This is a piece I wrote for our clients, trying to explain the issue to them. While it grossly oversimplifies in some cases, and is geared toward our particular business (wallpaper borders and murals), it may be helpful to some.

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A Quick Primer on Image Size, Printing, and Resolution

We frequently receive files to be made into borders or murals that are simply too small to be usable. There seems to be a lot of confusion out there about file sizes and printing resolution, so I'm going to attempt to clarify the subject here.

The first thing to understand is that most computer image files are made up of dots called pixels. These dots are very small, and very close together, and when we look at them they seem to make up areas of solid colors and shades.

The best way to describe how big an image is is to state how many pixels high and wide it is. For example, an image on your computer screen may be 900 pixels wide and 600 pixels high.

We can't really say how many inches high or wide an image is without specifying how we're looking at it. A computer monitor may fit 72 pixels onto each inch of the screen; in that case, our 900 x 600 pixel image is 12.5 by 8.33 inches. We would say that the picture is 12.5 inches wide at a resolution of 72 dots per inch.

If we were to print the image instead of looking at it on a monitor, it might be very different. Printers cram a lot more dots into each inch than monitors, so the image will be smaller. If it is printed at a resolution of 300 dpi, it would only be 3 inches wide!

The three properties are related like this:

    **Size in Inches = Pixels divided by resolution

    Pixels = Resolution times Size in Inches

    Resolution = Pixels divided by Size in Inches**

In other words, if we know any two, we can figure out the third.

The number of pixels is always fixed unless we resample the image. The resolution and the size in inches change in relation to each other. Higher resolution equals smaller size, and vice versa.

Printing Resolution

We've already mentioned that a lot of computer monitors display images at 72 dots per inch; a lot of magazines on slick paper are printed at a resolution of 300 dots per inch. We have found that we get the best results printing our borders and murals at a minimum of 150 dpi.

This means that a 9 inch high border, for example, should be 1350 pixels in height.

To figure out how big a file needs to be, just multiply the height you want it to print to by the resolution (in this case, 150 dpi); this will give the necessary vertical dimension in pixels.

One further note: taking an existing file or scan and sizing it up in Photoshop is not a substitute for obtaining a larger file; you are effectively reducing the printing resolution by doing this.

"I've wrestled with reality for thirty-five years, and I'm happy, Doctor; I finally won out over it!" -Elwood P. Dowd