Focus on an area of sky where the fireworks will be, or on an object the same distance away.
可以对焰火即将绽放的那片天空对焦，或者找个与烟花相同距离的物体进行对焦。
2
You need to set your focus to infinity. Try focusing on a stable object in the distance.
你要把焦点设为无穷远，对准远处一处固定物对焦。
3
Once you choose a lens whose focal length is closest to the focal length required by your imaging system, you need to adjust the working distance to get object under inspection in focus.