The next question in our quest to understand image resolution is the difference between pixels per inch (PPI) and dots per inch (DPI).  PPI is generally used to describe camera sensors, while DPI is generally used to describe digital printers.  We discussed in the previous blog how a pixel that is 1/200^th of an inch on a side is just about the smallest object that your eye can resolve.  So if you fill that little pixel with even smaller dots of color your eye will blend them together.  Early printers used only three colors, however modern printers can use as many as seven colors.  .  This is what enables them to achieve a more subtle range of color.  So if you are looking at 200 PPI you are going to need 7 X 200 = 1400 DPI out of your printer.  For my 200 PPI that would be 7 X 300 =2100 DPI. So basically, the number of DPI required is the number of colors) X (the number of pixels per inch).