This week I've had a two jobs where the files supplied to me by my clients wouldn't work for our projects. The images looked fine on screen but were too low of a resolution to be used for print. I spent a lot of time explaining why and I thought it might be helpful to create a quick primer.
There are two basic types of graphic files—vector and raster.
Whenever you see a photograph or a multi-toned illustration reproduced on your monitor—that image is a raster file. A raster image looks like continuous tone but in reality has been broken down into a series of square colored pixels. Because of those pixels, raster images are resolution dependent. Which is a fancy way of saying that when you enlarge that image you will literally be stretching the pixels which results in a blurry image.
It should go without saying that raster images should be avoided for logos and text. And raster images should NEVER be enlarged.
Raster images are created by cameras, scanners or built in photo editing applications like Adobe Photoshop and typically carry the file extensions: jpeg, tiff, raw, psd, gif.
Vector images use mathematical calculations to represent lines, points and curves that we see on screen. Since the math doesn't change, vector image may be resized without any loss of image quality. For this reason vector is the best format for logos, illustrations, type and line art.
Vector images are generally created through drawing programs such as Adobe Illustrator (see the right example above). Common file extensions: ai, eps and pdf. (Vector pdfs offer the most flexibility and can be viewed easily on every platform.)
Raster images for use on screen are generally 72 pixels per inch (or sometimes 96). That means there are 72 pixels of data both vertically and horizontally within each inch.
However, to print properly, even on a desktop printer, a raster image, i.e. a photograph, should be a standard 300 pixels per inch at the final print size. If you want your photo to print at five inches wide it must be at least 1500 pixels wide, or five inches at 300 pixels per inch. While the same five inch photo on your monitor only needs to be 360 pixels wide or five inches at 72 pixels per inch. (In fact, you want that file to be small for monitor viewing so the image loads quickly!)
What happens if you don't have high enough resolution? Well, your image could be blurry or fuzzy. Alternately it can look pixelated or jagged (see the middle example above) with stair steps of color where colors and tones change. It might also have jpeg artifacts—weird mottled areas of inappropriate color.
Rasterized line art
A piece of type or line art (solid blacks/color and white) that has been scanned or rasterized presents unique issues. And mostly I know this because I'm a dinosaur and remember the days when we had to scan a logo to get it onto the computer! Line art—type, logos, etc.—that are rasterized should be a minimum of 600 pixels per inch at the final size.
How I work with images
When a client sends me images the first thing I do is open up every image. Because I live in a print world I first check format—will it open in Illustrator (vector) or will it open with Photoshop (raster). If it is a photographic raster image I adjust the resolution to 300 pixels per inch and see how that effects the image size. If it is a logo that is rasterized I set the resolution to 600 pixels per inch. If all of my lovely photos and logos are now 1/4-1/2 inch wide—we have a problem and I have to go back to the client. Luckily, most of the time the images are larger than 45 inches at 72 pixels per inch. Swapping that image to 300 pixels per inch results in an image just around 11 inches—which works perfectly for a full bleed 8.5 x 11 page.