Resolution Is Not Image Size

• The resolution, otherwise known as the dots per inch (dpi - the same as ppi, pixelsIn digital imaging, a pixel is the smallest piece of information in an image. Pixels are normally arranged in a regular 2-dimensional grid per inch) or sometimes pixels per centimetre (ppc), of a photo, arguably makes the biggest difference to getting good quality prints from an image. Many people get horribly confused between image resolution and image size, so the following clear English explanation should help.

So What Is It?

• Imagine a square, like the one on the right, which is 3 inches along each edge, making a grid of 9 squares, each 1 inch by 1 inch. Now if the overall square had a resolution of 10 dpi, this means that you could expect to find 10 pixels in each of the 9 squares.
• The diagram, right, shows these 10 pixels as red squares. If the large square was printed at this resolution, it would appear on paper as a square of 3 inches by 3 inches. If you looked in a photo editing program it would show the image size as the same, but you would also notice that the image size in pixels was shown as 30x30 (perfect sense, as we have 30 pixels along each edge).
• Now imagine that you want to print this same square at the publication standard resolution, which is generally 300 dpi. Now remember we have only 10 dpi on our square. So we use an image editing program to simply change 10 dpi to 300 dpi. Easy. But you must then remember that you could not expect to still get a 3 inch by 3 inch square. Why? Well, we only have 30 pixels along each edge, and we would now need 300 pixels along each edge just to get a 1 inch by 1 inch square! So we'd actually end up with a square on paper which would be 0.1 inch by 0.1 inch, simply because we didn't start with enough pixels.

Almost There...

• Just to slightly complicate matters further, if instead of printing our square at 300 dpi, we wanted to publish it on a website, all we would need is a resolution of around 100 dpi, because computer monitors cannot show more dots in any given inch than this. So let's go back to our 30x30 pixel image - on paper at 10 dpi this would appear as a square 3 inches by 3 inches, on paper at 300 dpi it would be a 0.1 by 0.1 inch square, and on a computer screen, with a resolution of 100 dpi, it would cover an area of about 0.3 by 0.3 inches.

Resolution and MegaPixels

• Now modern digital cameras do not quote their image resolution in dpi, ppi or ppc. Instead they use figures such as 3MP or 6MP - 3 MegaPixels or 6 MegaPixels. This is because image resolution is very much in the hands of the user when they manipulate images at the computer - on this website most images are at 100 dpi or less, and yet when I print them I will change the resolution to 300 dpi. However image size is what the camera figures relate to, and this is covered in our Image Size feature.