Again, for the purpose of this tutorial, we will use a semi-manual mode. By selecting aperture priority, you manually set the aperture and the camera automatically adjusts the shutter speed to achieve a correct exposure. The aperture is the size of the light opening.

You should understand aperture because it controls depth-of-field. When a picture is in focus, a particular depth is the sharpest. A large aperture (small f-stop) creates shallow depth-of-field, while a small aperture (large f-stop) creates deep depth-of-field.

Let's think about a portrait. We focus on the subjects eyes, which is six feet from our camera. Focus softens as the distance from the sharpest point increases. Depth-of-field is the range that remains in relatively sharp focus.

Using f2.8 (large aperture), we can create shallow depth-of-field and keep the background out of focus. By doing this we can make our subject the focal point and minimize distractions in the background.

But if we were shooting a landscape picture, we would want deep depth-of-field so that everything is in focus and we can appreciate the nature scene as a whole. Look at the examples page to recognizing how depth-of-field enhances composition.

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