BetterPhoto Q&A
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Printing Image DPI

Is there a good place that explains why you only need to print the image at 250 - 300 dpi? Why a range of 250 to 300? And why do many printing houses tell me the art only needs to be 150 dpi for whatever size the art is being printed?

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October 01, 1999

However, in most printers, each dot is either on or off; there are no intermediate gradations. In order to give the "illusion" of different shades, printers usually use a technique called "dithering" (which comes in a number of flavours). The main idea is that, if you print certain patterns of very small on/off dots, the eye sees them as a single area of a particular shade. Various different kinds of patterns may be used, each of which is best for different types of images (you've probably seen options like "error diffusion" etc. in your printer driver). Inkjet printers use such a printing technique (each colour head either "spits" a droplet of ink or it doesn't).

Of course, there are certain printers that can produce dots with truly varying shades of colour, most notably the so-called "dye sublimation" printers. These use solid inks and transfer them onto the paper by applying varying degrees of heat. Heat vapourizes a portion of the (solid) ink (hence the "sublimation" part), which then is then "impressed" on the paper. Using this technique, dye sub printers can achieve even 256 gradations per colour. As a side note, most professional photo printers (eg. models from Kodak, which cost around $2K) are relatively hi-res dye sub printers...

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March 07, 2000