is: for parallel incident light, the distance (the image focal length) from the lens to its image focus; or the distance (the object focal length) from the lens to its object focus; or the distance from the mirror to the focus of the mirror. For a thin lens, the image focal length and object focal length are the same. [P6.3]
is: within the Cartesian sign convention a positive quantity for a convex lens or a concave mirror, and a negative quantity for a concave lens or a convex mirror. [P6.3]
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