Having been introduced to the fundamentals of how lenses create images, we can now explore the ways the controls on a camera lens allow you to alter the image. The principal control is the aperture (commonly referred to as f-number). The aperture adjusts image brightness and the range of subject distances you can focus sharply at one setting. It is very important in image-making to know when and how to create total sharpness, or to localize image detail. Some differences between cameras of different format (picture size) start to appear here too (Figure 3.1).