What is bokeh?
Bokeh (English. bokeh; YAP. 暈け or ボケ – “blurring”) is a characteristic of the aesthetic qualities of the image area that lies out of focus, i.e. outside the grip area. Bokeh has nothing to do with sharpness: two lenses that depict objects in focus equally sharply can have a completely different blur pattern, the subjective artistic merits of which are described by the term bokeh.
An ideal lens from an engineering point of view focuses light from a point source in the form of a regular cone. When the tip of the cone touches the plane of the matrix or the camera film, the image is in focus and the photo gets an extremely small point. The better the lens, the smaller the point and the sharper the image. If the source is out of focus, the cone of light is crossed by the matrix and the point in the photo turns into a disk, which is called a circle of blurriness or a spot of scattering. If the blurring circles are large enough, they begin to have a significant impact on the aesthetic perception of the photo as a whole.
Circle of confusion
This is how the scattering circle is formed.
Bokeh is more often seen in the background, less often in the foreground, due to the fact that the focus of the photo usually falls on the object closest to the photographer.
It is obvious that bokeh becomes noticeable at a low depth of field. Size of diffusion circles will be larger, the larger the camera format, the longer the focal length of the lens, the larger the relative aperture the greater the distance from the subject to the background and the less distance from camera to object.
The quality of the bokeh
Bokeh is a subjective concept and it is difficult to talk about bad or good bokeh, and even more so to measure it in some mathematical units. Personally, I like soft bokeh, in which the background blurs immaterially and does not interfere with the perception of the main elements of the photo. By bad bokeh, I mean hard bokeh that is conspicuous and distracting from the foreground.
An optically perfect circle of blurring has the appearance of a uniformly illuminated disk with fairly clear edges. Such a disk can be beautiful in itself, but being an element of a deliberately blurred background, ideal circles of blurriness create the illusion of unnecessary sharpness due to their sharply defined edges, and distract attention from the really sharp and plot-significant objects in the foreground.
Example of a neutral bokeh.
Unfortunately (and maybe fortunately) there are no ideal lenses, and real lenses are characterized by a number of aberrations or distortions, to some extent corrected by the developers. The level of spherical aberrations of the lens affects the distribution of light within the aforementioned cone, and therefore the uniformity of illumination of the scattering spot. In reality, the blur circle will almost always have a slightly blurred edge, and the brightness distribution from the center of the disk to its edges will vary greatly.
Beautiful (from my point of view) bokeh implies a smooth decrease in the brightness of the blur circle from the center to the edges according to the law of normal distribution. This is a consequence of not fully corrected spherical aberrations of the lens. In other words, the technical imperfection of the dialectically art moves in dignity.
An example of a good, unobtrusive bokeh.
The opposite extreme is spherical aberrations corrected beyond measure. In this case, the brightness of the disk increases to its edges. For my taste, ringed bokeh looks disgusting, but many, on the contrary, are delighted with such a specific, not to say “creative” drawing.
For many zoom lenses, especially those with high magnification, the quality of the bokeh may vary depending on the selected focal length and focus distance. This is normal, since the optical schemes of such lenses always imply compromise solutions. It is almost impossible to achieve the perfect bokeh in the entire range of focal lengths, and lens designers are guided by the circle of blurriness, rather from a mathematical than artistic point of view. In addition, if the bokeh pattern is optimized, it is usually for areas located in the background, while the character of the bokeh for the foreground plan often has to be sacrificed. There is also nothing wrong with this, since most people prefer to see the foreground sharp and the background blurry, which means that beautiful bokeh is much more in demand for the background.