Aperture is the variable size of the opening in the lens, letting in light at a rate that can be adjusted. It is expressed in “f-stops”. The smaller the number, the bigger the opening, hence the more light that the lens allows to strike the digital sensor. In fact as you decrease the number, each f-stop allows double the amount of light of the previous setting.
But there is also a side effect of adjusting the aperture that can either help you make much better photographs, or ruin the perfect shot. It is called depth of field. Depth of field, as you might guess, is the term used to indicate how far from the focal point the “in-focus” area of the photograph extends. The larger the aperture (smaller number) the less depth of field you have. So when you see those great portraits with just the person in focus and the background blurred, that is done by controlling the aperture (using a low numbered f-stop). When you see landscape photographs with the whole scene amazingly sharp, that is also done by choosing the appropriate aperture (using a high numbered f-stop).
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