By Raffi Darrow
November 2004
© R-Design, Inc.
I have often heard from people that they have tried to print a digital image that was on their PC or that they downloaded from the internet and it printed blurry. They want to know how to change settings on their printer or if the image needs to be printed professionally. A professional printer will probably tell you that their output of the image will be just as blurry. Why?
Images do not often print the way they look on a computer screen.
Images are made of dots of ink when printed or electronic pixels when viewed on-screen. Pixels are small, colored squares that together form a digital image, like tiles creating a mosaic. When there are enough pixels or enough dots of ink, and they are small enough so as not to be seen individually, a digital image can achieve photo quality. Increased magnification of a digital image will reveal the individual pixels. Rather than tiny dots that fool the eye into seeing one uninterrupted photo, you would see blurry squares. The photo becomes a mess. This effect is often referred to as pixilation.
Resolution refers to how many dots, or pixels, are in a given amount of space. The higher an image’s resolution, meaning the more dots there are in the same amount of space, the less pixilation will occur, and the better the image will appear. Another way to understand this is to say that the more information that’s packed into an image, the smoother its appearance.
The resolution of an image on your computer screen is often measured by ppi (pixels per inch). A printed image’s resolution is often measured by dpi (dots per inch). An image that is 72dpi contains less information than one that is 300dpi. Images seen on a computer screen are usually 72 ppi, because 72ppi is what most monitors display, but 300dpi is the “industry standard” for photo quality printing. This leads us to the two different levels of resolution that a designer uses; screen resolution and print resolution.
Screen Resolution: How you see an image on your computer screen depends on a couple of factors. The properties of your computer monitor need to be taken into consideration, as does the image’s quality. If an image has 16 million colors but you are viewing it on a monitor that can only display 16 colors, the image won’t appear as it should. Again, the common resolution for on-screen images is 72ppi. When a picture has more dots than the monitor can support, those dots are wasted. An image at 72ppi will look fine on your computer screen and its small file size will load quickly. A higher-resolution image (300dpi) won’t look better on your computer screen, and its large file size will make it take longer to load. Similarly, when a picture has fewer dots than the display device can support, the picture will not be as sharp.
Print resolution: This refers to how clearly an image will print. Laser printers, inkjet printers and imagesetters (used to output film for professional printing) require more information to produce a smooth and clear image than is available in a 72ppi file. Most commercial printers require an image to be at least 300 dpi or they can not guarantee the quality of the resulting printed piece.
There are two different ways to make the same digital image print at different sizes:
- Scaling – changing the physical size of the image for printing. An image has a fixed number of pixels. Imagine you have a 10-inch by 10-inch image at 72ppi, there are 720 pixels per line, and 5,184,000 pixels in the entire image. If you rescale the image to 2-inches by 2-inches, there are still 720 pixels per line, but the resolution would then be 360 ppi. The 720 pixels on each line are now only filling 2 inches of space, and therefore smaller. The image remains the same grid of 5,184,000 pixels with the same pixel data. Your image will print clearer, albeit smaller. Note that resolution and physical size are related to and directly affect one another. If you alter one, you alter the other.
- Resampling – adding or deleting pixels to change the resolution of a file. If you go back to our 10-inch by 10-inch image at 72ppi and attempt to resample the resolution to 360 ppi, the image would now have 12,960,000 pixels instead of its previous 5,184,000 pixels. The computer uses a mathematical formula, examining the current pixels to try to determine the color and location of the new pixels. The new pixels are not true to the image. They did not naturally exist in the image, and the printed result is often blurry or distorted.
Before sending any marketing materials to print, be sure to check the resolution of your image. A file created entirely in a word processing program instead of graphic editing software will most likely yield a 72 ppi image, and therefore a blurry print – making a poor impression on your customer.
More information on resolution and file saving can be found on our blog.