Mobile phone lenses are designed to meet high quality standards across a fairly wide viewing angle (e.g., 27mm focal length equivalence). These lenses (typically a stack of five or six lens elements housed in a lens barrel) have stringent manufacturing constraints that assure high yield and repeatability in mass production. Such design constraints result in lens designs having a total track length (TTL) that is larger than the effective focal length (EFL) of the lens assembly.
The TTL is the key parameter that determines the height of the camera and the thickness of the phone; thus, minimizing it is of great importance. Nonetheless, a typical wide angle lens assembly having an EFL of 4.2mm and wide aperture (e.g., F/1.8) results in a TTL of about 5mm and module height of 5.8mm.
In a 2x zoom dual camera, with wide and tele camera modules having the same pixel count, the tele camera, having a narrow angle of view (e.g., 54mm), requires one to have a lens assembly with a much larger EFL satisfying:
EFLτ =2×EFLω×(Pixel Size τ)/(Pixel Size ω)
For EFL=4.4mm, Pixel_Size=1.4um and Pixel_Size=1.0um, the tele camera EFL is 6.3mm. Typical lens designs having such EFL will result in a module height exceeding 8mm.
In order to enable zoom dual cameras in a slim form factor, Corephotonics developed and patented a new type of compact narrow-angle telephoto lens in which the EFL is actually larger than the TTL. While doing so, we keep the large aperture (up to F/2.2) and minimize the EFL to TTL ratio (down to 0.87) without any compromise in optical quality or manufacturability.
The result is a true 2x optical zoom camera with superior low light performance and high resolving power, which complies with the same module height standards (e.g., 6.0mm in the above referenced example) of single aperture cameras.