Answer:

The light is strong at day-time and the iris is small,the depth of filed becomes larger after focalizing.However,the iris becomes larger in the evening due to weak light and the depth of filed becomes smaller,leading to the object being monitored falling out of the depth of filed.The solution is that,please kindly set the camera to electronic shutter mode when trying to focalize using automatic iris lens(the automatic iris would be the largest),then change back to automatic iris mode. Or try to focalize under low illumination condition.