This is the size of the lens opening which determines how much light will go onto your camera’s sensor. It is measured in ‘f- numbers’ or ‘f-stops’ (explained later in this chapter), which are calcu- lated by dividing the aperture’s diameter by the lens’s focal length. In other words, a 50 millimeter aperture on a 200 millimeter focal length lens will have ¼ f-stop, which is commonly written as 1:4, f4 or F4. As a rule of thumb, the higher the f/stop, the lower the aperture value. The aperture value has a significant impact on the DOF (depth of field, also explained later in this chapter) of the picture. A smaller aper- ture value will result in a larger DOF effect. The way DSLRs can use var- ious values of aperture to control DOF is a distinct advantage over com- pact digital cameras that have neither the sensor size nor the focal length to match the DOF capability of DSLRs.

Depth of field

Simply put, depth of field (or DOF) measures how much of a picture, besides the main subject, stays in focus i.e. how much of the foreground and the background of the subject is sharp after the picture has been taken. Focal length, aperture and distance from the item all affect DOF. For example, a large aperture (an f/stop of 1:2) will have a shallower DOF ef- fect, with the large part of the background and foreground out of focus. This setting is typically used for portrait photographs where you want the picture to focus primarily on the subject. As you move closer to the subject, the DOF will decrease, whereas moving away from the subject will increase it. A DSLR camera equipped with a lens that has a smaller focal length will have a larger DOF. Your DSLR may also be able to preview the picture’s DOF before you have even taken the shot. This feature is known as ‘optical preview’ or ‘DOF preview’.