How a camera works

Learning goal

You will be able to explain step by step, by comparing it with your eye, how a camera turns light into a recording: how the lens gathers light, how the aperture adjusts the light coming in, how long the shutter speed lets the light in, and how the sensor stores the light. You will use the similarities between the eye and the camera to keep the settings in mind, and, knowing the differences, you will be able to explain why the camera sometimes can't record what your eye sees.

The question

One morning, when you open the curtains, your eyes are dazzled for a moment, then they adjust. When you walk into a dark cinema, at first you can't see anything; a few minutes later you start to make out the seats. Meanwhile, inside your eye, a small muscle makes your pupil bigger or smaller. Inside a camera there is a part that does exactly the same job.

How much is a camera like our eye? The answer: surprisingly so. The lens corresponds to the eye's lens, the aperture to the pupil, the sensor to the retina. This similarity is no accident; both solve the same problem: gathering the light that comes from the outside world and turning it into a sharp image. But there are important differences between them too, and those differences explain why your photos sometimes come out differently from what your eye saw.

Context

In the previous lesson we told the types of camera apart by the path light takes inside them. Now we get to know the stops along that path. This familiarity is the foundation of the settings you will see later in the exposure module. Aperture, shutter speed and sensitivity to light get muddled when they are memorized as abstract numbers; when they are tied to real parts inside the camera, they stick.

That is why the eye comparison works. You use your own eye every day and know intuitively how it behaves: that it is dazzled in bright light, that it struggles in the dark, that the distance goes blurry when you look at something close. When you tie every part of the camera to a counterpart in your eye, instead of learning a new system from scratch you are giving names to a system you already know. The differences matter just as much; the fact that the camera can't do some of the things the eye does on its own shows where the photographer has work to do.

Simple explanation

How does light enter the camera and come into focus?

In your eye, light passes first through the cornea, the clear layer at the front, and then through the eye's lens. These two bend the light and gather it at a point on the retina at the back. In the camera, the lens does this job: this group of one or more glass elements gathers the light coming from the scene onto the sensor. In both the eye and the camera, the image that forms is actually upside down; you never notice that in the eye your brain, and in the camera the processor, corrects it.

Sharpness comes from the light gathering exactly on the sensor or the retina. When you look at something close, your eye changes the shape of its lens; this is called accommodation, and it happens within seconds without your noticing. In the camera, some of the elements inside the lens move back and forth. This is called focusing. The faint whirr you hear when you tap the screen on your phone or half-press the shutter button on your camera is these elements moving. When the focus slips somewhere else, the image blurs, just as the writing in the background scatters when you fix your eye on a finger held close.

What decides how much light comes in?

In your eye, the colored iris makes the pupil at its center bigger or smaller. In bright sun the pupil shrinks and little light comes in; in the dark it widens and gathers as much light as it can. Inside the lens, the aperture does this: a ring of thin metal blades that leaves an adjustable opening in the middle. The bigger the opening, the more light reaches the sensor. The aperture has another effect too: it changes how blurry the background looks. You will see this in detail in the exposure module.

Besides the opening, time also decides the amount of light. Your eye is always open; it sees the scene as an unbroken flow. The camera, however, lets light in only for a short time. In front of the sensor there is a curtain that opens and closes when you press the shutter button. The length of time this curtain stays open is called the shutter speed. You can think of it like an eyelid, but with one difference: the eyelid closes to cut out the light, while the camera's curtain opens to take it in. This time can range from a thousandth of a second to several seconds.

How does light turn into an image?

On the retina there are millions of light-sensitive cells. Some work in low light but don't tell colors apart; that is why you see colors as faded at night. The others tell colors apart: some are more sensitive to red, some to green, some to blue. Your brain combines the signals from these cells and builds a color image.

The sensor works in a similar way. On top of each of the millions of tiny light collectors on its surface sits a small red, green or blue color filter. Each collector measures only the light of its own color, and the camera's processor combines the measurements of neighboring points to calculate the full color of each point. Here the most important difference between the eye and the camera appears. Your eye can see very bright and very dark areas at the same time, because as you move around the scene it constantly adjusts and your brain stitches the pieces together. You can easily see both the face of a friend with darker skin sitting by a window and the sunny trees behind them. The sensor, however, records at one instant with one setting; most of the time it can record correctly either the face or the window. The photographer's job is to decide what matters in that situation.

Visual evidence

Imagine two friends sitting at a table by the window in a café. One has darker skin, the other lighter; midday sun comes in through the glass behind them and the street glares white. When you look with your eyes, you see everything: the expressions on both faces, the cups on the table, the people walking outside, the tree on the opposite sidewalk. Nowhere is too dark or too bright.

When you photograph the same scene with a phone, you get one of two results. If the phone sets itself for the bright street outside, the window looks lovely but both faces turn into dark silhouettes; on your friend with darker skin, the expression is almost completely lost. If the phone sets itself for the faces, the faces show, but the window turns into a flat white area and everything outside is wiped out. The camera can't record in one frame what your eye sees at a single glance. There are ways to solve this: seating the two friends side-on to the window, changing the frame to leave the window out, or using techniques you will see later. But first you need to know where the difference comes from: the eye adjusts, the sensor records.

Common mistakes

Summary

A camera is surprisingly like our eye: the lens gathers light and brings it into focus like the eye's lens, the aperture adjusts the light coming in like the pupil, and the sensor turns the light into a color image like the retina. The big difference is this: the eye constantly adjusts and sees very bright and very dark areas together, while the camera records at one instant with one setting and asks the photographer to decide what matters.

Mini assignment

In this assignment you put your eye and your camera side by side.

1. Look at your own pupil in a mirror; switch the room light off and on, and watch how your pupil grows and shrinks.
2. Examine an object or person standing in front of a bright window first with your eyes, then photograph it with your phone.
3. On the phone, take two more frames, tapping first on the window and then on the object, and compare the three frames.
4. Write in two sentences the difference between what your eye saw and what each frame shows.

Check yourself

• Could you explain the similarity between the pupil and the aperture based on your own observation?
• Could you correctly name which areas were lost across the three frames?
• Could you explain why the camera can't record in a single frame what your eye sees?

Glossary

Mini quiz

1. Which part of the camera corresponds to the pupil in the eye?

• A) The sensor
• B) The aperture
• C) The screen
• D) The battery

2. How does focusing happen in a camera?

• A) By the sensor changing color
• B) By the aperture closing completely
• C) By elements in the lens moving back and forth
• D) By the curtain staying open longer

3. What is the most important difference between the eye and the camera?

• A) The camera uses no lens at all
• B) The eye has no light-sensitive cells
• C) The camera doesn't turn the image upside down
• D) The eye constantly adjusts and can see light and dark together, while the camera records with one setting

4. Why might the face of a person with darker skin in front of a bright window come out dark in a photo?

• A) Because when the camera sets itself for the window's brightness, it can't record enough light for the face
• B) Because darker skin damages the sensor
• C) Because the lens reverses the colors
• D) Because the window glass blocks the light completely

Answers

1. B — The aperture, like the pupil, adjusts the amount of light coming in by changing its opening; the sensor corresponds to the retina.
2. C — Some elements inside the lens move back and forth so that light gathers exactly on the sensor; in the eye, the lens does this by changing shape.
3. D — The eye constantly adjusts as it moves around the scene; the sensor records at one instant with one setting and often has to choose either the bright or the dark.
4. A — The problem is not the skin but the setting: when the camera sets itself for the bright window, it can't record enough of the lesser light coming from the face, and the face stays dark.

Summary card

• Lens = the eye's lens: it gathers light and brings it into focus.
• Aperture = the pupil: it adjusts the amount of light coming in.
• Shutter speed: how long the curtain in front of the sensor stays open.
• Sensor = the retina: it measures light through color filters and turns it into an image.
• The eye adjusts, the sensor records: you choose the most important area.

Bridge to the next lesson

Now you know that the sensor corresponds to the retina. But not all sensors are the same: there is a difference in size between the sensor in your phone and the sensor of a large camera, and that difference changes the photograph. The next lesson asks: same megapixels, so why a different photo? There you will see why a large sensor gathers more light, why it reduces noise, and why it blurs the background more easily.