Sunday 31 January 2016

Understanding Bokeh

Photography / Tips and Solutions

Understanding Bokeh

Specular out-of-focus highlights have appeared in photographs since the first photos were taken through lenses. In 1997, Photo Technique magazine’s Mike Johnston introduced the word “bokeh” to the photography world’s vernacular and the out-of-focus areas of photographs have been scrutinized ever since. Before the term, there was certainly discussion about the aesthetics of the out-of-focus specular highlights of a photograph, but until 1997, there wasn’t a good word in the English language to describe the phenomenon. Credit goes to Mr. Johnston, his friends Carl Weese and Oren Grad, the Internet, and a word that no one can agree on how to pronounce, for creating a blurry background craze.
Today, there are video and photography companies, movies, software, Photoshop plug-ins, film festivals, throw pillows, a Facebook page, creative cutout plates to cover your lens, Waterhouse plates, iPhone apps, and more named “Bokeh.” A smartphone recently touted its ability to produce “bokeh” as a selling point for its built-in camera. Other smartphone cameras have algorithms in their electronic brains that can create artificial bokeh on images when their lenses fail to produce the effect. There is even a nutrition website featuring an article on using bokeh in food photography. At press time, there are more than 1.1 million images on Instagram with the bokeh hashtag. Getty Images has more than 51,000 stock bokeh photos to choose from if you need some bokeh in your life right now. Do a Google search for “What is bokeh?” and you will get more than 3.5 million results—and more than 10,000 if you use quotation marks.
Let’s clear up a few things before we dive in.
“Bokeh” is an English word that is a translation of the Japanese word “暈け” or “ボケ” that means: blur—specifically out-of-focus blur. So, why not just use the already established word “blur?” Because the simple English word “blur” can be applied to motion blur as well. Also, the word bokeh also encompasses the Japanese word “ボケ味“ meaning “blur quality.” So, bokeh is more than the blur, it is a word used to describe the aesthetic quality of blur. Because of the dual meaning, you can say, “That photo has bokeh,” and you can also say, “That image has very pleasant bokeh.” The translation of those two statements is, “That photograph has specular highlights that are not in focus,” and “The out-of-focus areas of this photograph are pleasing to the eye.”
The word is a noun. It is only capitalized when beginning a sentence. “Bokeh” is not a proper noun.
How do you pronounce it? Most people say that it is pronounced “bow” (like a bow tie) and “keh” (like the “ke” in Kelvin) with equal emphasis on each syllable. Photo Technique added the “h” to the word “boke” to help readers pronounce it correctly as they understood it. However, are several articles on the Internet from Japanese speakers that say the last syllable should be pronounced like “kay.”
So, you might want to pick a pronunciation, make your own, or just skip the word all together and say, “Out-of-focus blur.”
The other thing feeding the bokeh obsession is the fact that the human eye, due to its excellent depth of field, does a poor job creating the kind of bokeh many viewers like to see in photographs or motion pictures. Therefore, big bokeh in photographs is, in a way, a unique visual experience that can only be had by viewing an image captured through an optical lens.

Photographs © Todd Vorenkamp


If there are portions of an image that are not in focus, that is bokeh. Of course, if that which is out of focus is pure black or pure white, you might not see it. But, in general, you should not over-think this. Even though the focal plane of a lens is razor thin (see this article on focus), when photographing distant subjects, the entire image may be in focus. Out-of-focus regions occur when portions of the foreground or background are outside the lens’s depth of field (DOF).

Bokeh as a Subject

One of the anomalies of bokeh, when you consider the amount of attention bokeh receives (guilty by virtue of writing this article), is that it is rarely the subject of the photograph. Therefore, we probably should ask ourselves why it receives as much thought as it does.

Bokeh as the subject in this shot, taken with a Nikon AF DC-NIKKOR 105mm f/2D lens.
In general terms, bokeh can be one of four elements in a photograph:
1. The subject: usually an abstract image comprising out-of-focus specular highlights
2. A part of the subject: the cliché bokeh-as-part-of-the-subject image is the cup or bowl with holiday lights “pouring” into it
3. A complement to the photograph
4. Distraction from the photograph.

Complement or Distraction?

When portions of the foreground and background are outside of the DOF of the lens, the light that reflects from objects in those regions will be reproduced as circles at the image plane. The DOF article linked above contains a thorough discussion of circles of confusion. This is what is happening in the image just described; points of light are not focused to a precise point at the image plane due to the fact that the object is outside of what is in focus to the lens.
Bokeh is the visual rendering of circles of confusion.

When a point of light is at the focal plane (middle illustration), it is reproduced as a point of light at the image plane. If the point is forward or aft of the focus plane, it is reproduced as a circle—not point.
Depending on how the lens elements are designed and how the aperture of the lens is shaped, the bokeh will have distinct characteristics. These characteristics will, in general, do one of three things: complement the image, be a non-factor in the image, or distract from the subject. Interpretations on the interaction of the out-of-focus areas of an image are as subjective as the photograph itself. Some find certain bokeh shapes and forms pleasing, others find some distracting. Regardless, it is subjective. When someone uses the terms, “Bad bokeh” or “Good bokeh,” know that you are hearing opinion, not fact.
As a photographer, you may want to experiment with your lenses and figure out which ones produce bokeh that is pleasing or non-distracting to your eye. You might also find that some lenses produce bokeh that makes you cringe. This is not to say that a particular lens is better or worse than another; it simply renders out-of-focus highlights in a different fashion. There are some legendary lenses that are known for their sharpness and color rendition, yet heavily criticized for their bokeh performance. Conversely, there are not-so-great lenses that create beautiful bokeh.
The bottom line here? Know your lenses and how to exploit their strengths and avoid their weaknesses. Shallow DOF portraiture usually demands pleasing bokeh. However, unless you are primarily a portrait shooter, my advice would be to not make bokeh a large part of your lens-shopping criteria unless out-of-focus abstractions are your photographic genre of choice or you are doing an art school project on the subject.

Bokeh is a Function of the Lens Design

The optical design of a lens—how the lenses are molded, polished, and placed—and the design of the aperture diaphragm are responsible for the way bokeh is rendered in an image. The next portions of this article will discuss how bokeh is created, but if you read this and crave an even more technical gaze into bokeh, feel free to absorb this linked document from our friends at the venerable lens manufacturer, Zeiss Camera Lens Division, titled Depth of Field and Bokeh, by H.H. Nasse.
For the purposes of this article, let’s try to keep the conversation relatively basic, because most of us want to spend our time making photos and not designing, molding, hand-polishing, or producing homemade photographic lenses.

Aperture and Shape

According to Schneider Optics, “Sometimes good things just happen. A good bokeh also might just happen, but it can also be designed. The shape of the iris is one of the aspects that needs to be considered.”
If you read my article on star effects, you know that diffraction caused by the shape of the aperture diaphragm will determine the shape of light passing through said diaphragm. The same diffraction applies to the shape of out-of-focus light.
On lenses with variable apertures, the diaphragm will be constructed of multiple blades that expand or constrict, depending on the selected aperture. The lens diaphragm aperture is generally designed to form a circle. The fact that several sections form the circle means that it will not be a geometrically perfect circle. Some aperture blades are rounded to maximize the simulation of a circle. However, the fewer blades the diaphragm has, the more difficult it is to form a circle, regardless of rounded edges.

You can see the Fujifilm XF 90mm f/2 R LM WR lens’s 7-bladed aperture creates a heptagon as the lens is stopped down from f/2 to f/16.
Therefore, for example, a lens with five aperture blades will produce, sometimes subtly, pentagon-shaped out-of-focus highlights, or bokeh. Depending on your aesthetic, this might be welcome or it may be objectionable. The more blades the diaphragm has, the more circular the bokeh will appear, especially when the light is passing through the diaphragm near the optical axis of the lens. With an eye on circular bokeh, some optics, like Rokinon’s XEEN lenses feature 11-blade apertures in an attempt to maximize roundness, even when stopped down.
Michael J. Hussmann, in Hasselblad’s Victor magazine, says, “From polygon-shaped, rather than round blur disks, one can easily determine the number of aperture blades. These shapes are rarely distracting, though, and increasing the number of blades doesn’t improve bokeh as much to be worth bothering.”
Some older lenses have aperture blades that form a kind of ninja star-shaped diaphragm that leads to a distinctive bokeh shape. For the eccentric photographer, you can create your own bokeh shape by cutting out a design of your choice in a piece of black construction paper and covering your lens with it. There are a lot of DIY videos online, suggesting different creative ideas for bokeh shapes but, if you prefer to skip the arts-and-craft project, you can just buy the DIYP Bokeh Masters Kit or the Lensbaby Creative Aperture Kit 2. Note that these custom shapes work best with bright out-of-focus regions in the photograph such as artificial lights outside of the DOF. If your out-of-focus region is devoid of small highlight regions, you’ll likely not see these shapes distinctively reproduced in the bokeh.

A star-shaped Waterhouse plate gives the Lomography Petzval 85mm f/2.2 lens’s bokeh a starry shape.
Getting back to the diaphragm aperture shape, the wider the aperture, the more round the diaphragm will be. At its maximum opening, the diaphragm is as round as it will get. The narrower the aperture, the more pronounced the diaphragm shape, if it has a shape, would be.

Simply putting your fingers in front of a lens changes the characteristics of the bokeh.

No Aperture: The Mirror Lens

The catadioptric or mirror lens—basically a reflector telescope—produces distinctive out-of-focus shapes popularly known as “doughnut bokeh.” Mirror lenses generally do not have adjustable diaphragm apertures. The distinctive shape of the bokeh comes from the design of the lens and the placement of the reflecting mirror in the center of the image. Light enters the optical tube, passes a small mirror, and strikes a concave mirror at the rear of the tube. The light is focused back toward the small mirror and reflected back through a hole in the concave mirror where the camera or eyepiece is mounted.

Doughnut bokeh from an LED street light, captured with a Nikon Reflex-NIKKOR 500mm f/8 (New) lens.
The result is bokeh with a bright edge and a relatively dark center. Some folks love this effect. Others despise it. Remember, bokeh quality is subjective.

Aperture: Depth of Field

There is nothing magical here, the shallower the lens’s depth of field (DOF) is, more regions of the image will be out of focus. Longer DOF will keep more of the image in focus and give you less out-of-focus regions.

Changing the aperture alters the depth of field and the aperture diaphragm shape to change the characteristics of the bokeh, in this series of images from the Fujifilm XF 56mm f/1.2 R lens.
DOF and aperture shape combine to affect the characteristics and prevalence of the bokeh. A narrower aperture gives you longer DOF and more diaphragm shape in the bokeh. A wider aperture gives shallower DOF and more rounded bokeh.

Optical Vignetting: Cat’s-Eye Effect

At wide apertures, light is entering the aperture diaphragm from a wide range of angles. Light passing along the optical axis of the lens will not have to be refracted to meet the image sensor. Light entering the edge of the lens must be bent toward the sensor. Because of the variation of the angles of light as you move from the center of the image to the edges, the shape of the bokeh changes. Instead of concentric circles formed on or near the optical axis, light coming in at an angle forms bokeh that is elliptical. These ellipses are known as “cat’s-eye bokeh,” because the shape resembles the feline pupil.

The shape of the bokeh changes ever so slightly as the tug and barge head upriver, in this series of images taken with a Nikon AF DC-NIKKOR 105mm f/2D lens.
As you extend from the middle of the frame, the introduction of the ellipse-shaped bokeh can cause a “swirling effect” in the image. Some lenses are specifically designed to produce this swirly bokeh effect, such as the Lomography Petzval 85mm f/2.2 lens (available in Nikon and Canon mounts). Lomography’s revival of the Petzval design was driven by the desire to create a lens with this bokeh effect. “For the Petzval Art Lenses, we were inspired to create them based on our love for the special swirly bokeh which Petzval lenses give. So bokeh was part of our inspiration for these lenses to create new Petzval lenses, which give the same special swirly bokeh,” says a rep from Lomography.

The Lomography Petzval 85mm f/2.2 lens is known for its intentionally swirly bokeh.
Another swirly lens is the Helios-44 lens family, manufactured in the former Soviet Union. The Helios lenses were based on a Zeiss Biotar design, also known for their swirly bokeh.

Spherical Aberration

Uneven shading of the bokeh shapes are determined by the amount of spherical aberration (SA) in the lens. SA is the optical effect caused when light entering lenses at different distances from the optical center are refracted more than the light passing through the optical center. Referring to SA, Fujifilm says, “One thing to note is that beautiful bokeh is the effect of beautiful contrast from smooth de-focusing area to the sharp focusing area.”

Spherical aberration causes light rays to spread unevenly and create variations in the shading of the disc.
If the lens is designed perfectly, all of the light entering the lens, no matter the distance from the optical center, converges at one place—no SA. With a lens devoid of SA, the circle of confusion discs will be uniformly shaded across the disc. If the SA is under-corrected, it causes an increase in light toward the center of the disc—a Gaussian distribution of light. The out-of-focus discs will be brighter in the center than on the edge. Finally, if the discs have more light-gathering to the edges of the disc, over-corrected spherical aberrations, you will get what is known as the “soap-bubble” bokeh effect. This bokeh resembles the aforementioned doughnut bokeh caused by mirror lenses, but it is created by aberrations, not by the blockage of light from a mirror.

Spherical aberration flavors. Over-correction causes light to build near the edges. Under-correction of SA leaves more light in the center. Some websites label these as “bad, neutral, and good,” but those terms are subjective.
The rub with SA is that, if you under-correct or over-correct it, you will get the opposite light distribution effect before and after the focus point. Therefore, an over-corrected lens that causes the soap-bubble bokeh in the background will have foreground-out-of-focus areas with a concentration of light in the center of the discs, and vice versa for under-corrected aberrations.
If you read other articles on bokeh, you may find that this is another area where opinion on bokeh gets thrown into the mix. The popular stance is that bokeh with bright edges is poor, bokeh with a bright center is wonderful, and neutral bokeh shading from lenses with corrected SA is, well, neutral. Again, this is just someone’s opinion.
Some photographers crave the soap-bubble bokeh effect in their images. In fact, some lenses, such as the Meyer-Optik-Gorlitz Trioplan 100mm f/2.8, are known for creating the effect. Other photographers detest the bubbles. And, as mentioned above, soap bubbles in the background means Gaussian discs in the foreground, so you need to prepare yourself for both types of out-of-focus regions if the lens is not fully corrected.

Aspherical Lens Elements

Lens designers generally try to combat SA and create “perfect” lenses. One way to remove the aberrations is through the use of aspherical lenses. A spherical lens is one that is virtually cut from a sphere; hence the name. An aspherical lens has a complex shape where the curvature changes based on the distance from the optical center. You cannot slice a sphere and create an aspherical lens. Aspherical lenses are much more complicated and expensive to manufacture than their spherical counterparts. Does your lens have aspherical glass? Many lenses containing these specially designed elements usually have nomenclature denoting the use of the aspherical elements.
Aspherical elements can also influence the rendering of out-of-focus regions by introducing what is known as “onion-ring” bokeh. This effect appears as a texture of rings on the bokeh discs and is caused by very slight defects in the lens surface. Adding to this, these imperfections can cause diffraction and result in different textures on the discs.

Onion-ring bokeh produced by the Fujifilm XF 35mm f/1.4 R lens
When asked about bokeh and lens manufacturing, Panasonic responded, “A unique technology that is deployed for the LUMIX G Leica DF Nocticron 42.5mm f/1.2 ASPH. Power O.I.S. lens (as well as the LUMIX G Leica DG Summilux 15mm f.17 ASPH., and the lenses in the LUMIX DMC-FZ1000 and LUMIX DMC-LX100) is a unique process for manufacturing the molds that are used to produce our aspheric optics. In the past, aspheric optics had to be hand-ground. Today they are molded, and the mold is created with a lathe that bores the shape of the lens. This process leaves concentric circles in the mold that in turn are pressed into the glass. These rings can be seen in the bokeh area of many lenses and some call it an ‘onion-ring’ bokeh. Our proprietary lens-manufacturing process requires hand-polishing of these molds to remove the rings from the mold. This gives us the look of a hand-ground aspheric lens for a fraction of the cost.”

Chromatic Aberration

Multi-colored light travels through space at different wavelengths. Different wavelengths means different speeds and, when this light passes through a lens, the different colors are refracted at slightly different angles. This causes color fringing to occur around objects in the image, especially near the edges of a frame where the light is bent more than light passing through the lens close to the optical axis. There are two basic types of chromatic aberrations (CA): lateral (left/right) and longitudinal (front/back).
Olympus examines many different bokeh factors when producing lenses, including CA. “Olympus’s optical engineers are particularly critical of bokeh design. At the beginning of the lens-design process, engineers separate bokeh into several components: shape, homogenization level of color on the inside, and color on the outline of the bokeh. Once the goal of each parameter is determined, work can begin to design the optics.”

The Nikon AF DC-NIKKOR 105mm f/2D lens shows slight chromatic aberration.
This CA can and will appear in out-of-focus shapes and can “color” the bokeh a bit. Lateral CA will color the edges of an object, including the edges of an out-of-focus disc. The longitudinal CA will produce a color tint on the entire disc. The more a lens is corrected for CA, the less color fringing you will see in both focused and out-of-focus regions of a photograph.


According to Sigma, “Designers and engineers pay attention to bokeh in design process. [We] design the aperture blades rounded to create nicer bokeh and the sharpness of the lens to balance between the optimal sharpness and the nice bokeh. In order to depict beautiful bokeh, our engineers try to not only minimize vignetting, astigmatic and chromatic aberration, but also look into the best balance for the concentration distribution of bokeh at the development stage.”
You may have heard your optician say the word “astigmatism” during an eye exam, and if you asked them to explain it, you might be fast asleep in only a few moments. Basically, astigmatism is a defect in a lens that affects the ability of the lens to accurately focus light coming in at different angles and wavelengths. A lens can often produce a sharper image in some regions than others.
The bottom line is that this can have an effect on the shape and characteristics of not only your subjects, but your bokeh, as well.

Lenses with Adjustable Bokeh

There are a few lenses in the world with systems that allow the photographer to specifically adjust the way the lens renders out-of-focus areas. These lenses include:
Nikon AF DC-NIKKOR 105mm f/2D – DC = Defocus Control
Nikon AF DC-NIKKOR 135mm f/2D – DC = Defocus Control
Sony 135mm f/2.8 – STF = Smooth Trans Focus
Fujifilm XF 56mm f/1.2 R APD – APD = Apodization Filter
• Sigma 135mm f/2.8 YS – Focusing System
The NIKKORs achieve manipulated bokeh by adjusting the lens’s SA to control the light distribution across the foreground and background out-of-focus discs. According to Nikon, its patented Defocus Control system was combined with its innovative rear focus system to “realize the ideal lens accepted by all of those who demand sharpness or want to utilize the out-of-focus foreground, and for those who call for a soft out-of-focus background.” The design features a movable “DC group” of lenses that can shift position inside the lens without affecting astigmatism and CA.

The Nikon AF DC-NIKKOR 105mm f/2D lens shows subtle changes to the foreground and background bokeh when cycling through its defocus-control settings from front to neutral to rear.
The Sony and Fujifilm lenses use a “reverse apodization filter” to adjust the qualities of the out-of-focus areas. The filter is basically a built-in graduated radial neutral density filter that has darker edges than the center; artificial vignetting in a sense. This helps soften the edge of the bokeh discs—to achieve the opposite of the soap-bubble effect.
Based on research on the older Sigma lens, it appears that there was a control designed to assist in macro focusing, but a side effect of that control was SA adjustment.

Engineering Bokeh in Lens Design

Does bokeh just happen? Are you curious about whether lens manufacturers design lenses with bokeh in mind, or if there are other priorities that outweigh how a lens renders out-of-focus regions, like SA removal, sharpness, color rendition, etc? In talking to 17 different lens manufacturers, it is apparent that their approach to bokeh in the design stage varies greatly in emphasis and execution.
Tokina says, “The primary concerns are sharpness, contrast, and chromatic aberration (or lack thereof). So the short answer is that it’s not that bokeh just happens, but it is a secondary consideration that is usually handled in the mechanics/aperture of the lens.”
Of course, with lenses known for a particular bokeh effect, like the Petzval and some Lensbaby lenses, the mission is bokeh. Mamiya, who designs its leaf-shutter lenses with Schneider-Kreuznach, states that bokeh is a design consideration, but not the most important one.

Cleaning the rear element of this lens removed ugly spots in the bokeh.
As with almost everything these days, computers are heavily involved in the design of optics and also bokeh rendering. Before computers, calculations were done by hand. In 1956, Fujifilm created the first domestic computer in Japan, FUJIC, to design lenses. The computer is now in Tokyo’s National Museum of Nature and Science.
Canon’s Lens Development Group shared the following: “In order to achieve high image quality, Canon utilizes its own unique simulation software in the design phase, to confirm how bokeh changes under different shooting conditions, such as distance from the subject and aperture setting.” The company would not discuss the design specifics of its lenses regarding bokeh, but Canon did say that it is notable that the optical engineers are “using [this proprietary] computer simulation software to predict and confirm the impact of a lens design on bokeh characteristics. This is the same process used to derive our MTF performance curves on Canon lenses, and it’s interesting to learn that computer simulation is used not only to map out the anticipated contrast and resolution a given lens design may produce, but even its out-of-focus area characteristics.”
Leica’s specially designed software optimizes more than 50 parameters in the optical design with accuracy down to the length of one light wave (0.5 micrometer) to help the task of removing all aberrations. Also, many Leica lenses have aspherical elements. This precision is important when the lenses are polished to an accuracy of 1/2000mm (1,000 times thinner than a human hair).

The Brooklyn Bridge is brought into focus using the Fujifilm XF 90mm f/2 R LM WR lens.

Testing Bokeh

The rubber meets the road, so to speak, when the lens is actually built and an image is created. Computer simulation has its limits when it comes to optical design. “Because of the inherent diversity of the ‘bokeh,’ we go through design-based simulation to prototype and engineering sample-based evaluation in the lab environment, then to a field test when the final tooled samples become available. At each stage, we find something in reality. All in all, the computer-aided simulation is an inseparable process of the design; however, that will not cover the whole ground for the real-life shooting conditions,” according to Tamron. In addition, the company says, “We have to follow through up to the final field test to make sure everything is well under control.”
Olympus designs in a similar manner: “Bokeh design is ultimately accomplished by simulating how bokeh changes in both the foreground and background when varying the camera-to-subject distance while the optics are fixed at a specific focal distance. This process is repeated through the focal range of the lens until engineers are satisfied with the results. Before a lens has reached the mass-production stage, optical designers will use a pre-production example of the lens in real-world situations to validate its design. It’s at this point where engineers can make sure that the bokeh created is the same gentle and natural appearance determined by the simulation.”

Keep Bokeh in Perspective

For almost 20 years, for every conversation and website about lens sharpness, there has been a conversation and website about bokeh. Neither sharpness, nor bokeh, is usually the subject of a photograph, but they certainly are the catalyst for a lot of discussions.
Playing with bokeh, creating it, or capturing it can be fun. Definitely feel free to explore bokeh with your lenses. Just keep in mind that if you find that your favorite lens is producing bokeh that is swirly, creamy, bubbly, bokethereal [You heard that here first], bokehlishious, bokehrama, bokeawesome, bokehgross, bokehyuck, or bokehugly, try not to lose the subject for the background… unless the background is a blob of really ugly bokeh.

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