You're in control - if you
choose to be!
By controlling your aperture, you control how much depth
of field you get. This is probably the most common way of taking
pictures. If you are taking a scenic picture with a foreground and
a background, you will often want ALL of it to be in focus. But if
you're taking a subject that you want to stand out from its background,
you will probably want to limit your depth of field - a very good
technique for taking a portrait. Almost every picture I take is
taken in "aperture priority" mode because most of the time I want to
control the depth of field.
In this image, a large lens opening combined with a
telephoto lens caused the background to go out of focus and makes the
lilacs stand out. Since the background is completely blurred, your attention goes
right to the flowers.
In this image, there is a foreground and a background
that I want to be in focus. So I used a small lens opening with a
wide angle lens. The result was that almost everything in the
picture is in focus.