Contrary to common experience, when an ordinary imaging lens is used to focus incoherent light from an extended source (such as the Sun), the brightest and most intense position of the focused beam is not at the image plane, nor is it at the focal plane. This specific effect is independent of diffraction and aberrations, and it would also apply to lenses with negative refractive index. In a new study published in the Journal of the Optical Society of America, Ronian discusses this interesting geometrical property of lenses and proposes a number of potential applications to signal detection, metamaterial lenses, and nonimaging optics [R. Siew, "Axial nonimaging characteristics of imaging lenses: discussion," J. Opt. Soc. Am. A 33, 970 - 977 (2016)].
Also, click on the image below to read about Ronian's simple "home-made" optics experiment where he demonstrates this focusing effect using a magnifying glass and an ordinary LED desk lamp!