Best part of the zoom lens is in the middle of the focal length so 4.7 or 5.1x (20.7 - 22.4mm) or in 35mm speak (112 - 122mm)
Now, that is the theory and don't know whether k zoom gives more range here or not. Did not test this out yet.
If you noticed, Olaf on the k zoom flickr group put up some nice portraits of his kids.
I tested at 2.1x zoom, ~50mm, it is very good, without any distortion, and the subject vs background distance are almost as what we see naked eye.
At 2.8x zoom / 4.0x zoom, ~70mm / 85mm, good too, no distortion, but the background perspektive was change from 50mm, which is good if you want to eliminate background subject.
at 10x zoom, 240mm, not so recommend, the portrait become flat, but if you like, it is good too.
half body head shot at 4x you are 2 meteres away.
Thing with portraits is you have to pick your background carefully otherwise it will interfere with the subject.
K-zoom's lens characteristics are different to what the usual calculators give for lens of a given focal length. Need to find one that takes crop factor into account.
eg. at 10x a lens of focal length of 44mm with aperture F6,3 should give DOF of 4cm if subject is 1 meter away. But when i did a little test DOF was like 3 metres behind subject.
and then you can close the f-stop all the way down to F18.4 at 10x and increase DOF further by 3x (!)
The aperture i wish could go larger than 3.1 as it would make isolation easier. Would crop factor (5.6x for k zoom) negate this if they make a 2.8 lens ?
But it increases as focal length like any cheap zoom. And that maybe is the thing here, to keep apertures large i don't know if its possible to do in such a small form factor or it will increase the cost a lot due to optics.