Torcal Milla, Francisco JoséSánchez Brea, Luis Miguel2023-06-172023-06-172017-050143-816610.1016/j.optlaseng.2016.09.008https://hdl.handle.net/20.500.14352/18972© 2016 Elsevier Ltd. All rights reserved. The authors thank J.A. Gomez-Pedrero for his help in this work. This work has been supported by project SPIP2015-01812 of the Ministerio de Interior of Spain and by SEGVAUTO- TRIES CM S2013/MIT-2713 of the Dirección General de Universidades e Investigación, Comunidad de Madrid (Spain).An accurate and simple technique for determining the focal length of a lens is presented. It consists of measuring the period of the fringes produced by a diffraction grating at the near field when it is illuminated with a beam focused by the unknown lens. In paraxial approximation, the period of the fringes varies linearly with the distance. After some calculations, a simple extrapolation of data is performed to obtain the locations of the principal plane and the focal plane of the lens. Thus, the focal length is obtained as the distance between the two mentioned planes. The accuracy of the method is limited by the collimation degree of the incident beam and by the algorithm used to obtain the period of the fringes. We have checked the technique with two commercial lenses, one convergent and one divergent, with nominal focal lengths (+100±1) mm and (−100±1) mm respectively. We have experimentally obtained the focal lengths resulting into the interval given by the manufacturer but with an uncertainty of 0.1%, one order of magnitude lesser than the uncertainty given by the manufacturer.engjournal articlehttp://dx.doi.org/10.1016/j.optlaseng.2016.09.008http://www.sciencedirect.com/open access535Focal lengthMetrologySelf-imagingÓptica (Física)2209.19 Óptica Física