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DPI, you hear the term bandied about everywhere from cellphone screens, digital cameras and of course printers. In the simplest terms dpi stands for Dots Per Inch and it used to describe how detailed an image is. In printing terms dpi directly affects how good your documents are going to look. I am going to teach you what it means and how to ensure you get the best dpi for your printing.
Once upon a time, everything was printed using carved blocks of material coated in ink and pressed into the page and the quality of the image was dependent on how good the carver was. These days everything is printed by the same sort of technology you find in your home printers. The printer lays images on the page line by line, each line made up of a series of dots. The finer/smaller the dots in this line, the smoother the images are on the page. They measure the fineness of dots by seeing how many of them fit into an inch, this being the dpi.
The human eye has a natural dpi limit, about 300dpi, where it’s impossible for it to distinguish the dots make up images on a page. Now most printers print at much higher than that, our primary printer at Copy Express is a 1200dpi machine for instance. While human eyes can only see 300dpi, they can see about more than 2 million colours (and have named about 100,000). Most printers only use 4 colours so they use a mixture of the finer dots to give us a better colour range because our eyes blend those dots to make the new colours.
Now how does dpi affect your document quality?
The biggest offenders we find here at Copy Express are internet bitmap graphics that have been scaled up. The dpi standard for web graphics is 72 or 96dpi or about a 1/4 of print standard. We get a lot of documents with internet graphics in them which are fine for viewing on a computer screen, but on a printed page they become extremely pixelated. If you have to use internet graphics, keep them small so you can get away with it. Read more about this in our entry in about Resolution.
Digital camera and cell phone images can also be a source of DPI issues. These devices allow you to set the image size and quality, losing details and size to give you more images in the device’s memory. Now if the images are to be uploaded to facebook or seen on a computer screen you will have no problems. For use in print you need to keep the camera at much larger image size or you will have the same blockiness that you get with web graphics. A good rule of thumb is for a 6×4” (15x10cm) image it has to be 2 megapixel image, for an A5 size it jumps to 4.5, A4 is 10mp. If you can set image quality when taking the photos, always set it to best quality because the other settings will remove the fine detail that you will notice are missing when you print the image.
Another issue with DPI is when we have to scale documents to a larger size. When printers enlarge an image they don’t actually look at the source in finer detail, they spread out what they have to fill the area and make guesses how fill the spaces between each dot. Now scaling to the next size works okay (i.e. A4 to an A3) as it only has to guess one dot out of every three. Going bigger than will add more chances for errors to occur, from A5 to A3 is a 200% enlargement so the printer has to make a guess about every second dot it prints. No matter how good the printer, the final print quality always drops the more you scale, so if you can you will always get the best results designing your material to desired size than trying to make it larger when you print it.
Finally a lot of programs give you the option to compress the images in the documents you create. This can either be in the properties you assign to each image in the document or when you export the finished version as a PDF. There is always the option for ‘screen use’ or ‘small size’ which is for documents that are for only ever to be viewed on the computer screen. Images are reduced to internet graphics dpi so are a poor choice for print. If there is a ‘commercial print’ option, choose that as it will give you a much better print finish, because the program will store all images in their original size and dpi ensuring the best print quality that they can give you.
There we go, a basic introduction to DPI, what can go wrong and how to avoid it in your next print job with us at Copy Express.