MT Shooter wrote:
It is because the DOF is so incredibly shallow and no matter how steady you may thing you are, the slightest movement on your part will cause the lens to try to re-focus. You were obviously using AF-Constant mode when this happened. Also, butterfly wings are seldom totally still, their slight movements will also cause AF to hunt.
Try using AF-Single mode and set your AF to SPOT, that will eliminate much of the issue you describe, or just focus manually.
Agree. If you are using Auto, then the D7100 will anticipate the subject and continually auto focus.