Photographers are often heard confusing pixels per inch (ppi) with dots per inch (dpi). In broad terms the two units of measurement may appear very similar. They are used to indicate the resolution of an image or print, and in some situations can be wrongly used without causing too much confusion. However, it is worth understanding the difference between the two units and why they both exist. Scanners and cameras are measured in ppi whereas printers are measured using dpi.
Dots per inch is a unit used to specify the output of printers. It sounds simple when one thinks of the number of dots of ink placed on a print for every inch of paper, but the reality is somewhat more complicated. The dots-per-inch specification is a characteristic of a printer and its driver software. It is not a feature of a digital image. It would be incorrect to say that a digital image had a particular dpi, but it would be correct to say that a print of a digital image was produced at a particular number of dots per inch. In general, the quality of a printed image rises as the number of dots per inch used in its production increases. However, this is only true to a limited extent. Once the dpi reaches about 1,200, the human eye is incapable of perceiving further increases in the number of dots per inch. It is impossible to quote a definitive limiting figure for dpi because perception varies from person to person, in different light and with different images. Some people claim to be able to see the difference between a print produced at 1,200 dpi and the same print produced at 2,880 dpi, but most observers cannot do so.
Inkjet printers are typically capable of printing 300 - 1,200 dpi although figures of 1,440, 2,880 dpi or more are claimed for some higher-end machines. However, the quoted figures can be misleading. Some printers are capable of placing dots of different sizes, intensities and shapes on to a page. Some also optimize the placement of ink drops to improve the quality of prints, a feature usually known as "optimized dpi". Those that claim high dpi specifications may be layering dots by passing over the same part of a print several times to place dots of different colors in one place. This technique produces good results but generally consumes ink at a higher rate.
It is important to realize that dpi and ppi are really quite different things. In particular, it should be realized that it is necessary to use a printer capable of perhaps 1,200 dpi to make the best print of an 300 ppi image. This is because each pixel of the image must be represented in a print by a number of dots. A typical printer has six or eight colours of ink available, and can consequently print only dots of six or eight colours - one of which is black. White dots cannot be printed as there is no white ink. White is consequently represented by no ink - the background colour of the paper. If, for example, a pink dot is required in a print, the printer must employ a combination of the available ink colours to create it - there is no pink ink. Pink can be simulated by combining red with white in the correct proportions but, as we have seen, there is no white ink. Magenta and yellow ink dots might be combined to produce red, and then combined with white space in the correct proportions to create what a human observer perceives as a pink dot. This whole process must potentially be repeated for each image pixel.
Pixels per inch (ppi) is a unit used to measure the resolution of an image created by a camera or scanner. The number of pixels in an image is determined by the pixel dimensions of the sensor, but the captured digital image has no physical dimensions. It is merely a matrix of coloured squares (for simplicity), perhaps 3,000 wide and 2,500 high, giving a total of 3,000 x 2,500 = 7.5 megapixels. If we now imagine these pixels projected on to a blank screen through a zoom lens, we can see what happens to the image and the ppi figure as the lens is zoomed in and out to change the size of the projected image. As the imaginary lens is zoomed out, and the linear dimensions of the image on the screen increases, so the the available pixels are spread out over a larger area. The number of pixels per inch also decreases.
Pixels per inch is therefore a measure of the resolution of an image when represented at particular linear dimensions. As the image is enlarged, so the resolution must decrease. This relationship can be seen clearly in the Image Size menu in Photoshop provided the resample box is not selected.