Why do the same photos taken from the same camera take up different memory space?

Ruben Jackson, Kottayam, Kerala

Some pictures take up less storage space in memory because patterns within the picture mean they can be represented more compactly.

A digital picture is made up of many small coloured dots called pixels. These pixels are packed so closely together that we do not see them individually, but rather they merge into each other to create the picture. Most digital cameras take photographs that are made up of millions of pixels.

Each pixel is represented by a series of numbers that contain information about the colour and brightness of the pixel

Mathematical techniques called compression algorithms are used to reduce the memory space required. The algorithms use patterns in the picture to reduce memory size. For instance, if there are 10 white pixels in a row, it is more compact to store this information as a code that means "10 white pixels" rather than simply repeating the same information for each of the white pixels. The algorithms are more sophisticated than this, but essentially they search for all sorts of patterns to make the information storage smaller.

Simple pictures can be stored in less space than complicated pictures. A simple picture has simple relationships among the pixels and these can be represented by simple codes. You can try this by taking a photograph of a white page and one of an ordinary scene. The memory size of the photo of the white page will be much smaller. No two photos will be exactly the same, and there will be different amounts of compression in each.

The compression algorithm may also approximate some parts of the picture to reduce the memory storage. This may lead to the loss of some of the detail in the picture, but often it cannot be detected by eye. Such algorithms are called "lossy", and the "jpeg" format commonly used in cameras is an example of this type.

When a photograph is stored in memory, it means that the numbers representing the pixels need to be stored. Therefore each picture requires the storage of many millions of numbers. However, large picture files are a problem since they take up more memory and are slower to copy and transmit.

Ian Cathers

Educational Consultant,

IID Community College,

Chennai