Focal length and FOV

What is focal length you may ask...well, the focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point. The focal point is located on the sensor or film ( the subject is in focus).
FOV stands for field of view, and is determined by the angle of view from a lens out to the scene.
It can be measures horizontally or vertically. The FOV depends on the film or sensor size, so it is not fix for all lenses.

But, to get an idea of what focal length and FOV is let's see some examples:

Focal length: 55mm and FOV of 28 degrees.

Focal length: 105 mm and FOV of 15 degrees

Focal length: 205 mm and FOV of 8 degrees.

So, the greater the focal length, the more closer the objects appear BUT less object in the scene. This means that the FOV is getting smaller.

That is it...