What is Digital Image Resolution?
Perhaps the most important technical concept to understand when starting any digital imaging project is image resolution. If you don't know the difference between a megapixel and a megabyte, you've come to the right place.
Pixels are the building blocks of every digital image. Clearly defined squares of light and color data are stacked up next to one another both horizontally and vertically. Each square (pixel) has a dark to light value from 0 (solid black) to 255 (pure white). That is, there are 256 defined values. A gradient is the gradual transition from one value to another in sequence.
Fine detail is rendered by having many, many of these pixels. Too few and the picture appears jagged....or pixelated as you can see from the photo on the right. It simply looks terrible. So we need to 
make sure that we use the correct resolution for the application we're working on. For most screen applications, we use 72 pixels per inch (72ppi).
So how many dots are needed to get the job done? If we want to make prints that look as good as conventional photographs made from film, we need to have a resolution of at least 240 dots per inch in both directions. If you are printing on glossy paper with a high resolution printer, you might need to go as high as 300 dpi.
What's is high, medium and low resolution?
Industry has divided resolutions into 3 areas:
- High Resolution - 300ppi
- Medium Resolution - 150ppi
- Low Resolution - 72ppi
These descriptions can be confusing as we perceive that low resolution is of lesser quality than high resolution. Simply not a true perception. High resolution (300ppi) is used for press applications by printing companies. Office inkjet and laser printers often use medium resolution (150ppi) and the web and most digital applications use lower resolutions (72ppi). These are generalizations, but they get you in the ballpark. Click on the link above, "DPI vs. PPI", for calculating correct print resolution.
