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.
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