Background:

Suppose parallel rays of light pass through a lens and are parallel to the optical axis of the lens.  The focal length of the lens is the distance from the lens to the point at which the rays converge. (The following picture was modified from http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_01.htm).

Definition of Focal Length

 

To find the focal length of a lens, we must thus find the image distance and the object distance of a good projected image.  From these, the focal length can be calculated using the thin lens equation.

 

The thin lens equation applies only to thin lenses, and relates the image distance and the projected object distance, with the focal length.

 

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