Bokeh is the aesthetic quality of 'blur' produced in the background or out of focus areas of your images. This is directly affected by the lens being used and its aperture blade design as well as the aperture your lens is set at.
Certain lenses will appear to render the bokeh smoother than other lenses which gives a softer or creamier look to the out of focus area in the image. This quality differs from lens to lens and aperture to aperture and can be more desirable than lenses rendering the out of focus areas in a more 'jaggy' or harsher appearing way.
A larger aperture will increase the amount of bokeh, while smaller apertures will produce less or no bokeh. This is why many photographers like to shoot portraits using lenses that have a "fast" maximum aperture, with as low of an f-stop number as possible.
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