The truth about file size

I’ve noticed over the years that many people, even many of those in the industry seem to have trouble understanding the facts about file size and how big of a file you need in your document so that it will print correctly. If nothing else, understanding this concept alone could save you hundreds, even thousands of dollars in costs to reprint a job because the quality was not up to par.

The easiest way to understand file size requirements is first understand that there are really only two types of image files. Understanding which type you’re using at the time is the first step to determining proper file size.


Vector Graphics
We’ll start with the easiest to understand first to get it out of the way. The first file type is called Vector graphics. Wikipedia.com defines Vector graphics as follows:

“Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygon(s), which are all based on mathematical equations, to represent images in computer graphics.”

The key words in this definition are “mathmatical equations”. What this basically means is that the shape or combination of shapes that make up the Vector graphic are made up of mathmatical equations. What this means to you if you are worrying about file size is, you don’t have to worry about file size at all if your entire graphic is Vector. You can enlarge or reduce it to your hearts content and it will never affect the resolution or quality of the graphic. Now keep in mind that if you place a Bitmapped graphic (explained later) into your Vector graphic, all bets are off and the Bitmap graphic rules will apply to the Bitmap portion of your graphic. The most common application for creating Vector graphics is Adobe® Illustrator.

The most common use for a Vector graphic would a logo. And the most common file format for a Vector graphic would be an .EPS which stand for Ecapsulated Postscript. You could, for example, create a logo in Illustrator and save it as an .EPS file. Then, everytime you place that logo into your page layout program, you could enlarge or reduce it accordingly without having to worry about quality degradation.

Another interesting fact about a Vector .EPS graphic is that the resolution it prints out at is dependent on the resolution of the printer you’re printing it out on. So, if you print your Vector Graphic on a 300dpi laser printer, it will print out at 300dpi. If you print it on a 2400dpi inkjet printer, it will print out at 2400dpi.
Below is an Illustration showing an enlarged comparison of the same graphic in Vector and Bitmap Versions. Notice as we enlarge the graphic the Bitmap version becomes pixilated while the Vector version remains clear.



Bitmap Graphics
Bitmap graphics require a little more understanding. Think of their composition as a bunch of tiny squares next to each other, all different colors, making up the image.

The higher the resolution, the more tiny squares per square inch.
The general rule for print quality graphics is as follows: Your Bitmap graphic should be as close to actual size as possible. Meaning, if your place a photo into InDesign and the photo is 5” by 4”, the image size in PhotoShop for example, should be as close to 5” by 4” as possible.

One common mistake I see with designers today is, “Well I’ll just open this photo up in PhotoShop and make it bigger.” The general rule of resolution is, “You can always go smaller but never go bigger.” The proper way to make your 5” by 4” photo into a 25” by 20” photo is to rescan it at that resolution and image size.

Another rule, the resolution or dpi (Dot Per Inch) should be (2) Two times the line screen. So now what is line screen. Take a look at a printed photo. If you look very closely, you’ll notice that the photo consists of small dots, generally in a pattern. The finer or smaller those dots the higher the line screen. Probably, the most common line screen used today for commercial printing would be within a range of 150LPI (Line Per Inch) and 200 LPI.

In my experience, for most practical commercial work a photo to size at 300DPI is more than adequate for a good quality reprooduction.
Using these general rules will help you in preparing your files correctly for print and save you time and money.