Thin lens: Definition from Answers.com

A lens can be considered a thin lens if d << f.

In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the focal length of the lens. Lenses whose thickness is not negligible are sometimes called thick lenses.

The thin lens approximation ignores optical effects due to the thickness of lenses and simplifies ray tracing calculations. It is often combined with the paraxial approximation in techniques such as ray transfer matrix analysis.

For a thin lens, in the paraxial ray approximation, the object ( $s$ ) and image ( $s'$ ) distances are related by the equation

${1\over s} + {1\over s'} = {1\over f}$

where $f$ is the focal length of the lens.

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