Large aperture camera lens with minimalistic refocus for smartphone portraiture photography

This paper describes the design of a small form-factor lens for use in portraiture photography. The current general requirement of mobile cameras having good all-round performance results in a typical, familiar, multi-lens design. Such designs have little room for improvement, in terms of the available degrees of freedom and highly-demanding target metrics such as low F-number and wide field of view (FoV). However, the specific application of the proposed portraiture lens relaxes the requirement of an all-round high-performing lens, allowing improvement of certain aspects at the expense of others. With a main emphasis on reducing depth of field (DoF), the current design takes advantage of the simple geometrical relationship between DoF and pupil diameter. To achieve high resolution in object space, the design has a large aperture of 4 mm, for which a typical F/2 lens would have a relatively large focal length compared to z-height, requiring refractive–reflective elements; hence, FoV is reduced. The system is folded perpendicular to the optical axis using two 45 ^∘ plane mirrors, with a 4:3 aspect ratio for the FoV maximising use of available space. Inverse sensitivity analysis was used to show that the lens has acceptable tolerancing limits. A minimalistic refocus mechanism supplements reduction of energy consumption from digital bokeh post-processing to give a low-energy, low-DoF, high-performing lens for future use in a multi-camera setup.