The term bokeh is used to indicate both the out-of-focus areas of an image and their aesthetic connotation. From a technical point of view, a homogeneous bokeh with gradual focus-out-of-focus transitions is considered perfect. In evaluating bokeh, great importance is given to the way in which highlight discs, also called bokeh balls, are played, that is the out-of-focus specular lights, whose design is determined by the shape of the diaphragm hole. In perfect bokeh, the highlight discs are circular over the entire field, show no sharp edge and decrease in brightness from their center outwards until they are completely faded. When the diaphragm is at maximum aperture it often happens that the bokeh balls lose their circularity towards the ends of the field due to mechanical vignetting, thus taking on an elliptical appearance defined in English cat’s eye shape.. In extreme cases, usually linked to super-bright parfocal lenses and short or medium focal lengths, bokeh balls can also appear cropped; this effect, being connected to a certain degree of mechanical vignetting, can also manifest itself as a consequence of the use of a mattebox, a flag or a lens hood.

That said, out-of-focus preferences are extremely personal. Many look for a bokeh with a marked character and capable of attracting attention, such as the so-called soap bubble bokeh, i.e. an out of focus with highlight disc vaguely similar to soap bubbles as they are darker in the center than at the edges and characterized by clear and bright outlines. The swirly bokeh is also quite popular, a blur with a swirling trend, capable of transmitting a sense of rotation that is perceptible more than anything else towards the edges of the frame.

Out-of-focus reproduction is closely related to the correction of spherical aberration and chromatic distortion. This is particularly noticeable in the appearance of the highlight discs, which mirror the circles of confusion. In out-of-focus backgrounds, in parfocal lenses that are undercorrected against spherical aberration, specular highlights have no edge and have a bright center that softly blurs. In these cases the blur almost always appears very soft. In parfocal lenses that are overcorrected against spherical aberration, the opposite happens: highlight discs are characterized by a darker center and a brighter edge. In both cases, the opposite behavior occurs in front of the fire plane. In lenses that are perfectly corrected for spherical aberration, out-of-focus specular highlights create smooth disc highlights, with sharp but not edged borders. When the lens experiences chromatic aberrations, the edges of highlight discs always appear colored.

In the catadriotic highlights disc take on a “donut shape”, called donut shape, hence the expression donut bokeh ; the edge of these discs is thick and sharp, as well as much brighter than the central area.

Not infrequently, highlight discs reveal a certain nervousness in their background. This can be due to numerous factors, related to the type of lenses used in the optical design of the lens but also to the presence of dirt or imperfections on the lenses themselves. Aspherical lenses are known for creating concentric circle highlight discs, producing what English is called onion-ring bokeh. Check for more lenses on dzofilm.com.

In conclusion, there is no lens capable of producing a perfect bokeh both in front of and behind the plane of focus, both as regards the texture of the out of focus in general and as regards the reproduction of the highlight discs; such a lens should have a diaphragm able to keep its hole always circular, be free from mechanical vignetting, be perfectly corrected against chromatic aberrations, as well as suffer the effects of the under-correction of spherical aberration behind the plane of focus and those the overcorrection of the spherical aberration in front of it; all this without using lenses with a particular shape such as aspherical or diffractive ones.