What is Bokeh in Photography?

Bokeh is a Japanese word which literally translates as “blur”. Bokeh is described as the aesthetic quality of the out of focus areas in a picture. It is how the camera lens renders points of light in a scene which are not in focus.

Photo by: Amir Kuckovic

As photographers we are familiar with the concept of Depth of Field; when a picture is shot using a wide aperture (shallow depth of field) objects that fall outside of the depth of field range will appear blurred.

Bokeh Tutorial

In the diagram given above light rays coming from the plane of focus will be perfectly sharp, those rays coming from within the range covered by the available depth of field will be acceptably sharp and those will appear as a sharp point in the sensor, but light rays coming from outside of the depth of field will appear as blurred circles.

This is what we generally call background blur (or foreground blur); bokeh is not the blur itself or the amount of blur present in the picture; rather it is the quality or feel of the blur and the reflected points of light.

Good Bokeh and Bad Bokeh

Photo by: Amir Kuckovic

Lens design is a complicated thing, a perfect lens will render out of focus points as evenly lit circles of light with sharp edges but that’s not what we call a perfect bokeh. To get perfect bokeh the lens should render the points of light as a Guassian distribution. Thus what is perfect for physics is not perfect when seen from an artistic point of view.

A technically perfect lens which has no spherical aberration focuses all points of light as cones of light behind the lens. When the sensor plane is at the exact location where the cone reaches its finest point the image is said to be in focus. The better the lens the finer the points will be. But if the sensor plane is not located where the con of light reaches its smallest point, then the point is rendered as a disk of light instead of as points. These discs are called "blur circle," or "circle of confusion".

In a lens with no spherical aberration this blur circle is an evenly illuminated disc, all out of focus points will look like perfect discs with sharp edges. But in reality no lens is perfect, the all have differing amounts of spherical aberration present and this aberration alters the way the lens renders the out of focus points of light. Thus even though all the light coming through the lens from a point may meet at a precise point on the sensor; the light distribution within the cone may be uneven. They may have more of the light collect in the middle of the disc or towards the edges.

It should be noted that Under corrected spherical aberration and Over corrected spherical aberration causes light to collect in two different ways. If light tends to collect towards the middle of the out-of-focus discs on one side of the cone, then it will collect on the outsides of the discs on the other side of the cone and therefore, a lens with great bokeh for backgrounds has awful bokeh for foregrounds, and vice versa.

Bokeh

In the above diagram it is clear that in the out of focus circle for poor bokeh and good bokeh both lenses display some degree of spherical aberration in the case of poor bokeh light is collected towards the edges and in case of good bokeh light is collected at the center.

Photo by: Quentin Descotte

Artistically most people tend to prefer sharper foregrounds and softer backgrounds. Blurred foreground elements are distracting while blurred backgrounds are perfectly fine. Therefore lenses with good background bokeh are said to be lenses with good bokeh.

Diaphragm Blades and Bokeh Shapes

The shape and number of lens diaphragm blades define the shape of the blur circle. The out of focus circles actually appear as shapes with as many sides as there are blades in the aperture diaphragm. Thus a diaphragm with five blades will produce pentagons, one with six blades will produce hexagons, one with seven blades will produce heptagons and ones with nine blades will produce nonagons and so on and lately lens diaphragms are being designed with curved blades to give a close approximation of a circle. The more the number of blades, the more the shape looks like circles thus more the number of blades better the bokeh right?

Camera lens aperture diaphragm

Not entirely, the shape of the circle is only one part of the equation, the other being how well is the light distributed. And when shooting with aperture wide open the diaphragm has absolutely no role in determining the shape of the circle.

How to Determine if a Lens has Good Bokeh or Bad Bokeh?

Photo by: Colton Witt

You can determine the quality of bokeh a lens produces either by looking at a picture taken with the lens or by observing a point of light through the lens.

If you find a picture that shows distant points of light at night or of light shining through leaves during day you could easily judge the quality of bokeh of the lens.

The lens has good bokeh if the points of light all blend together nicely and
The lens has neutral bokeh if the points of light appear as distinct circles.
The lens has bad bokeh if the points of light looks like donuts (dim center with brighter well defined edges).

You can also tell the quality of bokeh a lens could produce by looking at a light source through the lens. Point the lens at a light source and turn the focusing ring.

The lens has good bokeh if you see soft edged circles.
The lens has neutral bokeh if you see perfect round discs and
The lens has bad bokeh if you see doughnut shaped discs.

It should be noted that the above test of bokeh is not conclusive, the following parameter all influence how the bokeh is delivered by a lens:

Focal length
f-number
Picture format
Distance from camera to subject
Distance from the subject to the background or the foreground
Shapes and patterns of the subject
Aperture iris shape
Aberrations of the lens
Speed of the lens
Foreground/background brightness and
Colour

Changes to any of these parameters will change the way a lens delivers bokeh and this is the reason why we often hears different or even contradictory judgement about the bokeh of many lenses. So never judge a lens based on single observations, there are many effects that are attributed to the lens but in reality are caused by the subject in front of the camera. Differences between lenses are often very marginal but grossly exaggerated.

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