Definition: DSLR; Digital Single Lens Reflex Camera

Definition: DSLR; Digital Single Lens Reflex Camera  | Glossary entry

DSLR; Digital Single Lens Reflex Camera

‘DSLR’ stands for “Digital Single Lens Reflex” – a type of camera. The DSLR format provides the user with a direct view of the scene through the main lens. The viewer will therefore see the same scene the sensor captures.

Before the DSLR and film SLRs all cameras had ‘viewfinders’ through which to view the scene to compose the shot. Looking through the viewfinder, a lens in its own right, a user saw down a separate and parallel optical path to the main lens. This twin lens arrangement resulted in two important issues…

  1. The user had to guess at the effect of the main lens they were using.
  2. The scene the user saw was slightly offset from the scene the main lens recorded.

Using two separate lenses to view the scene made it difficult to compose the shot. The user was seeing a scene which never quite matched what was recorded by the main lens. (Diagram of a viewer camera). Some point-and-shoot cameras use this viewfinder format today. However, they usually have a main LCD screen to show the view through the lens.

The SLR and DSLR overcome these problems. The significant step forward was to redirect the optical path from the main lens to the viewfinder used by the photographer. This was done using a ‘flip-up mirror’ which directed the light up to a pentaprism lens. Today we call this TTL technology – through the lens technology. The diagram below shows the optical path in a modern DSLR.

The DSLR uses through the lens (TTL) viewing.

The DSLR uses through-the-lens (TTL) viewing. Users see the same scene as recorded by the camera sensor. Click to view large size.

An image that has passed through a lens is optically reversed horizontally and vertically. Light from the mirror with this optically reversed image is directed into the pentaprism. Modern DSLRs use a ‘roof pentaprism’. The shape of a roof pentaprism creates an optical path within the pentaprism that will reverse-flip the image horizontally and vertically. The photographer looking through the viewfinder (the part you look through on a DSLR) will see the image returned to its real-life orientation. More on pentaprisms on Wikipedia  External link - opens new tab/page.

When the user takes a shot, the mirror flips up. This movement is the ‘Reflex’ that allows the exposure to take place. While the mirror is up there is no optical path through the lens. The user is unable to see the scene in front of the camera.

Once the mirror lifts, and the shutter opens, the light from the lens has a direct path to strike the image sensor. The light falls on the sensor surface, which is the focal plane of the DSLR. The light falling on the focal plane forms an ‘image circle’. The image circle is where the image is focused by the lens into a circle forming an acceptably sharp image representing the scene in front of the camera.

The Digital Image Sensor (image sensor; sensor) is an Integrated Circuit Chip External link - opens new tab/page which has an array of light sensitive components on its surface – the sensor points. Each sensor point inside the image circle acts to convert the light to an electrical signal. The full set of electrical signals are converted into an image by the on-board computer.

There are two main types of image sensor. These are charge-coupled devices (CCD) and complementary metal–oxide–semiconductor (CMOS) active pixel sensors. Both types of sensor carry out the same task. There is no detectable image quality difference between the two sensors. However, there are clear economic, manufacturing and image rendering differences (beyond the scope of this article).

Modern DSLRs have a complex of sophisticated controls. Most of these are managed by the on-board computer. The controls are mostly pre-programed. They can carry out quite sophisticated shots in various automatic modes. However, these auto-modes tend to be based on ‘idealized’ shots. Thus, users do not have precise control over their shots. The computer can also provide full manual control to the user. This allows the user to quickly and very precisely adjust the camera to their needs and produce a quality exposure. This leaves the on-board computer to do the ‘developing’ of the file once the shot is taken.

The mechanical, computing and software standards in modern DSLR cameras are very high. They command a high price, but have become rugged and reliable. Most quality DSLRs are rated for over 100,000 shots and some much higher.

Most DSLRs, like SLRs before them, have inter-changeable lens systems. However, there is an intermediate class of camera called a bridge camera. It is so called as it fills a niche between point-and-shoot cameras and DSLRs. However, normally these cameras do not have interchangeable lenses. They not have pentaprisms or reflex mirrors.

The SLR technology is as effective today as when it was first released. It allows modern photographers to view the world as their lens and sensor would capture it. It is a simple mechanical technology which corrects the parallax errors of the earlier formats of camera. It allows us to more accurately frame and compose photographs.


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