By John Ruffi
Ok so here’s the thing: yes, pixel is a unit of measure. Problem is, a lot of people tend to use “pixel” interchangeably with “point,” or worse, they disregard the pixel size of a “rasterized” image altogether.
What’s “rasterized,” you ask? Rasterized simply means an image “made out of pixels.” This pretty much always means an image, as opposed to type (words and sentences), or “vector” images. Yes a rasterized image can have type in it, but it won’t be type you can independently alter just by changing font size or adding or deleting a word. In a rasterized file, the type has basically become a picture of the word or words. Vector art is created using mathematics to describe its appearance, which your computer then interprets and displays, as opposed to raster images. As a result, vector art can be displayed or printed at any size without changing its quality or characteristics at all, as opposed to rasterized images. And herein lies the subject of this missive.
A pixel might be a unit of measure, but not of physical dimensions, such as are found in the 3-dimensional world you and I live in. A pixel can be 1000 miles wide, or fit on the point of a pin. In order to see a pixel we need to decide how many will fit into an inch, or some other unit of dimensional measure. So, if an image is created at 100 ppi (pixels per inch), for example, then a 100 pixel by 100 pixel image will be 1 inch wide and 1 inch tall. Think of pixels as the ‘density’ of an image. Just as we can measure the density of any physical object, that measurement can’t be substituted for width, height, thickness.
Computer screens and monitors have been built to display 72 ppi for most of the existence of computers. This means a 300 pixel-wide image would appear 4 inches wide on your computer screen and smaller on your phone. Phones have more pixels packed into an inch and a 300 pixel-wide image will fit snugly on one. Printed materials are never measured in pixels BUT the number of pixels per inch (ppi) is important to make sure an image does not have jagged edges, is blurry, or looks “bitmapped.”
Generally speaking, an image should be 300 ppi to be safe for printing on any printer, although 240 is sufficient for most printing, and large format such as posters and billboards can be even lower – 115 or even 75 ppi. Very often, though, someone will grab something off the internet and want to use it for printing which generally means the image will have to be 1/4 its internet dimensions to be clear, or else the image will be of pretty poor quality once printed.
In short, always be aware of the resolution of the images you plan to use, and of the minimum resolution required for the desired application. As with everything, it’s always better to have more than you need than not enough.