DPI ...[Photo]

Image resolution and DPI explained

Image resolution dictates the printed output dimensions and quality of your image. Really printed output dimensions are dictated by DPI (Dots Per Inch), or sometimes referred to as PPI (pixels per inch) . This refers to how many pixels get printed in an inch. So, if you have a 600 pixel x 600 pixel image at 300DPI, it will output at 2 inches square. If you change this images DPI to 150, this will mean it will output at 4 inches square. So, changing the DPI of an image changes it output size.

However, lowering the DPI too low produces progressively lower output quality. 300DPI produces high quality output on any medium, but produces large files. For A3 images I would work to 300-200DPI. For larger items I would use 150dpi, and for really large items like exhibition panels I would use as low as 75-100dpi.
For large format digital prints you can actually get away with amazingly low DPI and still get extremely good quality prints (RIP, printer and operator allowing!), 100DPI is no problem for this kind of print.

Good quality Lytho (or similar) commercial printing really requires you to have your images in the 300-200DPI range.

For home inkjet or laser prints and even digital laser prints you can actually get away with images as low as 150DPI, dont think that because your inkjet printer has an output DPI of 2800 you need your images to be set to this resolution, this just refers to the printed ink density on the page!

On the flip side of this, if we where to take the same image, and scale it by 50%, we would end up with an actual output DPI of 600. You may think this is better – it is not! All this actually does is slow down the printing of the document (the RIP has to resize your image), and produce larger jobs to send out to print and archive – this really does you no favours!

If you are enlarging an image in your DTP application – again, go to Photoshop and resize the image for all the same reasons above, as well as Photoshops image resize algorithms are fantastic! You also will see how your image quality is degraded and have the ability to combat this in Photoshop.

This information only refers to printed material, image dimensions for web and multimedia usage are purely defined by pixel dimensions and they are traditionally set to 72dpi, although I usually prepare my images at 150DPI if im ever unfortunate enough to have to prepare images for Powerpoint – they will then print fine on an office laser or inkjet printer.