How optical zoom is achieved in mobile phones theoretically ??

 At a time when camera was first introduced in mobile phones, no one ever thought that mobile photography will gain this much importance  in future.Today,mobile photography is an area of vivid importance and wide acceptability that when one intends to buy a mobile phone,the first aspect he look into is the features of its camera...

But till recently,mobile photography lacked a main feature till when compared to high end and normal point & shoot camera ,and it was the presence of optical Zoom. 

Optical Zoom in mobile phone was not possible earlier due to  lack of lens movement inside the compact space of mobile phones. The only zoom available was digital zoom which when compared to optical zoom gave less quality images as we zoom in. However, with advent of technology, mobile companies have now overcame this hurdle,and optical zoom is made available in some of the recently launched phones..

Here is a copied detail from One Plus site on how they  achieved 2x zoom on their one plus 7t.  

" The one plus 7t telephoto lens offers an optical xoom of upto exactly 1.99x, which is described as 2x in the software for the sake of clarity and ease of understanding. When using telephoto lens zoom functionality ( effective 12 megapixels) the actual equivalent focal length is 51.37mm and the equivalent focal length of main camera is 25.75mm So 51.37/25.75 ~ 2x times. "

You can view the same on their site product page in camera section. The same technique is used by different manufactures using multi camera setup.

They used focal length of fixed telephoto and focal length of main camera to achieve the said zooming. Since there is no camera length due to restricted space in a mobile phone such achievement is really appreciable.

This feature of optical zooming is revolutionary technology that will change the fate of photography as a whole....