September 12th, 2011

In digital imaging a pixel is the smallest amount of data in any one image. The number of colours that can be shown by any one pixel is determined by the bit depth of the pixel. Most digital images today are either 8bits/channel or 16bits/channel. The list below shows how many colours are in 1 through to 24 bits.

· 1 bpp, 21 = 2 colors (monochrome)

· 2 bpp, 22 = 4 colors

· 3 bpp, 23 = 8 colors

· 8 bpp, 28 = 256 colors

· 16 bpp, 216 = 65,536 colors

· 24 bpp, 224 ? 16.8 million colors

Image resolution in a camera is determined by the number of megapixels the camera has in its sensor. A megapixel is equivalent to 1,000,000 pixels and can be used to show the number of pixels in an image or a camera’s sensor. The more megapixels a camera has, the larger you can print the final image without losing quality. For example a 2MP camera and a 10MP camera can both take excellent photographs and when printed at 6×4 inch it would be hard to tell them apart. However when you start making larger prints say 12×8 inches the 10MP camera will far exceed the quality of print from the 2MP camera. The reason being is that the mega pixel count of a camera tells you the size of the photo in PPI (pixel per inch) from this you can work out the size you can print and still retain a good quality finish. see fig1.

Fig 1. Table showing Estimate of width and height 3:2 camera ratio

Mega Pixel Count Image Size on Screen PPI Print Size (in) at 300dpi
2 1736 x 1160 5.78 x 3.86
4 2448 x 1632 8.16 x 5.44
6 3000 x 2000 10 x 6.67
8 3464 x 2312 11.54 x 7.7
10 3872 x 2584 12.9 x 8.61
12 4288 x 2848 14.29 x 9.49


For Print

The industry standard recognised for good quality prints is 300 dpi (dots per inch) this means that the printer will print the final image with 300 individual dots of ink placed into every one inch line of paper. Using a number less than 300 dpi will result in the image being more pixelated, for example suppose you setup the printer to print at 1 dpi, you would have a printed photograph made up of a series of dots 1 inch in diameter.


For Screen

Computer monitors have an optimum resolution that they should work at and this varies from monitor to monitor. If the screen has a high resolution set then you can view large image files without having to zoom out from the picture. For example a screen with a resolution of 600×800 will not be able to show an image of resolution 1200×1600 without scrolling or zooming out.

When viewing images on a monitor the accepted resolution is 72 dpi as most computer monitors work at this resolution. However as we have already established if you tried to print at 72 dpi the result would be less than satisfactory. In order to print something from the web first you would have to change the resolution to 300 dpi which would result in a much smaller image.

For example the following image is 8.3 x 12.5 inches at 72 dpi.


If I was to print this image at 72 dpi I would end up with something looking like this.



At 300 DPI the image is clear but a lot smaller


In order to get a good quality print I would need to set the DPI to 300 but this would mean the print size would be 2 x3 inches instead of 8.3 x12.5 inches.

Another reason for web images having a lower resolution is to do with file size, a 72 dpi image will take up a lot less memory than a 300 dpi image of the same dimensions.

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