>>If you Google [iphone camera hd aspect ratio] you get a couple of apps that claim to be able to do that.
>
>Does this mean the app take over the iPhone capability of taking picture?
>
>I still cannot believe such a phone would still take a picture which is not HD format.

Dunno - but if the phone could do it out-of-the-box, there'd be no need for an app.

If a camera uses circular/spherical lenses then the ideal sensor shape (and image format) is circular. Anything rectangular suffers from distortion and exposure problems towards the corners. The higher the aspect ratio, the worse these problems. There is actually something of a movement in digital photography towards 1:1 aspect ratio, for this reason.

Digital camera makers have to make a compromise - it's easy to make a sensor in a 16:9 aspect ratio, but that doesn't make good use of the optics. What they'll often do is use a sensor that's closer to square, and if they have lots of pixels, use the ones closest to the center for various image pixel sizes and aspect ratios.

Some 4S info at http://www.eoshd.com/content/4362/iphone-4s-camera-sensor-actually-capable-of-4k-24p-video .