icon account icon cash icon cart icon cart icon search main arrow arrow check icon camera icon light icon audio icon computer icon storage icon discount icon facebook icon twitter icon linkedin icon vimeo icon youtube icon instagram icon google plus icon share icon email icon print icon time icon phone icon email-m icon marker-m icon pdf icon remove icon calendar icon list icon comment icon out icon-status icon-star icon-switch pie-chart line-chart icon-user icon-user-1 icon-warning icon-heart expand-arrow-1 expand-arrow-2 icon-upload icon-download icon-none icon-date-scheduled icon-date-available icon-is-hazardous
Header image for article Lens Bokeh Explained

Lens Bokeh Explained

Bokeh is one of the more misunderstood concepts in optics. The term was coined for photographic use in 1997, from the Japanese word boke, meaning blur or haze. In recent years, "bokeh" has been simplified to refer to how the shape of a lens' iris is rendered in out-of-focus points of light in an image, such as streetlamps at night. Much is made of how many iris blades are used in a lens and how round the iris aperture is as it is stopped down, but these are only minor aspects of bokeh.

The bokeh of a lens defines how that lens renders the out of focus portions of the image. In representing a three-dimensional space on the two-dimensional field of a photographic plate, the geometry of the path light takes through the lens elements can become apparent. As elements within the frame move out of focus (further from the third dimensional "Z-axis' center of the image) and further from the middle of the frame (the two dimensional "X-axis" and "Y-axis" center of the image), the Z-axis distance can begin to interact with the X- & Y-axis distances, causing a curving of the image. The visual effect is to create a spherical or tunnel-like rendering of the image as it progresses from the plane of focus. The more pronounced this effect, the more "coarse" the bokeh. A highly corrected bokeh minimizes the rounding effect, rendering a more flat field as the image shifts focus.

In the video above, I demonstrate the differences in bokeh effect using three lenses of similar aperture size and identical focal length. Test lenses were the Zeiss Ultra Prime 50mm T1.9, Cooke 50mm S4 T2, and a 60-year old Cooke Speed Panchro Ser.II T2.3 lens. All were tested with their apertures wide open so that iris shape was not a factor. They were mounted to a Chrosziel lens projector, which has a lamp inside and interchangeable reticles to project a reference image onto a wall. I shifted the lenses quickly through their focus ranges to see how the projected image was altered. The Zeiss Ultra Prime appeared highly corrected, as the image did not change dimensionality much as it moved in and out of focus. The Cooke S4 was less corrected, as the shape of the image began to "cone" as it went out of focus, with the center remaining flat but the outer edges curving away. The elderly Speed Panchro exhibited a much coarser bokeh, with a pronounced curvature of the image.

Remember, a course or fine bokeh does not necessarily translate to a "good" or "bad" bokeh, as it is more of a personal preference. One man's artifact is another man's art.

If you'd like to learn more about bokeh and the other sometimes-indefinable characteristics of lenses, keep your eye out for my HD Essentials: Secrets of Optics class!

Mitch Gross


Intro image for article NAB 2016: New Cooke Anamorphic Lenses
Tech News
I stopped by the Cooke booth today and spoke with Rich Eilers about some of their new offerings at this year's show. Cooke has announced two new additions to their Anamorphic /i family of lenses.
Intro image for article Panel Discussion: 25 Years of Camera and Lens Advancements
Industry News
Over the last 25 years, the range of image acquisition tools has expanded exponentially and opened up many possibilities. At our 25 Year Celebration in Los Angeles, we put together a panel of seasoned image-makers and new talent to get their take on how technology has changed and continues to influence their craft.
Intro image for article At the Bench: An Inside Look at Zeiss Cinema Lenses
Tutorials & Guides
Most cinema cameras today can work with lenses designed for cinema use, as well as those designed for still photography, but there are still important differences between these lenses. I teamed up with Snehal Patel from Zeiss to answer the question "what makes a lens a cinema lens"?
Subscribe to our Newsletter