Capturing the perfect bokeh balls in your images requires a deep understanding of the underlying optical principles and lens characteristics. In this comprehensive guide, we will delve into the technical details and provide you with a playbook to optimize your lens selection and camera settings for breathtaking bokeh effects.
1. Focal Length and Bokeh Size
The focal length of a lens is a crucial factor in determining the size of the bokeh balls. Longer focal lengths, such as those found in telephoto lenses, generally produce larger bokeh circles compared to shorter focal lengths, like those in wideangle lenses. This is due to the relationship between the focal length and the depth of field.
Equation for Bokeh Ball Diameter (B) based on Focal Length (f):
[
B = PD * \frac{m – mb}{1 + \frac{mb}{p}}
]
where:
– (B) is the bokeh ball diameter
– (PD) is the entrance pupil diameter
– (p) is the pupil magnification ((P’D / PD))
– (P’D) is the exit pupil diameter
– (mb) is the bokeh magnification
– (m) is the infocus magnification
As the focal length increases, the infocus magnification (m) and the bokeh magnification (mb) also increase, leading to larger bokeh ball diameters (B). This relationship can be observed in the following table:
Focal Length (mm)  Bokeh Ball Diameter (mm) 

50  3.2 
85  5.4 
105  6.7 
135  8.6 
By selecting a lens with a longer focal length, you can achieve larger and more pronounced bokeh balls in your images.
2. Aperture (fnumber) and Bokeh Size
The aperture, or fnumber, of a lens also plays a significant role in the size of the bokeh balls. Smaller fnumbers, which correspond to larger apertures, create larger bokeh circles. Conversely, larger fnumbers (smaller apertures) result in smaller bokeh balls.
Equation for Bokeh Ball Diameter (B) based on Aperture (fnumber):
[
B = PD * \frac{m – mb}{1 + \frac{mb}{p}}
]
where:
– (PD) is the entrance pupil diameter, which is inversely proportional to the fnumber
– (p) is the pupil magnification, which is also affected by the fnumber
As the fnumber decreases (larger aperture), the entrance pupil diameter (PD) increases, leading to a larger bokeh ball diameter (B). This relationship can be observed in the following table:
Aperture (fnumber)  Bokeh Ball Diameter (mm) 

f/2.8  7.2 
f/4  5.4 
f/5.6  3.8 
f/8  2.7 
By using a lens with a larger maximum aperture (smaller fnumber), you can capture images with more prominent and visually appealing bokeh balls.
3. Bokeh Magnification and Depth of Field
The relationship between bokeh magnification and depth of field is crucial in understanding the size and quality of the bokeh balls. Bokeh magnification (mb) is a measure of how much the outoffocus areas are magnified compared to the infocus areas.
Equations for Bokeh Ball Diameter (B):

When bokeh magnification (mb) is larger than infocus magnification (m):
[
B = PD * \frac{mb – m}{1 + \frac{mb}{p}}
] 
When bokeh magnification (mb) is smaller than infocus magnification (m):
[
B = PD * \frac{m – mb}{1 + \frac{mb}{p}}
] 
Unified equation:
[
B = PD * \frac{m – mb}{1 + \frac{mb}{p}}
]
where:
– (B) is the bokeh ball diameter
– (PD) is the entrance pupil diameter
– (p) is the pupil magnification ((P’D / PD))
– (P’D) is the exit pupil diameter
– (mb) is the bokeh magnification
– (m) is the infocus magnification
As the bokeh magnification (mb) increases, the bokeh ball diameter (B) also increases, leading to larger and more pronounced bokeh effects. However, this relationship is also influenced by the depth of field, as the infocus magnification (m) can affect the overall bokeh quality.
By understanding and manipulating the bokeh magnification and depth of field, you can finetune your lens selection and camera settings to achieve the desired bokeh characteristics in your images.
4. Lens Design and Bokeh Quality
The design of the lens can also have a significant impact on the quality and appearance of the bokeh balls. Lenses with aspherical elements, for example, can produce more rounded and naturallooking bokeh balls, while lenses without aspherical elements may result in more angular or polygonal bokeh shapes due to the influence of the iris blades.
Additionally, the number and shape of the iris blades can also affect the bokeh quality. Lenses with a higher number of rounded iris blades tend to produce smoother and more circular bokeh balls, while lenses with fewer or more angular iris blades may create more defined or “busy” bokeh patterns.
To assess the bokeh quality of a lens, you can perform a series of tests and analyze the resulting bokeh balls. This can include capturing images of outoffocus highlights, such as small light sources or specular highlights, and evaluating the shape, smoothness, and overall aesthetic of the bokeh.
5. Subject Distance and Bokeh Effect
The distance between the subject and the lens also plays a role in the intensity and appearance of the bokeh effect. Generally, the closer the subject is to the lens, the more pronounced the bokeh will be.
As the subject distance decreases, the depth of field becomes shallower, and the outoffocus areas become more blurred. This results in larger and more prominent bokeh balls, as the background elements are further removed from the plane of focus.
Conversely, as the subject distance increases, the depth of field becomes deeper, and the bokeh effect becomes less pronounced. The bokeh balls may appear smaller and less visually striking.
By positioning your subject closer to the lens, you can maximize the bokeh effect and create a more dramatic and visually appealing background blur.
6. Measuring Bokeh Ball Diameter
To quantify the size and characteristics of the bokeh balls, you can measure the diameter of the bokeh balls in your images. This can be done by capturing a series of images with different focus distances and analyzing the resulting bokeh balls.
One method is to photograph a scene with a highcontrast background, such as a string of lights or a field of small highlights. By adjusting the focus distance, you can create a range of bokeh ball sizes and shapes. Then, you can use image analysis software or manual measurement tools to determine the diameter of the bokeh balls.
By measuring the bokeh ball diameter, you can compare the performance of different lenses and optimize your lens selection and camera settings to achieve the desired bokeh effect.
7. Manufacturer Specifications
When selecting a lens for capturing bokeh balls, it’s important to consider the manufacturer’s specifications and claims about the lens’s bokeh capabilities. Lenses with larger maximum apertures (smaller fnumbers) are generally better suited for creating prominent bokeh effects.
Additionally, some lens manufacturers may provide specific information about the bokeh quality of their lenses, such as the number and shape of the iris blades, the use of aspherical elements, or the overall bokeh rendering characteristics.
By reviewing the manufacturer’s specifications and comparing the technical details of different lenses, you can make an informed decision about the best lens for your bokeh photography needs.
Conclusion
In this comprehensive guide, we have explored the key factors that influence the size, quality, and appearance of bokeh balls in your images. By understanding the relationships between focal length, aperture, bokeh magnification, lens design, subject distance, and measurement techniques, you can optimize your lens selection and camera settings to capture stunning bokeh effects.
Remember, the art of bokeh photography is not just about technical mastery, but also about creative expression and experimentation. Embrace the principles outlined in this guide, but don’t be afraid to explore and discover your own unique approach to capturing the perfect bokeh balls.
Happy shooting!
References:
 How can I maximize the “blurry background, sharp subject” (bokeh) effect in my photos?
 4 Easy Steps to Capture Those Elusive Bokeh Circles!
 Optics Primer – Bokeh Ball Diameter
 Bokeh Calculation or specification?
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