Images - Colour & resolution

The effect of colour depth and resolution

Colour depth is used to describe the maximum number of colours that can be used to represent an image. The higher the number of colours, the more faithful will be the image. This will clearly also affect the file size of the image.

We can have monochrome (black and white), grey scale (usually 256 shades of grey), 16 bit colour and 32-bit colour (aka true colour), for example.

Resolution of an image is the number of pixels (picture elements) or dots that make up an image. The greater the number of pixels, the sharper the image will be, and the lager the file size of the image. Pixel density, measured in pixels per inch (PPI), is used to describe the resolution of a computer screen, camera or scanner.

An image from the internet is typically 72PPI, which is a low resolution. If you try to enlarge the image on the screen, the software makes up for the pixels which don't exist and you get a blurred image. The higher the resolution, the larger the image you can display on a screen without it looking blurred.

Calculating file size

An image captured in 256 colours, with a size of 2100 pixels by 1500 pixels is save on a memory stick. Wha is the size in bytes of the file?

size in bytes = (image width x image height x colour depth)/8

                  = (2100 x 1500 x 8) / 8 (256 colours = colour depth of 8 bits)

                  = 3,150,000 bytes (3.15MB)