Blurry photos are generally a bad thing. But although it's true that--with rare exception--no one likes camera shake in their photos, sometimes blur is intentional, such as when you shoot a photo with shallow depth of field so that the subject is sharp and the background is out of focus.
Within the blurry background, you often have lights or reflections--especially at night. In fact, the quality of the background on those highlights is something that photographers consider when taking the shot, and you can affect the look of your photo by tweaking that blurriness. There's even a name for this effect--it's called bokeh, and it comes from a Japanese word meaning "blur" or "haze." Here's what I'm talking about; consider this detail from the background of a photo.
These bright circles are a result of shooting lights or reflections out of focus, such as what you get in the background of a photo.
In a nutshell, bokeh refers to the visual properties of blur. But before I go any further, first, some simple science: Where are the blurry dots coming from? Without getting too technical, I can say that bokeh happens when light rays pass through the camera lens and diffract around the lens aperture. As a result, the shape of the aperture gets repeated all over the background of the image. Often, the blur is not quite round, because the aperture isn't perfectly round; typically it's made from seven or nine metal blades--small metal surfaces that open and close inside the lens like a diaphragm--that approximate a circle. But if the aperture is shot wide open at the smallest f-number, then the blur will be a perfect circle.
In this background, you can clearly see the lights take on the shape of the seven-blade aperture.
So how do you shoot to get this effect? The most important thing you need is a scene that includes fairly small lights or reflections. The lights of a city at night, Christmas tree lights, or bits of light reflecting off leaves are all good candidates. Large lights or reflective surfaces won't work if you shoot them too close.
Once you have the lights, you need to shoot them with the right settings. To bring out their inner bokeh, you have to keep them out of focus, so make them the background element in your shot. The more distance you can put between your subject and the background lights, the better the bokeh may be.
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