Sometimes I browser over there, and found they are selling a
bunch of cameras, but I don't get the next info in the specs or what
they are meaning
>Focus 2.8 mm 4.0 mm
>Lens Mount M12
>Angle of View (H) 101.5° 83.8°
>Angle of View (V) 56.9° 42.7°
>Angle of View (D) 117.4° 100.1°

I get the focus is the focal length, but what does it mean? they can see far from? and the angles relative to fucking what? is right to divide the angles to half to get a idea of the field of view of the thing?
Thanks