About the DSLR

If you were to strip away the electronic refinements and automatic features of any camera you would find the same basic design underneath – a lightproof container with a hole at one end over which a lens is placed and a holder opposite to accommodate either a strip of light-sensitive film, or a light- sensitive electronic chip, the sensor.

To produce a correctly exposed image in a variety of light intensities, the camera lens has an iris diaphragm that can be adjusted to leave a hole of varying diameter. This is called the aperture. On a fixed-lens compact camera, the lens also contains a shutter mechanism, known as a between-the-lens shutter, which opens to allow light to reach the film. The shutter allows you to choose the precise moment of exposure, and by selecting from a range of shutter speeds you can also control the length of exposure. The shutter mechanism on a single lens reflex (SLR) camera is located inside the body, behind the lens, just in front of the film or digital sensor, and is known as a focal-plane shutter as it is just next to the focal plane, where the sensor is.

Another common feature is the viewfinder. This is basically a compositional aid that allows one to aim the camera accurately
and to view what elements to include in the frame or focus on.

The Path of Light

Subject and light source

A light source to illuminate a subject is essential. Light rays reflected from the subject are transmitted through the camera to form a latent image on the sensor.



A simple lens consists of a convex disk of ground and polished glass that refracts the widening light rays traveling away from every point of the subject, so that they converge to form coherent points. The point at which the lens focuses these rays – the focal plane – coincides with the position of the film when the lens is correctly focused. 


Focal plane

This is where the rays of light refracted by the lens converge to form a sharp, upside-down image. Light traveling from different distances from the camera needs varying degrees of refraction to focus at the focal plane, so a focusing mechanism moves the lens toward or away from the back of the camera. The position of the film, or chip, and focal plane coincide if the lens is correctly focused. 


The diameter of the lens diaphragm can be changed by turning the aperture ring. This dictates the brightness of the image reaching the film. Moving to the next f-number either halves or doubles aperture size. Aperture size also affects depth of field 



The shutter can be set at different speeds, which determine the length of time the film is exposed. Moving the shutter speed dial to the next stop either doubles or halves exposure time. Shutters located between the aperture and the lens or behind the aperture have overlapping blades that spring open when the release button is pressed; focal-plane shutters consist of two metal blinds that open progressively. 


Direct vision viewfinders on compact cameras do not show exactly the same image the lens sees. With an SLR, light is reflected by a mirror and pentaprism to the viewfinder. Digital cameras usually also have an LCD TV monitor to show the image being projected onto the sensor. 


The unique feature of the popular SLR (single lens reflex) camera lies in the design of its viewfinder system. Light traveling from the subject enters the lens and strikes a mirror angled at 45 ̊. It is then reflected upward, through the focusing screen, and into the pentaprism, where it exits the camera via a rear-mounted viewfinder window. This means that no matter what focal length lens is attached to the camera, the scene the photographer sees through the viewfinder corresponds exactly to that seen by the lens.