To increase the depth of field of an optical system, an inversion of the chromatism is produced. This accommodates the fact that, in natural light, users prefer, for their photographs, taking long distance shots outdoors, where the illuminant is mostly composed of blue, and short-distance shots indoors, where the illuminant is mostly composed of red. While optical devices naturally focus the blue components at a shorter distance than the red components, which is unfavorable, with an inversion of the chromatism, a focus more in keeping with what is required is restored.