1. How to choose the FOV?
By default the FOV in specifications H:V is 4:3, if you use 16:9 sensor the FOV(H) will be wider, and FOV(V) will be smaller.
2. There are 1/1.8" 1/2" 1/2.5" 1/2.7" 1/3", Which one meets my needs?
The image format depends on the sensor format. The image format of lens should be equal or larger than the sensor format, otherwise there will be black area or corner.