Why Does the Bokeh Effect Occur in Photography? | Focus #344

The bokeh effect is an essential technique in photography to highlight subjects and add depth. However, its effect relies on the physical mechanisms of light behavior within the lens, beyond just the phenomenon of 'background blurring'.

This time, we will explain in detail the fundamental optical principles that give rise to bokeh and its close relationship with depth of field.

Bokeh occurs in areas outside the range known as 'depth of field', where the image appears in focus. When a camera lens gathers light and focuses, only subjects at a specific distance are accurately captured in the focus plane, while those in front or behind appear as 'bokeh'.

When the F-number (aperture value) is reduced to open the lens, the angle of light passing through the lens widens, and light outside the focus plane does not converge accurately at a single point on the lens, making the background appear blurred. This is the basic mechanism by which bokeh occurs, with light spreading outside the focus plane, contributing to the depth and three-dimensionality of the photo.

'Focal length' also significantly affects the strength of bokeh. In lenses with a long focal length (telephoto), the depth of field becomes shallower, making the background and foreground more likely to blur, resulting in beautiful bokeh around the subject. This is because telephoto lenses gather light at a distant focus plane, making the background appear farther away and emphasizing the bokeh.

Conversely, wide-angle lenses have a deeper depth of field, resulting in a wider range of focus and a tendency for weaker bokeh effects. Telephoto lenses also create a 'compression effect', making the background and subject appear closer together, resulting in more impressive bokeh.

The shape and texture of bokeh are also influenced by the shape and arrangement of the lens's aperture blades. Lenses with polygonal aperture blade arrangements tend to produce angular bokeh light points, while lenses with circular aperture blade arrangements are more likely to produce soft, circular bokeh.

Therefore, the 'quality' and 'softness' of bokeh differ from lens to lens, and in cases where bokeh plays an important role, such as portrait photography, the design of the aperture blades significantly affects the impression of the photo. Expensive lenses are often designed with this in mind, resulting in beautifully smooth background bokeh.