Several key factors influence the quality and characteristics of bokeh in photography, each playing a crucial role in how the out-of-focus areas of an image are rendered. Let’s talk about these elements with examples and understand how to control bokeh to achieve the desired aesthetic. Here’s a breakdown of the main factors that bokeh depends on:
Aperture Size (f-Stop)
Larger Apertures
Smaller f-stop numbers (f/1.4) produce a shallower depth of field, resulting in more pronounced bokeh. This allows subjects in focus to stand out distinctly against a blurred background. A portrait with a wide open aperture can blur the background into a creamy, indistinct canvas, making the subject’s face sharp and the background lights appear soft. It’s important to note that each lens focuses an image onto a curved surface rather than a flat plane (a field curvature).
Smaller Apertures
Larger f-stop numbers (f/16) increase the depth of field, bringing the background into focus and reducing the bokeh effect. A landscape shot where both the foreground and the distant mountains are in focus, with the bokeh effect being almost non-existent due to the deep depth of field.
Lens Optical Design
Number of Aperture Blades
Lenses with more aperture blades tend to produce a rounder aperture, especially when not wide open (how open depends on the lens, for example, f/5.6). This can result in smoother, more circular bokeh. Lenses with fewer blades or blades that don’t form a perfect circle can create more polygonal bokeh shapes. A lens with 8+ aperture blades renders out-of-focus highlights smoother, producing more circular bokeh compared to a lens with 5 blades, where the bokeh might appear more pentagonal.
Lens Elements and Groupings
The specific arrangement and types of lens elements (including aspherical elements, for example) influence how light is bent and focused, affecting bokeh quality. Some lens designs prioritize minimizing aberrations, which can also impact the smoothness of the bokeh. For example, a prime lens designed with aspherical elements to reduce spherical aberration might produce a softer, more uniform bokeh compared to a zoom lens at the same focal length and aperture. Some lenses have a spherical aberration control, like the Canon RF 100mm f/2.8L Macro lens. Its spherical aberration control (via the SA ring) allows you to vary the bokeh characteristics manually. Canon’s SA control ring is similar to Nikon’s Defocus Control (DC) system. Please refer to this article for a more visual explanation of Canon’s SA control.
Why Corrected Spherical Aberration Produces Smoother Bokeh?
Why Zoom Lenses Often Have Less Aspherical Elements?
When was the lens’ spherical aberration control first implemented?
What is the future of spherical aberration control?
Focal Length
Longer Focal Lengths
Longer focal lengths generally produce a shallower depth of field at a given aperture, leading to more pronounced bokeh. A wildlife photographer captures a bird in the wild at a 200 mm focal length. The bird is sharply in focus against a beautifully blurred green background, emphasizing the bird while the background fades into smooth bokeh.
Shorter Focal Lengths
Shorter focal lengths have a broader depth of field, making it harder to achieve significant bokeh effects, especially when subjects are further away. In a street photography scene, the photographer captures a crowded market at a focal length of 24mm, with people and stalls in focus throughout much of the frame. This creates a sense of place without strong bokeh.
Distance Between Camera, Subject, and Background
Subject Distance
The closer the subject is to the background, the more the bokeh effect is created by making the background more out of focus. For example, a photo of a flower taken up close with a garden several meters behind it shows the flower in sharp focus. In contrast, it turns into a colorful, unrecognizable backdrop.
Background Distance
A greater distance between the subject and the background enhances the bokeh effect by further blurring the background. For example, a group photo where everyone, including those slightly behind the main subjects, remains relatively in focus due to the proximity of the background to the subjects.
Shape and Quality of the Light Source
The appearance of bokeh is also affected by the characteristics of the light sources within the image. For instance, point light sources like street lamps at night can turn into soft, glowing orbs or more defined shapes depending on the lens’s aperture shape and optical quality.
Point Light Sources at Night
Capturing city lights at night with a fast lens, where the distant streetlights and car headlights turn into large, glowing orbs that beautifully contrast against the darker night sky.
Sunlight Filtering Through Trees
For example, in a forest, the sunlight filtering through the leaves creates a bokeh effect where the light spots appear as soft, ethereal glows scattered across the image’s background.
