Understanding the Bokeh Effect and How to Use It
With all the time and effort you spend making sure your subject is framed, exposed and focused correctly, it can be easy to forget about what the background looks like. Selecting a large aperture to blur the background can reduce distractions, but it also introduces the question of whether the out of focus area itself is aesthetically pleasing—a concept known as the bokeh effect.
The Mechanics of Bokeh
Bokeh is the way your lens renders the out of focus points of light in a photograph. Since each lens has its own unique optical characteristics, they will all render out of focus light slightly differently, giving a different aesthetic quality to the blur.
In the out of focus portion of the image, each point of light that shines through the lens becomes an image of the aperture of the lens. This image is a disc of light roughly circular in shape, but since lens apertures are formed by a number of metal blades, it is never completely round. An aperture made up of six blades will appear as slightly hexagonal bokeh, while one with eight blades will appear as octagonal.
The spherical aberration of a lens also plays a role in the appearance of the bokeh effect. It’s possible for the discs of light to be brighter near the edge, brighter near the center or evenly bright across the entire disc.
Bokeh is most noticeable in images with a shallow depth of field and a light source or bright highlights in the background. Lenses with focal lengths longer than 85mm and apertures of f/4 and larger are best for creating the shallow depth of field needed. While the quality of the out of focus area is noticeable in any image, a backlit area with many small point light sources—such as dappled sunlight through trees—truly shows off the bokeh of your lens and makes for particularly beautiful portrait backgrounds.
Good Bokeh, Bad Bokeh
Not all lenses—and therefore not all bokeh—are created equal. Inexpensive lenses often have fewer, straight-edged aperture blades that render as harsh, sharp-sided geometric shapes. Lenses with a larger number of blades and ones with curved blades will tend to render softer, rounder discs. Catadioptric or mirror lenses will render bokeh as rings or “donuts,” owing to the mirror in the lens blocking part of the aperture.
In the end, what makes bokeh “good” or “bad” is almost entirely subjective. What is most important is making sure that your bokeh complements your subject and enhances your image, rather than distracting or detracting from it.
Photo credit: Morguefile