Bokeh and Depth of Field

Whenever you read a lens review, you’re almost certain to come across an evaluation of the ‘bokeh’ of the lens. It’s an aesthetic quality that can be difficult to define and quantify. Most definitions refer to the way the lens renders out-of-focus points of light or the ‘blur quality’ of out-of-focus backgrounds. The term comes from the Japanese word ‘boke’ (pronounced bo-keh), which means ‘haziness’, ‘dizzyness’ or ‘confusion’.

Use of a long focal length and a wide distance between the subject and the background ensures attractive bokeh in this shot, even though it was taken with an aperture of f/8, a full stop down from the maximum aperture of the lens. (70-300mm lens at 300mm f/8 on a ‘full frame’ camera body.)

Bokeh is generally associated with a shallow depth of field. This is easier to achieve with long telephoto lenses but is also an essential parameter in macro photography. Bokeh can also be important for portrait photography where medium telephoto lenses (typically 85″“150 mm in 35mm format) are generally used.

In all these situations, the photographer will normally use a shallow depth of field to focus the viewer’s attention on the subject by blurring the background. In most cases, this is achieved with a wide lens aperture, which creates a relatively shallow plane of focus. Background regions become out-of-focus as a result.

The appearance of the background blurring can affect viewers’ perceptions of the attractiveness of a photograph. The lens plays a vital role in determining how smoothly background tonality is rendered. Differences in aperture shapes and lens aberrations can cause some lenses to blur out-of-focus areas in a way that pleases the eye, while others will produce blurring that is distracting or unattractive.

The image format, lens focal length, selected aperture, camera-to-subject distance, position of the subject within the scene, and shapes and patterns within the subject can also influence bokeh quality. So can tonal relationships with respect to foreground and background brightness and colours within the subject.

Backgrounds containing patches of brightness and darkness or strongly contrasting colours can become ‘busy’ looking, even with lenses that normally produce attractive bokeh.

Similarly, a lens with relatively indifferent bokeh can produce attractive-looking pictures when the background to the subject is evenly lit with soft and harmonious tones.

In principle, the best bokeh appears when the subject of the photograph dominates and the bokeh literally ‘retreats into the background’ and is unobtrusive. When the blurred background elements take your eyes away from the subject of the photograph, the bokeh is classed as poor.

There are a couple of ways to minimise the influence of potentially unattractive bokeh. Filling the frame with the subject is the most straightforward. You can also change your shooting angle to ensure bright highlights are excluded from the background because large contrast differences can make the bokeh appear ‘choppy’.

Bokeh and Lens Design

To understand bokeh you need to know a little about how images are formed so this section will be a bit technical. Each picture element (pixel) is generated when the beams of light passing through the lens aperture combine to form a light cone. The base of this cone is the lens aperture, which can be adjusted to produce exit pupils of different areas.

When the apex (point) of the cone falls upon a pixel in the sensor, the image is focused; when it falls short of or beyond the sensor, it is out-of-focus. The diameter of the cone of light at the point where it becomes a disk instead of a point is known as the ‘circle of confusion’ or ‘blur circle’.

An example of attractive bokeh. Note the smooth tonal transitions in the blurred background areas that enable the viewer’s eyes to concentrate on the main subject. (70-300mm f/4.5-5.6 lens at 300mm f/5.6 on a ‘full frame’ camera body.)

It is almost impossible to achieve attractive background blurring when you shoot with a small-sensor digicam, even though the lens can focus very close to the subject. (5mm focal length, f/3.3.)

Outlining of the blur circles can indicate the lens suffers from longitudinal chromatic aberration. (24-105mm f/4 lens at f/4, 73mm focal length.)

Filling the frame with the subject can minimise the influence of the background, particularly when the subject is five metres or more in front of the background. (75-300mm f/4.8-6.7 lens at 171mm f/8 on an M4/3 camera.)

A harshly-lit background created the choppy bokeh seen in this image. Even though the subject was many metres in front of the tree behind, the contrasty background could not be defocused enough at the aperture used to produce smooth bokeh. (75-300mm lens at 210mm f/5.6 on camera with an APS-C-sized sensor.)

In a perfect lens, blur circles should have sharp edges and even illumination. Real lenses are never perfect (although some can come close). Spherical aberration will cause light to be unevenly distributed within the cone and produce different intensity across the circle and, when attempts are made to correct it, they often compound the problem instead of solving it.

Chromatic aberration (particularly longitudinal CA) can produce rings around the boundaries of blur circles. The shape and number of the diaphragm blades in a lens define the shape of the blur circle but otherwise have no effect on bokeh. Shoot through a cut-out star shape or triangle and you will see how the shapes of blur circles are affected.

The reason some manufacturers try to create relationship between bokeh and diaphragm blades is because this can create a point of difference between lenses. Myth has it that nine blades produce superior bokeh, but that’s simply not the case. However, if the blades close to a circular aperture, they will produce genuine blur circles, regardless of the number of blades (although the more blades the easier it is to approximate a circular aperture).

Seeing Bokeh

If you want to check the bokeh of a lens, set it to the maximum aperture and focus it on a distant light source, such as a street lamp at night in a reasonably well-lit area where you can discern details in the surrounding area. Scan the scene, looking at out-of-focus areas.

Now adjust the focus moving between a point in front of the light through to one behind the light and check how the rendition of the out-of-focus areas changes. If you see soft-edged shapes, you have good bokeh; if there’s a noticeable difference in sharpness or brightness between one side of the shapes and the other or if you can see outlining of the shapes, the bokeh is inferior.

Practice looking for bokeh when you’re watching TV or viewing movies. The shapes of the blur circles can tell you a lot about both the bokeh of the lens and the type of lens that was used.

The best bokeh comes when all the blur circles blend together attractively. When they appear as regular polygons, the number of points corresponds to the number of blades in the iris diaphragm. (Note: odd numbers of blades can produce twice as many points as the number of blades while even numbers of blades always produce the same number of points.)

If the blur circles are circular around the centre of the frame but oval towards the edges, the scene was probably shot at full aperture. If they are vertical ovals roughly twice as tall as they are wide, it was shot with an anamorphic lens for cinemascope projection.

Catadioptric (mirror) lenses are often criticised because they can produce donut-shaped highlights in out-of-focus backgrounds. (500mm f/8 reflex mirror lens.)

Artificial Bokeh

Many recently-released cameras provide an in-camera effect that simulates background blurring. Some cameras can produce quite attractive results; others less so. In most small-sensor digicams and camera-phones, background blurring is achieved by moving the lens and sensor together while keeping focus on the subject. The end result looks more like motion blur than ‘proper’ bokeh.

Several image editors provide tools for producing background blurring. But most involve complex mathematical adjustments. The best simulations are produced by techniques like Gaussian blurring, which requires a fair bit of processing power and is never as effective as the real thing.

The Lens Blur tool in Photoshop can be fraught with difficulties because the inherent bokeh of the lens will influence the way the blurring is applied. An oval blur point will remain oval unless you smear out the background completely (which will create an unnatural result). It’s simpler and you’ll end up with more satisfying images if you get a lens with good bokeh and learn to use it effectively.

A photograph taken with the ‘Flower’ setting in the Scene modes in a compact digicam, which creates artificial bokeh by choosing a wide lens aperture, masking the subject from and defocusing the background.

 

This is an excerpt from Photo Review Issue 56.

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