Sometimes the branches of trees in the photographs obtained unpleasant forked. It happens and cases where thin branches are transformed into a kind of capillary tubes. Some photographers in such cases, sadly shake their heads and say: “The bad lens bokeh!» What then is their surprise when they learn that the photo was taken by a well-established lens from a reputable manufacturer. Unaesthetic bokeh can sometimes be seen on the photographs made by recognized masters (see, for example, a portrait of Albert Camus, executed by Henri Cartier-Bresson in 1947).
In this article we will talk about the causes of this uncomfortable ambivalence and try to explain why it is immune from virtually any object (with the exception of the special soft-lens). Also, there will formulate practical recommendations, compliance with which would avoid this unfortunate phenomenon.
In the process of reading the most inquisitive readers will be able not only to familiarize with the theory of matter, but, from their computer, a small demonstration experiments.
The Japanese word «bokeh» (bo-ke) is called the character image lenses in the area of focus.
Photographers has been noticed that the lenses appear differently blurred objects. Some of them blur the objects gently and plasticity, while others fill the mild zone unsightly bagels-shaped spots and double minded lines. Good bokeh, as a rule, characterized expensive lenses with fixed focal length. This feature is not less important than the resolution, contrast, halftones, distortion, etc.
However, it is important to understand the nature of the image unsharp zone depends not only on the lens, but also on many other factors. But let’s not get ahead …
«Ni-sen» – at least as used, but still found in the literature, the term. In translation it means “double lines”. This term is called the phenomenon of unpleasant to the eye ghosting lines on the photographic image. Thus, we can say that ni-sen – a type of bad bokeh.
How bokeh depends on the lens?
In this short article is impossible to describe in detail all the factors influencing the design of optical systems. Let us consider one, but very important (if not the main), property – on how the lens represents a point in the area of focus.
Point objects are as bright against a dark background and dark – a light. Bright point object can be obtained as follows. In a dark piece of cardboard big enough size do with a needle a small hole. For a piece of cardboard place a source of bright light – and a point object is ready!
Study spots, in which dissolve point in the zone blur depends on the shape of the diaphragm aperture. Despite the fact that this form is itself an important parameter for describing the bokeh, here would be considered mostly a round hole. This will be quite sufficient to describe the phenomenon as ni-sen.
As part of this article to describe the image created in the zone blur point source, we use the term “highlight”. Using such an extended interpretation of the term would seem justifiable in a popular article, as in real photographs blurred bright light source appears exactly in the form of highlights. A more rigorous scientific term “function blur point” (PSF or PSF) in the text will not be used.
The main types of glare caused by point objects in the zone blur shown in Fig. 1. Under each image shows the corresponding graph showing how the brightness changes along the diameter of light spot.
1. Type A. Uniformly colored stain. In a sense, it is almost perfect diffuse blur point. Most of this patch provide lenses successfully eliminated aberrations. Figure of lenses is both very sharp and very plastic. Nevertheless, these lenses can still form a not very aesthetic images of gradual objects. Under what conditions this might happen, we will discuss later.
2. Type B. Spots with halos. In some cases, this may be a bright halo around the dark nucleus, in others – the dark halo of light around the middle. This pattern lenses have been removed from unsuccessfully aberrations. Such lenses are usually quite sharp, but the sharpness is often perceived more unnatural, due to the fact that elementary point is displayed as a “donut”. By the way, and the filter Unsharp Mask, available, for example, in Photoshop, also based on the application to the image masks, similar in form to highlight the type B. It should also be noted that the presence okantovok in such sparkles almost always leads to unaesthetic bokeh, including to the splitting of the lines.
3. Type C. Spots with a smooth change of brightness from the border to the center. On the left shows the spot with the increase of brightness toward the center, and right – with a decrease in brightness towards the center. Often such blurring give lenses do not fully eliminated aberrations. The boundary of such reflections are not clearly delineated, and brightness within the flare varies smoothly. As a result, this type of lens forms a rather soft image. Compensating for this softness is a good bokeh, almost never associated with unpleasant ghosting lines.
Once again: the sharpness of the lens in a way detrimental to good bokeh. To obtain a sharp image requires that elementary point depicted in the form of sharp spikes. For a good image of the same bokeh elementary point should be quite different, namely: in the form of a smooth dome-shaped curve. Lens, which reached a good compromise on both these parameters, appreciate, for rare.
Sometimes in the literature mention the hard-edged, almost perfect lens, implying by these optical products to have been brought to the minimum aberrations. However, in the nature of the ideal things do not happen. First, the aberrations can not be completely eliminated. Secondly, the continued small spherical aberration can achieve a beautiful and plastic bokeh.
It is also worth recalling that the diameter of the flare is determined by the distance from the focus point to a point object in the area of focus. In the far zone can be calculated that the diameter of the formulas given in [1].
Kind of a particular lens flare is almost always depends on the size of the diaphragm. Therefore, to achieve a satisfactory result photographer should examine what types of glare that forms a lens at different relative apertures. It is also important that the image point in the zone of blur in different parts of the frame may be different.
In addition, we must remember that in the near field blur highlight may be the same type, and further – another. For example, Harold Merklinger in [2] cites the example of Rodenstock Imagon, which in the near field draws highlight type B, and further – highlight the type C. Given the fact that often the most photos on the front vague plan not available, we can expect that the lens is quite combines sharpness and beautiful bokeh. The only important thing to keep in mind that in the near zone should not get blurred objects.
Generally speaking, the type of flare depends on a large number of different factors. However, a detailed discussion is beyond the scope of this article.
Bokeh and visual illusions
Strange but true: sometimes the effect doublemindedness lines may be due to the peculiarities of visual perception.
The fact that the abrupt transition from the plot with the changing brightness to the site where the brightness remains almost unchanged, visually perceived as a line. These are the features of the visual perception of man. Indeed, look at Fig. 2, which shows the black and white stripes obtained using gradient fills.
The variation of brightness along the strips shown in the chart below. Visually, it seems that between the two arrows brightness slightly increased (blue dotted line) or slightly decreased (yellow dotted line) respectively.
In fact, the area between the arrows perfectly flat. The presence of two vertical lines in places marked by arrows, no more than an illusion of perception.
In general, the blurring of objects in the zone blur often associated with the appearance of gradients, and gradients in turn are connected to many optical illusions. Consider, for example, in Fig. 3, in which the same phenomenon appears in a different, more obvious form.
It depicts a square, bathed in gradient fill from the center to the sides. Observed bright diagonal in some sense illusory. If we look at how the brightness along the line A-A ‘, then, contrary to expectations, do not fix no bright bursts at the points of intersection of this line c diagonals.
In the following sections, we have several times met with the visual illusion of this kind.