This is the ratio of the apparent size of an object in comparison with what a viewer sees with the naked eye. For instance, an object 1,000m away viewed through a Fieldscope with 50x eyepiece looks about equal in size to an object 20m away viewed with the naked eye. The magnification can be changed with a mounted eyepiece.

Calculation of magnification:

• Magnification= Effective objective diameter ÷ Exit pupil diameter
  = Focal length of objective lens ÷ Focal length of eyepiece lens