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DPI and PPI - what does it mean?
DPI stands for Dots Per Inch and PPI stands for Pixels Per Inch. On their own these terms mean very little and most people use the terms incorrectly. The 2 terms are mostly used when referring to a printed image.
When Printing a photo you are probably most concerned with the PPI. This actually allows you to determine how big you can print a specific image (with some basic Maths). We look at the specific details in our article on preparing images for print. The DPI is a measure of how many dots of ink an inkjet is capable of printing. Remember, a Pixel is potentially made up of millions of colours, but an inkjet printer has to mix the 3 or more inks for each pixel. So, a printer, to print at 300PPI would need to be able to print at at least 900 DPI, assuming that each pixel is made up of 3 dots of ink (1 of each primary colour). It is a bit confusing as most people say DPI when in fact they really mean PPI.
In our example, a printed photo at 300 PPI is a High resolution image that will show a lot of fine details. A photo less than 300 PPI will show less details in print, where more than 300 PPI will not greatly improve the quality.
A Computer screen typically displays at 96 PPI and it is a trade off between providing the best possible quality v's the price. To make a computer screen with a higher pixel density is very expnsive.
The DPI information within an image is only used when it comes to printing out a picture because as we mentioned a computer screen displays in pixels and not inches. On a screen you get just the pixels at the screen resolution of 96PPI.
For more in-depth technical details read more about DPI in the Wikipedia