Q:  What is F2.0, f 3.4mm stand for, how do I choose lenses from these number.

A: F stand for aperture of the lenses is on  F stop 2:1 and f 3.4mm stands for the focal length of the lenses is 3.4mm
a lenses with F 2.0 and f 3.4~4 is most economical  to build, hence most inexpensive. it is widely using in board camera. A lenses with F 2.0 aperture will collect about half of the light then human eye. a lenses with f 3.4mm will have about 60 degree of view angel on a 1/4"  CCD and  90 degree on a 1/3" CCD , which is pretty close to view angel of one human eye. Human with two eye will cover more view angel as an ingenious design of God. It is generally in the range of 150 to 180 degree from people to people. however please keep in mind that F stop and f focal length is the fundamental spec of a lenses but it doesn't stands for the quality. A good lenses with same F stop and focal length  can be 100 time more expensive then a bad lenses in same spec. Please check next Q and A for more information

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More:
F 1.4  means F stop is 1.4:1 , it is the ratio of the opening of a lenses. a lens with F1.4 will collect 2 times more light them a lens with F 2.0 . the basic formulas is that  square power of  2 equal to 4 and square power of 1.4 equal to  2 , and 4 is 2 times bigger then 2   ( 2**2 / 1.4**2 = 4/2=2 ) In another word a lens of F 1:1 is 100 times brighter then a lens with F 10:1 most of the CCTV camera will choose to use lenses in the range of  F1.4 ~ F 4 for economical reason. the smaller the number the better the lenses and of course more expensive.

f 3.4mm mean the focal length is 3.4mm. a single glass lenses with focal length will focus a image 3.4mm from the optical center of the lenses. a f 8mm lenses will throw a Sharpe image 8 mm from the optical center of the lenses. the bigger the number the narrower the view angel. a lenses with f  8 to 80 mm is normally understand as telescope type. a lenses in the range of f 2~4 is generally having view angle of 40 to 90 degree and most close to what human eye will see. A lenses with f 1~2 mm or smaller is considered as wide angle lenses.