How A Digital Image Is Constructed

1. The image

Resolution

A digital image is made up of pixels. A good analogy would be a tiled floor forming a mosaic. Each pixel has a particular colour, as the pixels are very small the colours appear to blend together to form a continuous image. Medium to high end cameras now have resolutions between 2.1 and 3.3million pixels. Some cameras offer higher resolutions via interpolation, try and avoid these or use the non-interpolated figure in your comparisons before buying a camera.

Interpolation

Some cameras claim higher resolutions via software enhancement or interpolation.

Interpolation adds extra pixels between the existing pixels in order to increase the resolution. The 'value' or colour of these new pixels is estimated from the value of adjacent pixels.

Compression

This term is often refered to and denotes the degree to which the data which makes up the image is shrunken by using special algorhythms. A higher level of compression can lead to a degradation of image quality though.

How does it work?

Each pixel has a numerical value which denotes its colour. Compression turns adjacent pixels of the same colour into smaller values.

e.g 234565,234565 (11 digits) vs 234565x2 (8 digits)

At low compression levels only similar numbers are combined, as compression levels are increased, the width of colours combined is increased. This may produce artifacts as slightly different shades may combine to form a solid colour block under high compression levels. Therefore it is best to use a low level of compression with photographs used for dental photography as quality is of prime importance.

How is the Image Captured?

A digital camera works via a CCD (charge coupled device). This is basically a chip which is normally around 1/2 inch square. The chips is split up into many separate areas. Each one represents one pixel on the captured image. High end digital video cameras may have 3CCD badges on them. These give a better colour accuracy by spliting the light entering the camera with a prism (Red,Green and Blue). Each colour band is then directed at a separate CCD.

Image Storage

The digital picture is stored on a memory card within the camera. This can be removed to allow it to be inserted into a card reader for transfer to the main computer.

Compactflash

This is the standard used by Nikon cameras. The main advantage of this system is that the card can be inserted into a PCMCIA adapter which allows the data card to be used.

Smartmedia

This card is used by Fuji range of cameras and is smaller in size than the compactflash card but less robust and slightly flexible.

Memory Stick

This is a Sony card and has not been adopted by any other manufacturers.