Aperture

Learning goal

You will be able to explain how the aperture inside the lens sets the flow of light going to the sensor by making its opening bigger and smaller. You will show that the f-number is a fraction, and that this is why a small f-number means a big opening and a big f-number a small opening; you will know that each of the full steps such as f/2, f/2.8 and f/4 halves the light, recognize the aperture's effects other than on light, and use the aperture as a light tool when photographing different skin tones in bright or dim light.

The question

Imagine looking at the lens of a camera from the front and changing its setting. Behind the glass, thin, curved metal blades slide over one another, leaving a hole in the middle. As you turn the setting one way the hole grows, and the other way it shrinks; the edges of the blades sometimes draw a hexagon, sometimes a circle.

How does the "pupil" inside a lens work? You know that your pupil grows when you walk into a dark room and shrinks when you step out into the sun. This opening in the lens does the same job; but your hand sets it. In this lesson we will talk about what the aperture does inside, what markings such as f/2 and f/8 on the camera count, and how to read this marking at a glance.

Context

In Lesson 8.1 we compared exposure to buckets filling under a tap: how wide the tap is open is the aperture, how long it stays open is the shutter speed. This lesson focuses on the width of the tap. The aperture is the first corner of the exposure triangle and one of the settings photographers change most often, because besides light it also clearly changes how a photo looks.

In many languages the aperture is called the diaphragm, a word of Greek origin carrying the sense of a partition that divides something in two; the same word is also used for the muscle between the chest and the abdomen. In English this opening is called the aperture, meaning an opening. On cameras the aperture can usually be adjusted; on phones, on the other hand, it is usually fixed. The opening of a phone's main camera is usually wide and can't be changed; that is why the real levers of light decisions on a phone are the shutter speed and ISO. This lesson therefore applies most to cameras with an adjustable aperture, but learning to read the f-number is useful on every camera.

Simple explanation

Why does the f-number look backward?

The size of the aperture is described by the f-number and is written on the camera as f/2, f/4, f/8 and so on. It has a rule that looks odd at first glance: the smaller the number, the bigger the opening. f/2 is a wide aperture, while f/16 is a small opening. The reason is that the f-number is actually a fraction. The "f" here describes the focal length of the lens, and f/2 means that the diameter of the opening is half the focal length.

Let's look at an example. On a lens with a focal length of 50 millimeters, at the f/2 setting the diameter of the opening is 25 millimeters. On the same lens at the f/8 setting, the diameter drops to an eighth of 50, that is 6.25 millimeters. The bigger the divisor, the smaller the opening. This fraction form has a convenience: on different lenses, the same f-number means roughly the same amount of light reaching the sensor per unit of area. On a wide-angle lens and on a long lens alike, f/4 is f/4 as far as exposure is concerned.

How does each step change the light?

On a camera the f-numbers move in a series: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. These numbers aren't random. Each step in the series halves the area of the opening; so the light reaching the sensor halves too. When you go from f/2 to f/2.8 the light drops to half, and when you go to f/4 to a quarter. In the other direction, going from f/8 to f/5.6 doubles the light.

You may wonder why the numbers look so irregular. The area of the opening grows with the square of the diameter; to halve the area you need to divide the diameter by about 1.4. That is why each number in the series is about 1.4 times the one before, and every two steps the number doubles: f/2, f/4, f/8, f/16. Photographers call each doubling or halving of light a "stop"; you will see this idea in detail in Lesson 8.8. Many cameras add two in-between steps between these full steps, for example f/2.2 and f/2.5; these change the light by a third of a stop each.

What else besides light does the aperture change?

The aperture doesn't only change light; it also changes how a photo looks. Its most noticeable effect is the zone of sharpness: with a wide opening, only the plane you focused on and a thin slice around it are sharp, and the front and back soften; with a narrow aperture, a much wider area from front to back looks sharp. The name of this effect is depth of field, and it is the subject of the next lesson. For now know this: when you choose the aperture, you choose not only the light but also how much of the photo will be sharp.

Sharpness also changes with the aperture. Many lenses give a slightly soft image at the edges and slightly dark corners at their widest opening; when stopped down two or three steps, sharpness and evenness increase. On the other hand, at very narrow openings, for example f/16 and f/22, light bends away at the edges of the small hole and the whole photo softens slightly; this is called diffraction. A narrow aperture also creates spiky rays around bright light sources: a streetlight or the sun turns into a small star, depending on the shape of the aperture blades.

How is the aperture linked to skin tone?

The aperture is the first tool that decides how much light reaches faces. In a dim setting, for example a kitchen in the evening or a wedding hall, because dark skin reflects less light in the same light, catching the detail of the face takes more light. Opening up the aperture is the cleanest source of light at hand, before turning to raising ISO and grain or to lengthening the shutter speed and shake. A wide aperture helps dark skin be recorded in its own color and texture.

In bright sun, the opposite can happen. A very wide opening takes in so much light that even the camera's shortest shutter speed may not be enough; the photo is overexposed and the forehead, nose and cheekbones of light skin turn into pure white patches. Then stopping the aperture down a few steps, cutting the light to a half or a quarter, rescues the bright areas. If there are people with dark and light skin in the same frame, bringing the light with the aperture to a point that keeps both prevents at the shoot losses that can't be rescued in editing.

Visual evidence

Imagine photographing a friend sitting at a table under an arbor twice with a camera on a summer evening. The shutter speed and ISO are fixed; you change only the aperture. First you shoot at f/2. Your friend's face is lit by the last light filtering through the vines hanging from the arbor; the warm color of their dark skin and the lines of their smile can be clearly made out. The garden behind them has turned into a soft green blur.

Then you stop the aperture down to f/16 and shoot again without touching anything else. From f/2 to f/16 there are six steps; the light has been halved six times, that is, dropped to a sixty-fourth. The photo is almost completely dark; your friend's face is a faint shape in the darkness, and only the streetlight behind is shining. A small six-pointed star has appeared around the lamp. The same evening, the same settings; only the width of the tap changed, and the photo went from light to dark.

Common mistakes

Summary

The aperture is a partition inside the lens that sets the flow of light going to the sensor by making its opening bigger and smaller, and it is described by the f-number. Because the f-number is a fraction, a small number means a big opening and more light; in the series that goes f/1.4, f/2, f/2.8, f/4, each step halves the light. Besides light, the aperture also changes the zone of sharpness, sharpness and the shape of bright points; it is opened up to catch dark skin cleanly in dim light, and stopped down to rescue the shines on light skin in bright sun.

Mini assignment

For this assignment you need a camera with an adjustable aperture, or a mode on your phone that lets you choose the aperture; if not, do the assignment on reading f-numbers.

1. Put your camera in A mode and photograph the same subject at f/2.8 or your lens's widest opening, at f/8 and at f/16; note how the camera changes the shutter speed.
2. Put the camera in M mode, keep the shutter speed and ISO fixed, change only the aperture, and compare the brightness of the three frames.
3. In the dark, photograph a streetlight or a bright point at f/16 and see whether the light turns into a star.
4. In frames with a face, write in one sentence how the detail of the skin changed at a wide and a narrow aperture.

Check yourself

• Can you say, without thinking, which f-number is the wider opening?
• In the three frames in M mode, were you able to observe each step halving the light?
• Did you notice the difference in how faces look at a wide and a narrow aperture?

Glossary

Mini quiz

1. Which describes the widest opening?

• A) f/16
• B) f/1.4
• C) f/8
• D) f/4

2. How does stopping the aperture down from f/4 to f/5.6 change the light reaching the sensor?

• A) It halves it
• B) It doubles it
• C) It doesn't change it
• D) It cuts it to a quarter

3. At a very narrow aperture, for example f/22, why does the whole photo soften slightly?

• A) Shake
• B) Overexposure
• C) Diffraction
• D) Wrong white balance

4. In a dim hall, what is the first aperture move to catch the detail of a dark-skinned face without grain?

• A) Stopping the aperture down to its narrowest setting
• B) Leaving the aperture fixed and raising ISO to its highest
• C) Brightening the photo afterward without changing the aperture
• D) Opening up the aperture

Answers

1. B — The f-number is a fraction; the smaller the divisor, the bigger the opening, which is why f/1.4 is the widest opening.
2. A — In the series of full steps each step halves the area of the opening; going from f/4 to f/5.6 drops the light to half.
3. C — Light passing through a very small hole bends away at the edges; this diffraction reduces the sharpness of the whole photo a little.
4. D — A wide aperture gathers more light without bringing grain; the detail of dark skin is caught cleanly at the shoot.

Summary card

• The aperture is the lens's pupil; its opening sets the flow of light.
• The f-number is a fraction: small number = big opening = more light.
• f/1.4, f/2, f/2.8, f/4, f/5.6, f/8: each step halves the light.
• The aperture also changes the zone of sharpness, sharpness and light stars.
• In dim light, open it up for dark skin; in bright sun, stop it down for light skin.

Bridge to the next lesson

When you open up the aperture, you don't only get more light; the back of the photo also starts to soften. Part of the secret of professionals' portraits where the background turns into a patch of color is here. The next lesson shows why the background melts away, and how a wide aperture, a close distance and a long focal length together narrow the zone of sharpness.