Double-folded catadioptric lens for smartphone portraiture photography

Smartphones are increasingly utilising a dual-camera setup, introducing the possible use of one or more specialised camera modules. The current work presents a smartphone camera designed for portraiture. In portraiture, it is desirable to have a natural Bokeh effect, resulting from shallow depth of field (DoF). Because DoF is inversely related to the square of diameter, the proposed lens has a large entrance pupil diameter and low F-number. To fit such a camera within typical smartphone housing, the design is doubly folded, resulting in a compact z-profile (phone thickness) of less than 5 mm, with a total ray path of 13 mm. Due to physical limitations, the field-of-view (FoV) is reduced; this is acceptable, considering the specialised application of the camera and attractively large entrance pupil diameter. With a focal length of 7.6 mm, the system is retrofocus, but could be considered telephoto in terms of its z-height, when compared to typical smartphone lenses. This proof-of-concept design is demonstrated here using glasses. Axial colour is controlled using two doublets. For compactness, the aperture stop is positioned between the first doublet and first fold mirror, also allowing balancing of lateral colour and distortion. For very compact applications, a fixed-focus version of this system was designed. However, considering the large focal length and diameter, the hyperfocal distance is unattractively large; hence, a slightly longer refocussing system was also designed. Refocus is achieved by moving the the second doublet, the power of which must be increased relative to the fixed-focus design|the power increase disturbs the aberration balance about the stop, and hence lateral colour and distortion increase slightly. Distortion remains below 5% and lateral colour should be possible to correct digitally.