Bokeh is a photographic term to describe the blurry “out-of-focus” areas of a photo. It comes from the Japanese language and means ”blur.”

In photographic lingo, people use it to refer to the quantity and quality of the blur as different lenses may generate different types of blur. Out of focus highlights have the shape of the lens aperture.

Commonly called “background blur,” the Bokeh effect is used to visually detach the subject (in-focus) from the background (out of focus). It is often seen in portrait photography, which is why Bokeh Mode is also called “Portrait Mode” in many camera apps.

The blur does not have to be in the background. It could show up in any place which is out of focus. If you focus on something far, things that are close to you may appear blurry. Bokeh is the result of a “shallow depth of field,” which means a short space around the focus plane in which things appear sharp.

The depth of field is the distance between the nearest and farthest object that appear sharp in a photo. For example, smartphones have a large depth of field (everything is usually in focus). The depth of field is a topic in itself and a bit outside the scope of this article.

Blurring is a result of how lenses work. Because they are curved, light from the scene will gather in a way that the image will appear sharped along a “focus plane.”

The idea is to have the sensor right on that focus plane. Objects from the same that are outside of the depth of field will be projected in a blurry way onto the sensor.

The size of the blur (how blurry things get) depends on the size of the circle of confusion, or how blurry can a single point in the scene get in the photo, while still being distinguishable.

Several things can influence how shallow (or not) the depth of field is. Remember, the shallower, the blurrier:

  1. Lens to Sensor distance
  2. Camera to Subject distance
  3. Lens Focal Length
  4. Lens Aperture (f/x)

To adjust the focus plane, it is possible to change the distance between the lens and the sensor. This is how auto-focus (AF) work on a mechanical level. Different AF techniques (contrast, phase, laser) are just ways to decide if the lens-sensor distance should be shortened or lengthened.

Additionally, when the camera’s aperture is increased (smaller f-stop number) light is gathered on a curvier surface, or at a wider angle if you prefer, making the out of focus areas even blurrier.

If you want to spend more time learning about this, you can start navigating the Bokeh Wikipedia page and follow critical topics that are required for a deeper understanding on this topic. The videos below may help as well:

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