Sigma 70-300mm DG Macro lens

Recently added a Sigma 70-300 DG Macro lens to my Nikon D5100. This lens will not "lock" into focus when attempting to focus on objects at the extreme end of the telephoto (200-300mm). The focus will lock in focus and then immediately go out of focus and then re-lock. I am using a tripod and have tried several different setting on the camera. All settings on the camera and lens have been checked and double checked. Nothing seems to solve the problem. This is my first Sigma lens so any help or advice anyone can give would certainly be appreciated

Use manual focus.

​What focus setting are you using?

I believe this lens has two switches on it are they both in the correct position,ie fully home.

Have u try lock it to macro...minimum focus 200mm...

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All switches on lens and camera are in normal position for shooting telephoto. Camera is stationary. Subject can move (animal). Camera should automatically track movement and refocus. Camera performs correctly when using the Nikon 55-200mm lens. Using the Sigma lens, the focus will not lock on the camera, it "hunts"