The resolution of image files that are used for print is one of the most common misunderstandings to lead to disappointment, especially when a print job so often only exists digitally right up to the point of production. Whereas the computer screen might be displaying what appears to be a quality image at the right size, when printed the image becomes a blocky, perhaps illegible smudge on the printed page.
Understanding the concept of resolution can avert much disappointment when a design goes to print.
Vector vs. Raster
Firstly, it is important to note the difference between a vector graphic — composed of shapes — and a raster (aka bitmap) image — composed of pixel data.
The “shapes” in a vector image are mathematical descriptions of each object (like a circle or rectangle), unlike a raster image where a grid of pixels, each of a particular
Of course, you view both types of files on a computer screen, which uses pixels to represent their appearance. It is at the print stage that the difference can become most obvious. Vector graphics always print with a clarity that is not affected by how large or small they are scaled while bitmap images that are
There are some file types that can be a combination of vector and bitmap (EPS, PDF, AI), depending on how the file is constructed in a graphics program.
Here is a graphic of our mascot hard at work. The original art is a vector graphic.
A raster version of the same graphic looks much the same, except that zoomed in, it reveals how the pixel data affects the image clarity.
At normal (100%) pixel size, you see the same image but printing a raster image usually appears less sharp than an identical vector image.
Note: many vector file types (e.g. PDF, EPS, AI) can contain both vector and raster data
Pixels vs. Dots
The computer screen is composed of pixels while print is generally composed of dots. The steps that translate pixel data to print dots rely heavily on the resolution of the original image. Quality problems can most often be traced back to the picture file used in a design.
Demonstrating the Differences
Here is a nice looking JPG image (original is a TIF) that we will use to demonstrate how resolution affects print quality.
We’ll zoom in on that rectangular area to see more closely what we’re providing for the printing plates.
Original is 300 dpi (print quality)
The colour changes between pixels still look quite gradual, and edges are well defined.
Here is the image with a line screen applied, as would be used for press plates. From our 300 dpi image, this printout would appear very clean and detailed.
A close-up of the leaf edge. This shows how dots of ink will form the image.
Original is 72 dpi (website quality)
Pixels are very apparent, and there is a “staircase” effect (blockiness) along edges.
The 72 dpi image, however, is visibly inferior. While the print dots help a bit with breaking up those hard pixels, the results are not so pleasing.
Our screened web image appears more blurry and the edge definition is blocky.
It is probably intuitive to know that “an image of greater resolution will look better in print than one of lesser resolution.” It is just the trick of understanding how an image on screen translates to print since what you see is not always what you get.