[to keep it simple, all focal lengths noted are 35mm equivalents; crop factors for phone cameras are about 7-8x, give or take]
As far as I can tell most new phone cameras have a fixed, medium-wide lens about 30-35mm with max aperture around f/2 to f/2.8, and support some digital zoom, though the quality degrades quickly with digital zoom (e.g. iPhone 5 ~35mm f/2.4 8MP, Samsung S4 ~31mm f/2.2 13MP). These sensor + lens packages are pretty small, like the Sony 8MP cameras from the iPhone 5 & 4S below [from ifixit]; the unit on the left is 8.0 x 8.7 x 5.2mm.
Sure, there's the Samsung S4Zoom with optical zoom (essentially a consumer digital camera), but I'm wondering if some of those improvements in camera can be had with less weight/size/etc.
I'm wondering whether a longer (and no doubt slower) prime lens might be possible in a similar physical form-factor to the existing fixed wide-angle lens+sensor units, so that a phone could have two cameras, one wide one telephoto.
I guess its kind of analogous to using 2 DSLRs with a 35mm and 85mm prime vs one DSLR and 24-105mm zoom. Various trade-offs, which aren't really the point here, but different ways of tackling the same problem.
Along side the typical ~35mm camera, a ~70mm telephoto one would give you the effect of (instantly) changing between the two prime lenses. It'd be a big quality bump for telephoto photos on camera phones, even if you did still use a little digital zoom/cropping.
Design & Trade-offs?
The physical size of a lens tends to increase with maximum aperture (a physical necessity) and, at least in the 35mm/film world, it increases as you move away from about 50mm. I imagine there's a similar 'sweet spot' for the tiny camera-phone sensors too (probably around 30-35mm by design, though perhaps more flexible given they don't need a fixed flange focal distance). As I understand it, this is (to some extent at least) driven by the different lens designs needed to achieve high quality at specific focal lengths & aperture combinations (wide-angle and telephotos share some of their design, only mounted in reverse).
So my question...
So I'm keen to know if a similarly small lens+sensor package could be designed with a focal length around 70mm or so, keeping similar image quality to say the Sony/iPhone sensors above. Is this an impossibility due to lens design limitations? Would it need to be many times larger? Or have smartphone manufacturers simply not had any need for a telephoto lens (with a single fixed lens, wider is easier and more useful in a consumer phone), so simply not bothered?
So anyone who understands lens design for those tiny sensors, how realistic is something like a ~70mm ~f/5.6 (equivalent) sensor+lens without significantly expanding on the form factor of the phone cameras above? (at least one dimension ~6mm, even if it gets a bit wider or uses a mirror and is mounted sideways) What design factors will likely ruin this idea?