This part is more related to the aesthetic quality of a lens, and is the shape of the blurred image produced in the 'out of focus' parts of an image. More commonly seen in 'out of focus' highlights, the Bokeh can form part of a pull focus shot from an 'out of focus' object to 'in focus image, and the shape of the highlights can be crucial to the shot working.
The shape of the bokeh can create the highlight blur in the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting. In out of focus areas, each point of light (or cone of light circle of confusion) produces an image with the same shape as the lens aperture, so the shape of the aperture is crucial to creating good bokeh. The main differences between how different lenses perform is mainly due to the shape of the aperture, how circular the diaphragm is defined with how many leaves form the shape of the opening inside the lens.
The shape of the aperture has a massive influence on the quality and shape of the bokeh. When a lens with bladed apertures is stopped down smaller than its widest aperture it creates 'out of focus' points the same shape as the aperture, and for this reason lenses have many aperture blades and/or blades with curved edges to make the aperture more closely approximate a circle rather than a polygon. Some lenses have less leaves and have straight edges, so will often create a hexagon shape, compared to others with many leaves creating more of a circular image.
Good bokeh is particularly important for long telephoto lenses because they produce a shallow depth of focus. The out of focus areas are significant within the frame so the subject stands out sharply against a non distracting blurred background.
One lens used sometimes in still photography, but very rarely on cine cameras is the catadioptric telephoto lens, or 'mirror' lens, and this literally creates unique distinct circles without a centre - more like a doughnut shape or a ring. The reasons for this is the way the lens is designed with a series of mirrors and the light path is bent and reflected back. There is a mirror in the centre of the front element to fold the light back to the sensor. There are advantages and disadvantages with this design. the advantage is the lens is short and lightweight, with the disadvantage being it has a fixed aperture, perhaps f5.6 or f8. The nature of this means the design is only used for long telephoto lenses for stills cameras, and is also used on a larger scale in the Hubble space telescope.
There are more considerations too, such as the lens aberrations and colour fringing around highlights, and this can create some effects that are pleasing and some not so pleasing, such as the highlight turning from green to red as it changes focus.
Cinematographers will often deliberately shoot with the aperture of the lens wide open, creating a shallow depth of focus. This is actually a well known cinematic technique to create images with prominent 'out of focus' regions so the viewers eye is drawn to the subject, so 'out of focus' highlights need to be as unobtrusive as possible, so circular and a solid colour throughout. Lens design is difficult, that's why cine lenses are priced reassuringly expensive!
Bokeh is most apparent when there are small background highlights in the shot such as specular reflections or small light sources, such as on reflections on water or lit candles for example, or perhaps a Christmas tree lights.
Bokeh is not limited to highlights , however, as a degree of blur occurs in all out of focus regions of the image, it's just not as apparent. The shape of the bokeh is defined by the lens diaphragm, and whether the lens is opened to its maximum wide aperture. It also depends on how much the highlight is out of focus and the distance the subject is from the camera.
John Keedwell GBCT