There has been a great deal confusion among many people, even among photographers and those working on digital images as to some of the terms used in digital photography. In particular, we are looking at two of the most often used terms, the DPI and the PPI.
What adds to the confusion is that some in the digital imaging field tend to use these terms interchangeably, something brought about for the most part by a lack of understanding what these two terms are about and what they mean.
This post hopes to enlighten you on what these terms are about for you to be better informed about digital imaging and photography.
Pixels Per Inch (PPI)
If you’re someone who is working with digital images like Carl Glancey, your primary concern is going to be the image’s Pixels Per Inch or PPI. DPI, which we’ll discuss next, refers to a technical aspect of printing devices which is more of a printer’s concern rather than yours.
So when people say “DPI”, they really mean “PPI” most of the time, and it’s become so much commonplace that annoying as it may be, you just have to put up with it. What’s more important is to know is whether someone talking about DPI really means PPI.
To better understand PPI, we should know first what a pixel is. Pixel stands for “picture element”. It’s the smallest physical element of a digital display device that the eye can discern. You will see these pixels when you zoom in the photo on your screen; these are the rows and rows of tiny little squares.
Note that pixels are physical things of a fixed size, even though that size itself varies. Thus, the number of pixels per inch (PPI) on your screen is a fixed quantity and cannot be adjusted.
What can be adjusted though are the size of the pixels, making the actual images themselves become bigger or smaller by an adjustment of how many pixels can be accommodated per inch.
Remember though that this is only a relative gauge of quality; if you were to stand further away, the image would appear as clear as it did before. The absolute resolution of the image has not changed as there are still as many pixels relative to the picture as there were before. So if you’re looking to have higher resolution, the only way to go about that is to produce an image with more pixels, not increase the PPI.
Dots Per Inch (DPI)
As was mentioned earlier, if you’re a designer, DPI barely concerns you and your work. Still, it’s an important concept to know what it is about.
You see, printers reproduce an image by spitting out tiny dots consisting of a mix of four colors, Cyan, Magenta, Yellow and Key (black), which combine to create a range of hues by the subtractive color model. There is bound to be some space between these dots, and this is what DPI measures: their density.
For example, if you are printing a 150ppi image at 600dpi, each pixel will consist of 16 dots (600 dots/150 pixels = 4 rows of 4 dots per pixel).
This matters to the client more because, as a rule of thumb, the higher the DPI, the better the image’s quality. But on the flipside, it will also use more ink and take longer to print, so keep that in mind for printing such images on your home printer. Just to give an idea, 150dpi is generally considered the minimum standard for high quality photos in books and magazines, 85dpi for newspapers, and 45dpi for billboards.
However, higher dpi does not necessarily mean higher quality because there is no standard dot size or shape, meaning that one manufacturer’s dots might look as good at 1200dpi as another manufacturer’s dots do at 700dpi.
As mentioned a few times here before, you have no control over DPIs. But you can refer a client to a professional print shop and have the shop, which will know the specifications of its machines, take it from there, so to speak.