Question

why our phone has two sensors beside front camera ? This is not in any book​

Answer 1

There are two reasons for dual-camera phones to exist. The first is simple – the second camera is different to the primary camera and excels in a different aspect of photography. It therefore complements the primary camera as a standalone, additional unit. The second reason is much cooler.

Answer 2

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The field of view on this type of lens is narrower than others, but by using a telephoto lens with a second lens and a sensor, your phone's camera can get closer to the action without any moving parts. Telephoto lenses are also used to create those fancy bokeh or focus effects in portrait mode.