A question of Focal length in optics

Hey CEans!

I have recently entered the realm of aerial photography, and know nothing about it at all 😉

My aim is to capture an image of the ground from high altitude, from 500m till 5km (and 100km if things go according to plan). The digital camera I aim to use is a basic CMOS mega pixel sensor. I cannot use a commercial one (like Cybershot), as I need a UART output.. though it is certainly possible if I had ample time.

The issue I'm facing is, what is the appropriate specification of the lens to use in this type of application? Theres are some lens that are available with the CMOS sensor:

F/#                            2.8  2.8  2.5 2.0  2.0  2.0 1.6  1.6
Focal length (mm)              4.63 4.63 2.2  2.8  3.6 3.6 6.0 6.0
Field of View Diagonal (deg)   41   41   109  81   50  50   43   43

Now, professional aerial cameras have focal lengths of many inches, which is a heck more than the ones in the table. But is it still possible to get a decent image with a 6mm focal length? And, should the FoV be smaller or bigger? I'm thinking that the bigger it is, the more terrain I can capture.. but the image distortion would be more obvious.

Thanks in advanced 😀