Found a nice tool for figuring out what camera you need with what lens

CCTV Design Lens Calculator is a free tool to show you what lens / resolution combo you need in order to:

• Monitor and Control - 5%; A figure occupies at least 5% of the screen height and the scene portrayed is not unduly cluttered. From this level of detail an observer should be able to monitor the number, direction and speed of movement of people across a wide area, providing their presence is known to him; i.e. they do not have to be searched for.
• Detect - 10%; The figure now occupies at least 10% of the available screen height. After an alert an observer would be able to search the display screens and ascertain with a high degree of certainty whether or not a person is present.
• Observe - 25%; A figure should occupy between 25% and 30% of the screen height. At this scale, some characteristic details of the individual, such as distinctive clothing, can be seen, whilst the view remains sufficiently wide to allow some activity surrounding an incident to be monitored.
• Recognize - 50%; When the figure occupies at least 50% of screen height viewers can say with a high degree of certainty whether or not an individual shown is the same as someone they have seen before.
• Identify - 100% With the figure now occupying at least 100% of the screen height, picture quality and detail should be sufficient to enable the identity of an individual to be established beyond reasonable doubt.

Note those percentages are based on the old analog cams of about 400 vertical lines. With this tool you feed it the camera specs (including lens) and it will tell you how close it needs to be to the place you are watching for the level of recognition you need.

For instance if I want to read license plates at my main gate. 
With a 720p Escam QD300 with a stock lens, the camera would need to be 34.6 feet from the gate.
Oddly with a Escam QD900 with its larger 1080p sensor but the same 3.6 mm stock lens, the camera would need to be 32.4 feet from the gate. This is because the larger sensor causes the view to be wider even though the lens is the same.
If I swap out the lens though to a 16 mm, that max camera distance jumps to 144 feet. which works.
Put that 16 mm lens in one of the Hikvision 2K cameras and I should be able to read plates at 185 feet. Which be across the street or back in the trees so that is over kill.

To me the most useful part of this tool is seeing what you get for usable video for instance here is a compare of lenses for one of my cameras

You can also import your floor plan and yard objects into it to find blind spots in your layout and try various placements of cameras for the best coverage before installing any.
See their screenshots here