Bokeh

Bokeh in a photo with an aperture of 1.2

The bokeh (from Japanese 暈け , also written ぼけ or ボケ , boke "blurred, blurred") is a term used in photography for the quality of a blurred area. Blurred areas of a photo (or film) are created by a lens by projecting onto an image plane (typically a light sensor today ). They are, so to speak, "inventions" of the lens; the eye does not see the blurring or sees it differently. Bokeh tries to characterize the mostly subjective aesthetic quality of this lens-dependent blurring; It is not about the strength of the blurring, but about how the blurring areas look like rings or circles.

Details

When photographing certain motifs , the background is deliberately kept out of focus in order to reduce the distraction of the viewer from the main motif and to create a three-dimensional effect of the picture. Depending on the lens used, bright light phenomena in the form of rings, circles, ellipses, etc. are created in the areas that are outside the focus point.

The bokeh can vary significantly between a blurred foreground and a blurred background. A pleasant rendering of the background is often accompanied by an unattractive rendering of the foreground and vice versa.

Bokeh is a decidedly subjective quality that is difficult to measure and is controversial . With some experience, many photographers subjectively choose lenses with good bokeh, without realizing why exactly they prefer a certain lens to a lens that is otherwise equivalent in terms of photographic parameters: "It just takes the nicer pictures". Outside of specialist circles, the discussion about good or bad bokeh has only flared up since the mid-1990s and is far from over.

Influences of construction on the bokeh

Circle of confusion

The properties of the bokeh are largely determined by the appearance of the circle of confusion. In blurred areas, every point of the light becomes disk-shaped and takes on the shape of the aperture or the entrance pupil of the objective. With some lenses this "disk" is evenly illuminated, with others it is brighter near the edge of the image or in the center of the image, whereby ring-shaped brightness gradients appear rather restless.

The properties that cause a lens to produce a pleasant bokeh could not yet be clearly defined.

The number or shape of the aperture blades, for example, is in itself a very uncertain indicator of a naturally perceived, foggy instead of harsh bokeh, although a circular aperture image (without jagged edges) is usually less noticeable. Many manufacturers equip the iris diaphragms of the lenses with at least seven, often nine or ten lamellas and give them a special contour so that the pupil comes as close as possible to the circular shape at all diaphragm settings. Classic lenses from the 1960s or 1950s (e.g. Meyer-Optik Trioplan , Zeiss , ISCO , Pieker Berlin) are particularly valued due to their special bokehs and the up to 19 aperture blades, and some of them are also being produced again.

In general, zoom lenses tend to produce poor bokeh, as do - in relation to the focal length - rather short so-called telephoto lenses compared to classic long-distance lenses, the length of which roughly corresponded to the focal length. That is not always the case, however, and there are great zoom lenses out there when it comes to bokeh as well. Many photographers also find the bokeh of old, optically rather simple lenses to be advantageous, even if this is accompanied by the loss of other desirable properties such as sharpness or contrast.

Certain basic optical constructions in lens construction seem more inclined than others to good ( Sonnar ) or bad bokeh ( Tessar ), but there are also examples of Tessars with acceptable bokeh and Sonnare that do not exhaust the apparently existing potential.

Restless bokeh with double contours in the blurred areas

Up until a few years ago, different manufacturers pursued very different optimization goals for lens design, which also had an impact on bokeh. Traditionally z. B. said that many Nikon and Canon lenses have a rather restless bokeh; Nikon lenses in particular were optimized for extreme sharpness at an early stage, which is usually detrimental to balanced bokeh. Minolta's design philosophy was to achieve the best possible balance of color and contrast across the entire range of lenses. At Leica (and later also at Minolta) the focus was on an optimal balance between microcontrast and general contrast (i.e. highest MTF values for 60 lp / mm and (only) good values for the important range of 10–30 lp / mm), which gives the image a certain three-dimensionality through the reproduction of the finest surface structures with an otherwise rather soft display and helps to so-called liquid colors - and often also to a very pleasant bokeh, for which many Leica and Minolta lenses are famous. Zeiss lenses have been optimized, partly at the expense of contrast and neutral color rendering, to achieve an MTF value that is as uniformly high as possible up to the edge and a certain "kink" point at line pairs / millimeter depending on the focal length and application.

Since around the beginning of the 1990s, however, these overarching design ideas have been used less consistently by most manufacturers, a special look can in some cases hardly be made out, which is also due to the fact that digital photography places different demands on a lens and some negative effects can be accepted, since they can also be reliably corrected using electronic image processing. In the meantime, all manufacturers have lenses with rather average bokeh, but also lenses that are optimized with regard to particularly beautiful bokeh.

A pleasant bokeh is particularly important for bright lenses, as they have a particularly shallow depth of field at the largest aperture. Bokeh is also important for portrait lenses (medium telephoto lenses ), as the photographer often deliberately strives for a shallow depth of field in order to blur the background of the image and to emphasize the subject being portrayed.

Special constructions

Some lenses are specially designed to create particularly appealing blur areas that can improve the overall quality of an image.

The portrait lenses STF 2.8 / 135 mm [T4.5] Minolta / Sony , Laowa STF 105 mm f / 2.0 from Venus and Fujinon XF56 mm f / 1.2 R APD of Fujifilm use apodization filter in the vicinity of a second circular aperture to to generate an ideal Gaussian brightness distribution within the circular circles of confusion, which promises a good bokeh both in front of and behind the sharpness plane and at the same time a particularly harmonious transition between the area of sharpness and that of the blur. This results in a course of the depth of field that cannot be compared with the course of conventional lenses. Minolta / Sony calls its lens Smooth Trans Focus . The effect increases with the aperture. The STF is not, as is often assumed, a soft focus lens - the lens is even considered to be very sharp. The disadvantage of the STF lens is the high loss of light through the apodization filter of up to 1.5 f-stops. The corresponding loss of light may be less of a factor with the faster Fujinon lens.

From Nikon , there are two Defocus Control -Porträtobjektive Nikon DC 2.0 / mm 105 and DC 2.0 / 135 mm with variably adjustable spherical over- or under-correction. In this way, either the background or the foreground can be optimized in relation to the bokeh (but not both at the same time), whereby the bokeh at the other end suffers.

Soft focus lenses with screen diaphragms such as the historic Rodenstock ( depth builder ) Imagon lens series (and similar designs by Fuji and Sima) or Seibold's Dreamagon with a slit segment diaphragm also produce a very special (not necessarily good) bokeh .

Annular blur circles and double contours in a mirror lens lens

Mirror lens lenses produce a particularly noticeable bokeh . By using a secondary mirror in the beam path, fuzzy points are not shown as a disc, but as a ring (donut). With this type of lens, the foreground and background are therefore reproduced uncomfortably uneasy; This means that long, narrow objects often appear with a double contour. If these ring structures cannot be included in the design for imaging purposes, mirror lens lenses are therefore mainly used for shots of distant subjects (without a visible foreground) or with a background that is as uniform as possible (e.g. for macro shots ). Within these framework conditions, of course, excellent recordings can also be made with other lenses that are noticeable due to less beautiful bokeh.

Image examples

The same object was photographed in the four images below. Pictures 1 and 2 with a mirror lens and pictures 3 and 4 with a high quality lens. In pictures 1 and 3, the focus was on the forsythia , in pictures 2 and 4 on the respective background. Figures 5 and 6 show what a bokeh created using methods of digital image processing can look like.