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Print Resolution Explained

Digital Scaling & Resolution


Resolution is a measurement of the output quality of an image, usually in terms of pixels, dots, or lines per inch. A digital image is made up of many small squares of color called pixels (short for picture element). The number of pixels across the width and height of the image is called the resolution. An image with 640 pixels across the width and 480 pixels high has a resolution of 640×480. The more pixels in the image file, the higher the resolution.

When it comes time to print, you need to specify what resolution you want to print the image. There are, of course, standards for this. At Pixus, we use the most technically advanced equipment available today to produce vibrant graphics from your digital files. In order to produce a “photo quality” print, we must print at 100 ppi (pixels per inch); have at least 100 pixels for every inch of the print. Smaller resolution files can be printed, but the results will be less than optimum.

So how do you determine if the size and ppi of your photo are adequate to produce a large graphic?

Resizing Image Box

Let’s say you are designing a 22″x28″ poster. If you want your image to be 22″ wide, then purchase, scan, or take your photo with this dimension in mind. Plan ahead and have your image at the right resolution and size.

To determine what resolution to use before you take or purchase the photo, consider your final output. Pixus needs 100 dpi to get a photo quality print. The photograph that you would like to use is 5.5″x7″. You will need to enlarge your photo 400% to be large enough for your background. To check the resolution, open the original photo in Photoshop or other photo program. Different programs use different approaches to resolution, so you may need to find your users manual for specific instructions. In PhotoShop, go to Image>file size (figure 1). This will display the size and the resolution/pixels per inch. If your photo is 400 ppi or larger, it is ready to be enlarged to be used as your background. If the photo is less than 400 ppi, you will need to rescan the original photo or retake the digital photo to use at this large size.

Here’s an example: Original photo: 5.5″x7″@ 72 ppi, required photo size: 22″ x28″ @100 ppi.

5.5″x7″(original photo) x 4 = 22″x28″(poster size) This means our photo should be enlarged 400%. We also know that we should have 400 ppi to allow us to enlarge the photo without losing resolution, but our original has only 72 ppi. From these calculations, we know that we will need a higher resolution photo to use as a background.

TIP: Once artwork becomes electronic, it CANNOT be enlarged without loss of quality. For BEST quality images for use in print, begin with the high resolution photographs when the project is designed. To save time and memory, you can always resize the image when the project is finalized.

Resolution: sample images

As you look at these examples, notice that as the ppi is decreased, the quality of the photo decreases. Examine the effect on the image edges and overall clarity.

Image Degredation through image enlargement

Vector vs. Raster

Have you ever wondered about the differences in bitmaps and vector graphics? Bitmaps (also known as raster images) are pixels in a grid. Each pixel contains tiny dots of color which, when combined, make up the pictures you see on-screen. Vector graphics are composed of scalable objects, such as lines and curves. These are defined mathematically, so no matter how much they are scaled, a high-quality result is assured.

Vector graphics usually are easily modified within the creating application and generally are not affected by scaling (enlarging or reducing their size). Vector graphics are commonly created in Adobe Illustrator, and converting vector graphics to raster images is quite simple.

Raster images are produced by digital image capture devices: digital scanners or digital cameras, or by pixel editing programs like Adobe Photoshop. Raster images are composed of a grid of digital picture elements (pixels). Pixels are squares or rectangles described as black, white, gray or color. Converting a raster file to vector is difficult, and often is not possible. As well, raster graphics are impacted by scaling.