The interior of each lens is made of different curved glass, called elements.
Sphere lenses are called spherical because their lens elements are circular. When light passes through a sphere lens and hits a digital sensor or film, it produces a regular, uncompressed image. Anamorphic lenses are different in that the rear element of the anamorphic lens is a regular spherical surface, and a cylindrical lens element is added in the front. When looking through the lens, these lenses appear oval rather than round. When light hits the sensor through an anamorphic lens, it creates a squashed image, and the image is restored before being presented to the viewer.
The most notable difference is that the widescreen aspect ratio is captured through an anamorphic lens. The aspect ratio of traditional sphere lenses is closer to square. Common super 35 aspect ratios are 1.33:1 and 1.85:1. Anamorphic lenses produce wider aspect ratios, such as 2.35:1 or 2.39:1. Some DPs, like Roger Deakins, like to shoot with sphere lenses and then crop the top and bottom of the frame to get a widescreen aspect ratio.
Another most recognizable characteristic of anamorphic lenses is the bokeh they produce. Bokeh refers to that in photographic imaging with a shallow depth of field, the image falling outside the depth of field will gradually produce a loose and blurred effect. Since the glass element of a sphere lens is circular, it produces circular bokeh. The element of an anamorphic lens is curved and cylindrical, so it produces elliptical bokeh. The more subtle but very important differences in the picture are the distortion, flare and sharpness of the lens. Since sphere lenses have less glass for light to penetrate and they have a simpler construction, they tend to produce sharper images with minimal distortion throughout.
Sphere lenses have faster shutter speeds. Because they have wider apertures, such as T/1.3 to T/2, and thus let in more light. Anamorphic lenses usually have apertures between T2.8 and T4, that is, they transmit less light. This means that stronger light sources are required when using slower anamorphic lenses. Since anamorphic lenses are widescreen, this means they show more of the frame. The narrower the lens, the less the frame needs to be retouched. The fine sphere lens keeps distortion to a minimum, which makes the picture in the movie vivid, natural, immersive and true to life.