How a Camera Works

In this lesson we’ll cover some of the fundamental ways in which a camera actually works.  We’ll take a look at the major components of a camera, and get a basic understand of what each one is for and how it works.

On a basic level we can think of a camera as a kind of artificial eye. Like an eye, a camera captures incoming light and focuses it on a receptor, film, or sensor (in our eyes this is the retina).  A camera records the image formed. and this recording can be reproduced (either digitally or through chemical processes) as a photograph.  The most fundamental characteristic of a camera is that it is a device for capturing and recording light.

The image below shows a diagram of a DSLR camera.  DSLR cameras are a common type of digital camera, but most other varieties of camera will contain the same elements.  When a photographer wants to take a photograph, he or she will usually point the camera at the scene they wish to photograph and prepare the photograph by looking through the viewfinder.  On some cameras this is a LCD screen, on others it is a little window into which the scene is reflected by mirrors.  Once the camera is positioned correctly, the photographer presses the shutter release, which triggers the shutter in the lens to allow light to enter the camera.  This light is focused by the lens onto a sensor, which reacts to the image formed upon it.  The sensor then stores this image in some way, either to a digital memory card or onto a photographic film.

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The major components of a DSLR camera.


The lens is an optical component specially designed to bend and collect light.  If you look at the diagram below, you can see how light coming in from the surroundings (represented by the straight lines on the left) is bent (focused) and directed towards a small area called the sensor (positioned close to the point labelled “F” in the diagram).  In this way the camera forms an image of the light entering the lens on the sensor.

Depending on the distance between the sensor and the lens, and the specific shape of the lens, images can be made larger or smaller. This is a particularly powerful concept – by careful design lenses can be made that allow us to zoom in on very distant objects (the most extreme examples of this are telescopes), very small objects (for example, microscopes) or that allow us to capture light from a much wider angle than our normal field of vision.

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We will talk about lens in much more detail later on in the course.


The sensor is the part of the camera that captures light. In analog cameras this is usually a photographic film, a material that reacts chemically when light hits it. This film can be removed from the camera, and following a chemical process the image can be recreated as a photograph. This was a quite time consuming and sometimes expensive process!

Nowadays most cameras are digital, and the chemical film has been replaced with an electronic light sensor. This uses an electronic process to store information about how much light has hit the sensor. The key advantage over analog cameras is speed – the photograph can be produced almost instantly and for no cost, meaning photographs can be rapidly reviewed.

A sensor is a vital part of any camera. When an advert talks about the number of megapixels or the resolution of a camera, they are actually describing properties of the camera’s sensor.  A sensor has a fixed size, and usually the bigger the sensor the better – the more light it is capable of capturing and so the higher the image quality.  Every digital sensor is made up of a grid of millions of pixels, each of which can be thought of as a little bucket that captures light from a specific part of the image.  Megapixels refers to the number of these pixels contained on a sensor – for example an 8 megapixel camera will have 8 million pixels on the sensor.  Taking both the sensor size and the number of pixels we can arrive at the resolution – the number of pixels per square inch (or per square cm).  A sensor with a higher resolution will normally produce better quality images.

Camera Body

The camera body connects the sensor and the lens, and also carries things like batteries, memory cards, viewfinders and LCD screens. Depending on the exact type of camera some of these features may be different or may not even be present.

DSLR cameras contain a mirror that reflects light from the lens up to a viewfinder. This means that when you are composing a photograph you can see exactly how it will look through the viewfinder. When you press the button to close the shutter the mirror lifts up and light travels straight through to hit the sensor and produce the image.

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With the mirror down light is reflected into a prism and then into a viewfinder.

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When a photograph is taken the mirror rises and light is free to hit the sensor, allowing a photograph to be taken.

In cameras without a viewfinder the LCD screen will directly show the image. Although many cameras have both a viewfinder and a LCD screen it is generally preferable to use the viewfinder when possible, as the LCD screen can quickly drain the battery.

Memory Cards

Memory cards are an important feature on all digital cameras. Most current digital cameras will support the use of a type of memory card known as an “SD card”.  There are three types of SD card – SD, SDHC and SDXC. The only difference is in terms of capacity, with SD offering up to 2GB, SDHC up to 32GB and SDXC up to 2TB. Some cameras may only accept one type of these, so make sure you check before splashing out a lot of money on a high capacity card!

You may also come across “CF cards”.  CF (Compact Flash) cards were traditionally used in DSLR cameras, but are often now replaced by SD cards.  In theory a CF card will allow faster data transfer speeds than a SD card will, but in reality there is little difference with modern cameras.  The biggest advantage of CF cards is their durability – they are much harder to break than an SD card, but this comes at the disadvantage of a higher price tag.

Photograph Attribution:
Camera diagram: By Astrocog (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons
DSLR mirror images: By Rodrigo.Argenton (Own work) [CC BY-SA 4.0 (], via Wikimedia Commons

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