Image Resolution and Press Printing

The first lesson I learned as a budding pre-press technician was that you can not pull images from the web and have them print as well as they look on screen. The reason is that the optimum resolution for posting images on a screen is much lower than the optimum resolution for print images. Below is a definition of Resolution posted on About.com by Sue Chastain:

Resolution is a measurement of the output quality of an image, usually in terms of samples, pixels, dots, or lines per inch. The terminology varies according to the intended output device. PPI (pixels per inch) refers to screen resolution, DPI (dots per inch) refers to print resolution, SPI (samples per inch) refers to scanning resolution, and LPI (lines per inch) refers to halftone resolution.

http://graphicssoft.about.com/od/glossary/g/resolution.htm

The reason you cannot use web images on a printed piece is that the optimum resolution for web images is 72dpi and the optimum resolution for print is 300dpi. The lower the resolution the worse the images look in print.

Ok now that you know that 300dpi is essential to print your images well on a press, how can we figure out the dpi for an image? Well most designers have access to photo editing software that will tell them exactly what the DPI is. If you don’t have software to figure this out, there are a couple of simple math equations that can help you out with this.

Pixels/Inches= DPI

Basically if you know how many pixels wide and tall your images is, as well as the measurement in inches you can figure out the DPI.


Below is an example

To find out the image DPI just take the pixel width : 1500 and divide it by the Inch width: 5.


1500
   5    = 300


The result is 300, which is your width DPI. The width and height DPI is always the same so if you make the calculation for only one dimension you don’t have to make the calculation for the other dimension.

You can use the same formula if you know the pixel size and the DPI, to figure out what size in inches the image actually is. Just switch the equation around as below:

Pixels/DPI= Inches

This is a handy tool when you are trying to print the image to a printer.

How does all of this really effect printing? The more pixels you have in the width as compared to the width in inches the higher the resolution. For example if you have an image with 1500 pixels in the width but the inch width is 15, then if you divide 1500 (pixel width) by 15 (inch width) the result is 100 DPI which is too low for printing.

Can you resize an image to have a higher DPI? Yes and No. You can only re-size an image to have a lower DPI which is why you can not use images from the web in a print design. However some software does allow you to increase the DPI (I really wish they would eliminate that option in the software!), but if you start with a low resolution to begin with, the fuzziness of the original photo never goes away. See the image below to find out why. I have zoomed in on a portion of the image to show you what the image looks like at print resolution and how it will print:

A simple way of thinking about this is using cameras. Everyone has probably tried to purchase a digital camera in the past few years and has heard the term megapixels per inch. The higher you go in megapixels per inch with a digital camera the clearer the image is. The same goes for DPI and images for printing. The higher the DPI the clearer the image will print.

The Bolder the Better

I can remember the moment I realized that there was no such thing as white ink in offset printing. It was a flash of embarrassment as I learned the truth. Such a simple idea, so obvious and yet the concept never crossed my mind. How embarrassing to have a printer explaining the concept to me as a young graphic designer. I don’t know why it never occurred to me that all white type was actually a color printed with the type “knocked out” or blank to show the white paper through the colored ink. Knocked out type can be an effective design element but can also go very wrong if you don’t follow a few rules.

Rule #1: Try not to use knock out type when the color or image you are knocking the type out of is built from 2 or more ink colors. It seems ridiculous that you can use knock out type in some instances but not in others but I hope the image and explanation below will show you why:

Image #1                          Image #2                          Image #3

In Image 1 type is knocked out of a single ink color: 100% black ink. This design is very easy for a press operator to keep looking crisp and professional since it shows up on only 1 plate. Offset printing combines 4 different colors to create the color spectrum you view on full color printed pieces.

Image 2 shows type knocked out of 2 colors: 100% Yellow and 100% Cyan ink. Image 3 is zoomed in on one piece of image 2 and shows what the final printed piece might look like. The fuzziness of the type is due to the press the piece was printed on being “out of registration”. Out of registration means the Cyan Plate is not positioned in the exact same spot as the Yellow plate on the press before printing the pieces. In press printing the plates are hung on a press and separated by sometimes over 12”. The sheet of paper runs over the first plate and picks up the proper amount of cyan ink that the cyan plate is holding, then runs over the second plate and picks up the proper amount of yellow ink that the yellow plate is holding. If either plate is off by millimeters, image 3 is the result. It makes the white type look fuzzy with a yellow and blue shadow. This will not happen with every printer you send your printing to, but if you design with Rule #1 in mind, your pieces will print crisp and clean every time no matter what kind of press your printer is working with.

Rule #2: You should never knock out type that is smaller than 8pt because it becomes very difficult to read. Paper is a naturally absorbent and fibrous material. Knocked out type at 8pt or under in size tends to fill in when printed due to ink spreading and filling the gaps between fibers in the paper. If you choose an un-coated stock (which is very popular in print graphic design these days) the ink will spread more on the sheet due to the extra absorbent nature of the paper than if you choose a coated stock which is treat with an extra coating to help minimize ink spreading.

Rule #3: The Bolder the Better. You can see this in Image 1. The thinner the font chosen, the more difficult it is to read. Even though you might be able to read the font just fine on screen that doesn’t mean that it will print as clear as you see it. This rule comes back to the ink spreading issue discussed in Rule #2. Even if you picked a font larger than 8pt, if it is too thin, it too will fill in due to ink spread.

Keeping these tips in mind when designing for print will keep your finished product looking crisp, consistent and professional.