F is for Focal Length
Focal length is a term which determines how much the lens can capture. Focal length is the measurement of distance (in milimeters) from the point at which the light converges to the sensor or film on which the image is captured.
Human vision has a standard range of vision equivalent to a lens with a focal length of 50mm. A side effect of this is that photographs taken with a focal length of 50mm are ideal for reportage style photographs, but lack dynamism in artistic photography.