Bokeh

Definition of Bokeh

Bokeh is the blur that happens in the area of the image that is out of focus. It is happens in the area of the image that is outside the depth of field.

Bokeh and Aperture

Bokeh in an image is greatly effected by aperture. It plays a role in how much bokeh is in the image, the larger the aperture the more blur there will be. In addition to the amount of blur, it also determines the shape of the blur. A round aperture will create round circles of blur.

Some lenses have blades that are not round when the aperture is stopped down. This produces other shapes depending on the number of blades. A common example is that a lens with five blades produces a pentagon shaped bokeh. What is more interesting is that bright lights will produce star shaped bokeh. Even number of blades will produce a star shape with the same number of points. For example a eight bladed lens will create a star with eight points. Odd number of blades will produce double the amount of points on the star shape, so a lens with seven blades will create a star with fourteen points.

Quality of Bokeh

Quality of Bokeh is hard to measure. Depending on the lens some bokeh produces circles that are darker in the middle with a sharp bright edge around it. This is considered poor bokeh because it will create a sharp and harsh looking blur. Other lenses will produce bright circles with a sharp edge. Mathematically this is perfect bokeh but visually it is not. Although this type of bokeh is a lot better, the sharp edges can still make the bokeh a bit harsh. The best form of bokeh (visually speaking) is where the circle is brightest in the center and gradually fades as it approaches the outside.

As mentioned in the section above about aperture, bokeh can take on different shapes. Usually higher quality lenses will have blades that are rounded so that as the aperture changes, it always remains a circle. Sharp corners produces harsher bokeh so rounded aperture is usually preferred.

Shaped Bokeh

Some specialty lenses allow you to fit a disc in front that have a shape cut out. The bokeh will take on whatever the shape  is. You can also create this effect by cutting out a shape from the lens cap and take a photo with the lens cap on. A good way to see this effect is to take pictures of lights (like Christmas lights) on a dark night.

Conclusion

Bokeh is the out of focus, blurred area of an image. The amount and shape can be controlled by the aperture of the lens. Quality of the bokeh is hard to measure but some bokeh is more pleasing to the eye than others. Good Quality bokeh is smooth and gradual compared to bokeh that has sharp edges and harsh. The shape of the bokeh is determined by the aperture of the lens or by creating a cover over the lens with a small shape cut out of the cover.

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