The photograph is taken through the main lens however the view from the lens and the viewfinder is slightly different, this difference is called parallax error.

Some compact cameras have a zoom lens, these models are often fitted with zoom viewfinders that mimic the lens. Parallax error increases as the subject gets nearer to the camera therefore some viewfinders have 'close up' frames marked on the viewing screen to enable the subject to be reframed.

Before the widespread use of autofocus the coupled rangefinder was an important camera design. The rangefinder camera has two windows on the front, instead of the usual one, this enables two overlapping views to be seen on the focusing screen. The rangefinder is coupled to the focusing ring of the lens. Moving the focusing ring on the lens causes the overlapping images to move in the viewfinder. When they coincide exactly the subject is in perfect focus. 

Digital cameras usually incorporate an LCD (Liquid Crystal Display) viewing screen that displays the scene viewed through the lens. However as this process takes power from the battery and the refresh rate of the viewing system is quite slow an alternative direct viewfinder is often provided.

Digital cameras, in their simplest form, use the principle of lens, shutter and film as found in traditional cameras. However instead of silver halide based film, as described in Chemical Image Processing,  the image in digital cameras is usually captured by a block-array Charged Coupled Device (CCD) or Complimentary Metal Oxide Semiconductor (CMOS) sensor.

A block-array image sensor is a matrix of light sensitive receptors.  These receptors produce a continuous analogue electrical signal which is passed through an Analogue to Digital Converter (ADC). The ADC translates the fluctuations of the sensor into digital signals. 

After adjustment and compression the image is usually stored on a removable storage device, for example Compactflash or Secure Digital.

The image recording mechanism within some digital cameras have variable speed ratings like silver based films. The normal speed or sensitivity of the sensor can be increased by an amplification process. 

Due to the relatively small size of a block-array sensors a digital camera's standard lens, as described in Focusing Light is about 24mm. The standard lens for a 35 mm camera is 50 mm and significant lens distortion can be introduced with very short focal length lenses that have not been specifically designed for use with digital cameras.

For specialist applications ‘scan backs’ are available for larger format cameras. These cameras place a scanner type linear array sensor in the film plane. Although increased resolution can be obtained there is an increase in time taken to record the image. Therefore ‘scan back’ cameras are only suitable for still life photography.
 

The cutaway diagram on the right of a Sony DSC-S70, introduced in 2000, shows the typical characteristics of a digital camera. Light enters both the lens and the viewfinder, behind the lens is the CCD. Images are stored on a removable memory card, in this case Sony’s proprietary memory stick on the left of the diagram. The half removed lithium battery powers the camera for up to 2.5 hours.

For more information please refer to the Digital Image Processing section.