An image’s resolution is a count of the number of points or dots composing it.
A Mosaic of Dots!
A digital image is like a mosaic, e.g. an image composed of many colored dots, called “pixels”, a contraction of the words “Picture Elements”. A million pixels is a Megapixel (more details in an upcoming article).
Click to enlarge: Simulated image, resolution 64 x 48 (actually 640 x 480). © Charles Martel 2015
In principle, the more an image has pixels, the more detailed and precise it is. It is also “heavier” for the computer to handle (e.g. it takes longer to process), as each pixel “weighs-in” with a certain number of bits. It is therefore very important to adjust the resolution of the image depending on the use that will be made of it.
Web use
For example, to show a photograph on the web, a fairly low resolution version will be sufficient. Most computer displays (screens) work at 72 dpi (dots per inch) resolution. So an image that displays at 10 in. x 5 in. (25.4 cm x 12.8 cm) onscreen needs only be 720 x 360 pixels.
Print use
But if you need to print it to a large size, e.g. poster size, lets say 20 in. x 10 in. (50.8 cm x 25.4 cm), for example, then you’ll need a LOT more resolution in order to get a decent, sharp and detailed print. You’ll need to print at least 300 dpi, so that’s 6000 x 3000 dots! Big difference! If you used the same image used for the web, you’d get a very blurry print indeed!
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© Charles Martel 2015