This is a handy little app that will allow you to calculate what lens is needed to achieve a particular field of view. This is particularly useful when you know how far away the subject is and what screen height you need, the calculator will then allow you to slect a suitable combination of lens and sensor size.
The screen height descriptors are those used in the UK CCTV / law enforcement communities but the equivalent heights in meters is also included. A screen shot of the app is shown below