Adobe Photoshop CS5 for Photographers - Martin Evening [132]
lpi: lines per inch
This is the number of halftone lines or ‘cells’ in an inch (also described as the screen ruling). The origins of this term go back way before the days of digital desktop publishing. To produce a halftone plate, the film exposure was made through a finely etched criss-cross screen of evenly spaced lines on a glass plate. When a continuous tone photographic image was exposed this way, dark areas formed heavy halftone dots and the light areas formed smaller dots, which when viewed from a normal distance gave the impression of a continuous tone image on the page. The line screen resolution (lpi) is therefore the frequency of halftone dots or cells per inch.
dpi: dots per inch
This refers to the resolution of an output device. For example, let's say we have an imagesetter device that is capable of printing small, solid black dots at a resolution of 2450 dots per inch and the printer wishes to use a screen ruling of 150 lines per inch. If you divide the dpi of 2450 by the lpi of 150, you get a figure of 16. Therefore, within a matrix of 16 × 16 printer dots, an imagesetter can generate individual halftone dots that vary in size on a scale from zero (no dot) to 255. It is this variation in halftone cell size (constructed of smaller dots) which gives the impression of tonal shading when viewed from a distance (see Figure 5.2).
Figure 5.2 Each halftone dot is rendered by a PostScript RIP from the pixel data and output to a device called an ‘imagesetter’. The halftone dot illustrated here is plotted using a 16 × 16 dot matrix. This matrix can therefore reproduce a total of 256 shades of gray. The dpi resolution of the imagesetter, divided by 16, will equal the line screen resolution. 2400 dpi divided by 16 = 150 lpi screen resolution.
Desktop printer resolution
In the case of desktop inkjet printers the term ‘dpi’ is used to describe the resolution of the printer head, and the dpi output of a typical inkjet can range from 360 to 2880 dpi. Although this is a correct usage of the term ‘dpi’, in this context it means something else yet again. Most inkjet printers lay down a scattered pattern of tiny dots of ink that accumulate to give the impression of different shades of tone, depending on either the number of dots, the varied size of the dots, or both. While a correlation can be made between the pixel size of an image and the ‘dpi’ setting for the printer, it is important to realize that the number of pixels per inch is not the same as the number of dots per inch created by the printer. When you send a Photoshop image to an inkjet printer, the pixel image data is processed by the print driver and converted into data that the printer uses to map the individual ink dots that make the printed image. The ‘dpi’ used by the printer simply refers to the fineness of the dots. Therefore a print resolution of 360 dpi can be used for speedy, low quality printing, while a dpi resolution of 2880 can be used to produce higher quality print outputs.
Choosing the right pixel resolution for print
In the past it has been suggested that the optimal pixel resolution for making an inkjet print output should ideally be the printer dpi divisible by a whole number, i.e. if you intended printing at 2880 dpi, the following pixel resolutions could be used: 144, 160, 180, 240, 288, 320, 360. More recently, it has been shown that there isn't any need to make the pixel resolution match any particular formula in relation to the dpi setting used on the printer. The ideal resolution for print is anything in the 180–480 pixel per inch range.
Megapixels to megabytes
If you multiply the ‘megapixel’ size by three you will get a rough idea of the megabyte size of the RGB image output. In other words, a 12 megapixel