All camera lenses for all formats have a focal length - 
It is the measurement of the distance from the lens to the film when focused at infinity in millimeters or inches.

Measure your magnifying glass focal length by focusing a distant light source on any surface. This is called convergance.
Measuring that distance equals the focal length.

Lens Focal Length - __________mm
_________ inches

The f/stop is the focal length divided by the diameter of the lens (or lens opening).
Example 1 - if your lens focal length is 100mm and your lens diameter is 25mm... your f/stop is f/4
Ezample 2 - if your focal length is 10 inches and your lens diameter is 2 inches your f/stop is f/5