Depth of field

Learning goal

You will be able to predict how much of a photo will look sharp from front to back by looking at four factors: the aperture, the distance to the subject, the focal length, and the distance between the subject and the background. You will decide when to choose shallow or deep depth of field according to what the photo tells, explain how the zone of sharpness spreads in front of and behind the focal plane, take everyone into the zone of sharpness in group photos, and make keeping the eyes sharp a priority with shallow focus, especially on faces where light is scarce.

The question

Leafing through a wedding album, you stop at a photo. The bride's henna-painted hands are sharp; you can even make out the traces of henna on her nails and the scratches on the ring on her finger. Behind her, though, the tables, guests and candles have turned into soft patches of color; only a few round balls of light are shining. In the photo you took with your phone on the same night, everything is sharp: the hands, the tables and the exit sign at the back.

Why does the background melt away in professionals' photos? There is no magic in it; there is a result built by a few factors together. In this lesson we will talk about how sharpness forms a slice in a photo, what narrows and widens this slice, and how to set the slice according to what the photo tells.

Context

In Lesson 3.3 you defined depth of field as the front-to-back depth of the area that looks sharp in a photo, and saw why a big sensor makes melting the background easier. In Lesson 8.2 we said that besides light the aperture also changes the zone of sharpness. This lesson combines those two clues and turns depth of field into a tool you can control.

While talking about simplicity and background, we said that when you call a subject a few steps forward from the foot of a wall, the wall softens, and we noted that we would come back to the physics of this softening later. This lesson is the answer to that. Depth of field isn't just aesthetic decoration; it is a powerful tool that decides where the viewer's eye will be sent. What is sharp comes forward; what softens steps back. That is why where you put the sharpness and how wide you keep it is an answer to the question of who the hero of the photo is.

Simple explanation

What kind of slice does sharpness form in a photo?

When the camera focuses on a point, it actually focuses on a plane: like an invisible sheet of glass containing everything standing at the same distance from the lens. This is called the focal plane. Everything in this plane is fully sharp. As you move in front of or behind the plane, sharpness slowly decreases; but up to a certain distance the eye doesn't notice this decrease. The whole front-and-back area that the eye still accepts as sharp is the depth of field.

This slice usually doesn't spread equally in front of and behind the focal plane. At middle distances, for example when you focus on a person a few meters away, the zone of sharpness stretches further behind than in front. When you get very close to the subject, on the other hand, the slice thins and spreads more evenly to both sides; when you focus on a flower petal from a few centimeters away, the tip of the petal can be sharp and the stem blurred. Four factors decide how thin the slice will be, and the photographer can change each of them.

What narrows and widens the slice?

The aperture is the best-known setting of this slice. A wide opening, for example f/1.8 or f/2.8, narrows the slice of sharpness and softens the background. A narrow opening, for example f/8 or f/11, widens the slice and keeps much more sharp from front to back. The distance to the subject also changes the slice. The closer you get to the subject, the thinner the slice; that is why at the same aperture a flower shot up close looks against a blurred background, while a landscape shot from far away looks sharp from start to finish.

Focal length is involved too. When you shoot from the same spot with a long lens, the slice narrows. If you step back to keep the same frame, the depth of the slice ends up roughly similar to the shorter lens's, but because the long lens enlarges the background more, the background still looks softer and fuller. The factor that is often missed is the subject-to-background distance. The further the background is from the subject, the more it softens. Calling a person a few steps forward from the foot of a wall is the easiest way to melt the background without touching the aperture. If you add sensor size to this, it becomes clear why phones with small sensors find it hard to melt the background.

Shallow or deep?

You answer which one is "right" not by the question but by looking at what the photo tells. Shallow depth of field separates the hero from its surroundings and gathers the eye in a single place. In a portrait the face, in a food photo the front edge of the plate, in a detail shot the fingers of a hand stay sharp; everything else turns into a softness of color and light. Photographers call the quality of this softness, in which bright points in the background turn into round balls of light, bokeh.

Deep depth of field, on the other hand, tells of relationship. A rock in front and a distant mountain being sharp together in a landscape, people and their surroundings being readable at the same time in a street photo, the whole room being visible in an interior: these call for deep focus. In documentary photography, deep focus is often the more honest choice for showing context, because it offers the viewer the whole scene. Shallow focus wipes out context; sometimes this is exactly what you want, and sometimes it hides important information from the viewer.

Where do you put the sharpness when photographing people?

When you photograph a face with shallow focus, the slice can be so thin that the tip of the nose is sharp and the ears blurred. In that case put the sharpness on the eyes; in a portrait the viewer looks at the eyes first, and if the eyes are soft, the photo looks flawed however good the rest is. If the person is turned sideways to the camera, focus on the eye closer to the camera. In dim light, the detail around the eyes on a dark-skinned face is already low in contrast; if the focus slips slightly from this area, the detail is lost quickly. In situations like this, zooming in on the screen to check the sharpness or stopping the aperture down one step is a safe way.

In group photos, shallow focus is a trap. When you photograph a group standing in a few rows at f/1.8, the front row stays sharp and the back row blurred. Stopping the aperture down a few steps, for example to f/5.6 or narrower, and lining the group up at as equal a distance from the camera as possible takes everyone into the slice. Think about who you put in the back row too: the back row both gets less light and falls at the edge of the slice of sharpness; always placing a dark-skinned person at the back makes their face get lost twice. A phone's portrait mode imitates shallow focus by calculation; it can go wrong while guessing what is in front and what is behind, and in particular it can blur the edge of curly hair.

Visual evidence

At a henna night, you move closer to the bride's henna-painted hands with a camera. Her hands are on the edge of the table in front of her, palms facing up; behind her, two or three meters back, guests are sitting and candles are burning on the tables. First you shoot at f/1.8, moving closer to the hands. The dark orange patterns of the henna, the lines of her palms and the gold ring on her finger are sharp. Even the bride's face has softened slightly; the guests behind have turned into unrecognizable patches of color, and the candles into round, warm balls of light. The photo speaks only of those hands.

Then you stop the aperture down to f/11, lengthen the shutter speed to make up for the light, and shoot once more from the same spot. The hands are still sharp, but now the bride's face, her smiling mother, the henna tray on the table and the faces of the guests behind can also be made out. The candles are small, sharp flames. The photo no longer speaks of a detail, but of a night and the people together on that night. The same moment; the width of the slice of sharpness changed, and the story grew from the hands to the room.

Common mistakes

Summary

Sharpness forms a slice around the focal plane in a photo, and the depth of this slice is the depth of field. A wide aperture, a close distance to the subject, a long focal length and a background far from the subject narrow the slice and melt the background; a narrow aperture and a far distance widen it. Shallow focus separates the hero, while deep depth of field tells of relationship and context; when photographing people, you need to put the sharpness on the eyes, take everyone in a group into the slice, and push no one to the edge of sharpness and light.

Mini assignment

In this assignment you will move the slice of sharpness with each of the four factors separately.

1. Put an object, for example a cup, on a table with other objects in front of and behind it, and photograph it from the same spot first at the widest aperture, then at f/8 and f/16.
2. Keeping the aperture fixed at its widest, focus on the object first from half a meter, then from two meters, and shoot.
3. Photograph a friend first at the foot of a wall, then a few steps away from the wall, at the same aperture.
4. Photograph a group of three or four people in two rows first at the widest aperture, then at f/8, and compare the faces in the back row by zooming in.

Check yourself

• Were you able to see in your own frames which factor changes the slice of sharpness most?
• In the portrait frames, is the sharpness on the eyes?
• In the group photo, is every face in the back row, whatever its skin tone, sharp and well lit?

Glossary

Mini quiz

1. Which of these narrows the slice of sharpness?

• A) Stopping the aperture down to f/16
• B) Moving closer to the subject
• C) Moving away from the subject
• D) Bringing the background closer to the subject

2. What is the easy way to soften the background more without touching the aperture?

• A) Moving the subject a few steps away from the background
• B) Raising ISO
• C) Shortening the shutter speed
• D) Shooting the photo vertically

3. In a portrait taken with shallow focus, where should the sharpness be put?

• A) On the tip of the nose
• B) On the ears
• C) On the background
• D) On the eye closer to the camera

4. If, when photographing a group standing in two rows at f/1.8, the faces in the back row are blurred, what should be done?

• A) Open the aperture up even more
• B) Stop the aperture down a few steps and line the group up at an equal distance from the camera
• C) Send the people in the back row further back
• D) Sharpen the photo afterward

Answers

1. B — The closer you get to the subject, the thinner the slice of sharpness; at the same aperture a close flower looks against a blurred background, and a far landscape against a sharp one.
2. A — As the background moves away from the subject, it falls further outside the slice of sharpness and softens without the aperture changing.
3. D — In a portrait the viewer looks at the eyes first; in a thin slice, putting the sharpness on the eye closer to the camera keeps the face alive.
4. B — A narrow aperture widens the slice, and lining people up at an equal distance takes everyone into the same slice; that way no face in the back row gets lost.

Summary card

• Sharpness is a slice; its depth is called depth of field.
• What narrows the slice: a wide aperture, a close distance, a long focal length, a far background.
• Shallow focus separates the hero; deep focus tells of relationship.
• In a portrait the sharpness is on the eye; focus on the eye closer to the camera.
• In a group, a narrow aperture and equal distance; push no one to the edge of sharpness and light.

Bridge to the next lesson

The aperture was the width of the tap, and it also decided the slice of sharpness. The second corner of the exposure triangle is time: the shutter speed. This duration decides not only how much light is gathered, but also how everything that moves during that time will look in the photo. The next lesson asks how we freeze time.