In photography, this concept refers to the subjective quality of a lens due to the aesthetics of the blurred areas created in a photograph.
In a more vivid and practical way, this can be understood as the classic image in which the main element is the only one that appears focused and the remaining elements disappear, completely out of focus, or in the form of indistinct luminous light surfaces, with which the composite framework is working.
Above Bokeh in a photo shot with diaphragm f / 1.2 and a focal length of 85 mm.
So it’s not the blur a target creates, but how it is? When photographing certain subjects, it is interesting that the background is out of focus to avoid distractions when viewing the photo and to highlight the subject in this way.
While some objects display unfocused objects as circular patches, other shapes, colors, and contrasts do. These shapes as soft patches of unfocused objects characterize the bokeh of a lens.
It is therefore a subjective quality. The properties by which a lens produces a pleasing blur to the eye are not yet clear. The number of membrane plates per se is not a reliable indication of whether a bokeh in the form of a pleasant haze or a hard Bokehs is obtained.
In general, lenses tend to blur mostly, though this is not always the case. A pleasant bokeh is particularly important for very bright targets, as the membrane in their larger openings can produce a minimum depth of field.
It is also very important for portraiture because the photographer of portraits prefers short depths of focus to make the bottom area disappear and emphasize the reason.
The catadioptric lenses, due to their mirror construction, create a special bokeh that reproduces the defocused objects in the form of rings rather than spots. The thin and elongated objects in the background often appear as a double contour.