Aperture and Depth of Field Quiz

Depth of field is the zone in front of and behind the focus point that looks acceptably sharp. Deep depth of field keeps most of the scene sharp; shallow depth of field keeps only a thin slice sharp. It has nothing to do with brightness or color.

Shallow depth of field means only a thin slice of the scene, usually the subject, looks sharp while the foreground and background fall into blur. It is not everything blurry; the point of focus stays sharp.

f/2 is a wide aperture, and a wide aperture gives a shallow depth of field, so a subject can be sharp while the background falls out of focus. Higher numbers such as f/8 or f/16 keep more of the scene sharp.

A wide aperture like f/1.8 produces a shallow depth of field that blurs the background, separating a portrait subject from distracting surroundings and drawing attention to the face. Narrow apertures keep more of the background sharp.

Landscape photographers typically want the entire scene from foreground to distant horizon in sharp focus. A mid-range aperture around f/11 provides enough depth of field to achieve this without introducing softness from diffraction.

Depth of field shrinks as you move closer to your subject, regardless of aperture setting. A macro photographer working at very close range may find depth of field measured in millimeters even at small apertures. ISO, white balance and file format have no effect on it.

The closer the focus distance, the thinner the zone of sharpness at any given aperture. A head shot from one meter has far less depth of field than the same lens focused on a building across the street, which is why close-up and macro work needs careful focusing.

Long focal lengths magnify the subject and give shallower depth of field when shot from the same distance. That is why telephotos are good at blurring backgrounds behind wildlife and portraits. Wide-angle lenses are the ones known for deep depth of field.

Out-of-focus points of light take the shape of the aperture opening. Lenses with more, rounded blades make round, smooth bokeh, while fewer straight blades make polygon shapes, especially when stopped down. Exposure is set by the size of the opening, not the blade shape.

Bokeh is the quality of blur in the out-of-focus areas of an image. It depends on aperture shape, focal length, and distance to the background. Smooth, creamy bokeh is prized because it isolates the subject without distraction.

Small apertures like f/22 create the widest depth of field by limiting the cone of light entering the lens. The tradeoff is that diffraction at very small openings can reduce overall sharpness, so many photographers stop just short of the minimum.

Diffraction is light spreading as it passes the edges of a very small opening. At tiny apertures such as f/22, that spreading softens fine detail across the whole frame. Wide apertures do not suffer from it.

In aperture priority mode (often marked A or Av) you set the aperture and the camera chooses the shutter speed for a correct exposure. It is the mode to use when depth of field is the main concern. Shutter priority is the reverse.

Changing the aperture changes how much light comes through the lens and how much of the scene is in focus. Opening up gives a shallower depth of field; stopping down gives a deeper one.

Aperture is measured in f-stops, such as f/2.8, f/4 and f/5.6. Each full stop halves or doubles the light that gets through. Millimeters measure focal length, seconds measure shutter speed, and Kelvin measures color temperature.