The word bokeh comes from the Japanese word for blur. In photography though it is more than this, it’s the quality of the blur. There are certain elements however that determine if it is bokeh, or just plain old blur.
Have you ever looked at a portrait photo in a magazine, book or online and been amazed by the soft out of focus and blurry background. This is usually the result of focusing on your subject and using a wide open aperture. Or in other words, an aperture with a low f/ number. A wide aperture helps to isolate the subject and throw the back ground out of focus. As a rule anything from f/4 and lower will create this effect. Having said that, the lower the f/ number at say f/2, the more the background will be thrown out of focus and look blurry.
If you look closely at some of these photos, you may notice more than just a soft out of focus blur and wonder what and how this was done. How did the photographer get those really cool out of focus circles? What are they?
Light passing through the lens and touching the aperture blades creates Bokeh. The more open the aperture and the more light entering the lens, the easier it is to make. Lenses on most cameras can create bokeh, even the lenses of compact cameras. However some lenses are better than others at achieving this and can give an almost milky effect to your photos.
When comparing bokeh in different photos, the more circular the bokeh, the better quality it is considered to have. This is where the lens you are using can make all the difference. The number of aperture blades the lens has will effect how the bokeh looks. The more aperture blades, the more round and better quality the bokeh. Lenses with few aperture blades, tend to create a more hexagonal bokeh than lenses with more blades.
An easy way to experiment with this is to go outside and look for an area with light filtering through the leaves of trees. Place your subject facing you with their back towards the trees. Remember that the further away the subject is from the back ground, the more out of focus the background will appear. Using a wide open aperture, somewhere around f/2 to f/4, focus on your subject and take the shot. Have a look at the photo on the camera’s screen. Can you see the bokeh effect happening?
Once you get the hang of this, try using different backgrounds to see the effect. Water coming from fountains, streetlights and even rain can reflect light in certain ways that help the photographer to achieve bokeh in their photos. For more information on aperture please see my post The Exposure Triangle: Part 3 Aperture