Lesson 5: Aperture: Controlling Depth of Field

The aperture is the adjustable opening inside the lens, made of overlapping metal blades. It does two things at once: it controls how much light reaches the sensor, and it controls depth of field — how much of the scene, from near to far, appears sharp. Of all the exposure controls, aperture is the one that most changes the look of a photograph, which is why so many photographers begin here.

Aperture is measured in f-stops, written as f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. The numbers are counter-intuitive at first: a smaller f-number means a larger opening. The reason is that the f-number is a ratio — the focal length divided by the diameter of the opening — so f/2 is a wider hole than f/16. Each full stop in the sequence halves or doubles the light: f/2 lets in twice as much light as f/2.8, and half as much again as f/1.4.

Depth of Field

Depth of field is the zone of acceptable sharpness in front of and behind the point you focus on. A wide aperture such as f/1.8 gives a shallow depth of field: your subject's eyes are sharp while the background melts into soft blur. That blur has a name — bokeh — and portrait photographers prize it because it separates the subject from distractions. A narrow aperture such as f/11 or f/16 gives a deep depth of field: a landscape can be sharp from the flowers at your feet to the mountains on the horizon.

Three things affect depth of field besides aperture: how close you are to the subject (closer means shallower), the focal length (longer lenses give shallower depth of field at the same distance), and the sensor size (larger sensors give shallower depth of field). This is why a phone, with its tiny sensor and short lens, keeps almost everything sharp, while a full-frame camera with an 85 mm lens at f/1.8 can blur a background into cream.

Using Aperture in Practice

A useful habit is to choose the aperture for the depth of field you want, then let the other settings follow. For a portrait, try f/1.8 to f/2.8 and focus on the nearer eye. For a group of people in a row, f/5.6 keeps everyone sharp. For a landscape, f/8 to f/11 is usually the sweet spot: deep enough for the scene, and near the aperture where most lenses are at their sharpest. Very narrow apertures such as f/22 can actually soften the image slightly through an effect called diffraction, so they are best reserved for when you truly need maximum depth of field.

Try this exercise: photograph the same subject at f/2, f/5.6 and f/11 without moving. Compare the three frames and you will see the background change from a wash of colour to recognisable shapes to full detail. That single comparison teaches more about aperture than any diagram — though the diagram below shows the idea too.