VF, and the image went in and out of focus. This blows!

Just curious if anyone has experience anything similar, and what it took to repair it, etc. Thanks in advance for your comments.

Based on what you're experiencing I doubt you cause that by manually focusing the way you did (not that you should have, but you know that).

javig999 wrote in post #11696517
When I try to AF its smooth one way, but its struggles in opposite direction - you can actually hear the motor stressing to overcome it. It eventually will lock focus, but not without a slight delay, and what seems like the lens barrel jerking a bit.

While I do not know the internal workings of that lens what you are describing sounds like a gear that has been run over.

What can happen when there is a large ratio between two gears say a small one on a drive motor and large one on the object to be driven, is if the drive power is reversed from the driven part to the motor the reverse torque can damage one or both of the gears bending the tips of the teeth. One or more things can happen as a result of this type of damage, one is the bent teeth while not affecting motion in one direction will produce additional friction in the other direction causing sluggish or sticky operation in that direction and or the other is due to the ramp like effect of the bent teeth cause the drive gear to skip over the teeth of the driven gear resulting in a jerky operation.

Both types of damage are not reversible and will eventually become fatal in the end requiring replacement of the damaged parts and the removal of any chips or flakes that may come off the gears as a result of the damage.

Wayne

Sigma has poor quality control..,,,,.,