Why are camera lenses always round in shape?

The lens in a camera focuses the light constituting the image onto the recording screen like the photographic film in a conventional camera or the CCD array device in a modern digital camera. The distance between the lens and the screen is a few millimetres or at best a few centimetres. And so, the focal length of the lens has to be of that order. This is achieved by combination lens systems. 

The combination lens also corrects for two known sources of image distortion, the spherical aberration and the chromatic aberration. As a consequence of spherical aberration, the outer parts of an image appear unnaturally bent and the bending decreases as you approach the centre of the image. This is quite noticeable in pictures taken using a wide angle lens. And the chromatic aberration, as the term suggests, causes the light of different colours to focus at different distances from the lens. Consequently, the image gets blurred unless the lens is corrected for the aberrations. The region of the lens farther from the centre contributes more to these aberrations. Thus, the correction needed also has the circular symmetry. The aberrations are corrected by appropriately shaping the curvature of the lens. Therefore, it is convenient to correct a lens of circular shape. 

In fact, you see a few new model cameras with a rectangular shape of the lens, but actually the lenses of these cameras are also circular but a rectangular mask is fitted in front which presents a rectangular shape. This mask is designed to restrict the light in different directions of the field of view, so as to capture natural brightness all over the image. If the lens is made in a rectangular shape, the aberration compensations also have to be done in the same symmetry. In the case of combination lenses it would be practically impossible to provide the necessary aberration corrections differently in different directions; instead the job at hand is conveniently and accurately accomplished by choosing lenses of circular shape.

Source: thehindu.com