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Published online by Cambridge University Press: 24 October 2008
The method of determining the focal length f of a thin converging lens by finding the minimum distance Δ0 between image and object is well known. When the thickness of the lens is negligible, f = ¼Δ0.
In the general coaxial system, the media at the two ends have refractive indices μ1, μ2, and there is a finite distance t between the principal or unite plances. The two focal lengths are unequal, since, f1/f2=μ1/μ2, and the nodal points do not coincide with the principal points.
* Searle, Experimental optics, Chapter ix.
† The focal length f is positive or negative, according as the image of a distant object is inverted or erect. This is the definition used by practical opticians.