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.
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.Home - Equipment - Composition - Light - Shutter - Aperture - Examples - Questions