Pixel mysteries

Pixels, point, dots per inch, high resolution, low resolution... help! Every industry has its jargon, its specifications and its units of measurement. A pilot understands Level 290 to mean 29,000 feet. 30 gusting 60 means that winds are a steady 30 knots and gusts have been registered at 60 knots. What's a knot?

The same applies to the printing, design, video and computing industries. We group them because they share a common set of units. We will not cover them all, considering some are hyper-specialised and others have simply fallen out of use. You rarely come across picas anymore. We will highlight those that are most confusing. We will also use non-technical terminology that makes its meaning simpler to grasp. Our apologies to the techies who are looking for the precise definition.

Pixel - A pixel is used when describing either a computer screen or a digital imaging sensor like the one found in a digital video or still camera or even in a portable telephone camera. A pixel can be described as a single image point. You put enough points together and you have an image. Pixels are always laid out in rows and columns. The result is a grid of pixels. To either record a proper image or to display it on a screen requires many pixels, usually in the millions. There are two ways of describing how many pixels there are - also known as pixel count. You can either describe how many horizontal and vertical pixels there are or give the total. The computer screen on which this article is being typed is 1280 pixels across by 800 pixels vertically. This is usually written out as 1280 x 800 pixels. The same screen can be described as having 1,024,000 pixels. A PAL television video image has 720 x 576 pixels for a total of 414,720 pixels. Modern high-definition still cameras are now approaching 15,000,000 pixels and will soon go well beyond.

Dots per inch (DPI) - This one is probably the most important one to know. It is used by designers and printers. It is relevant to images, PDF files or plates. Dots per inch (dots per centimetre is rarely used, even in Europe or other metric countries) means how many dots there are on one side of a square filled with dots. Each one of those dots can be of different diameter or even of a different colour. Together, as with pixels, they make up an image. The most common quantity is 300 dpi. We will understand in a moment when we talk about lines per inch (as though we needed more confusion). Back to dots per inch. For years, laser printers used 300 dpi. Now they use 600 and even 1200. That's fine for laser printing. The more points there are, the smoother the image will look. However, when it comes to professional printing, you rarely need more than 300 dpi; 400 is the absolute maximum. There is simply no need for more. Why is this? Shouldn't more points mean, as with laser printing, smoother images.

Offset printing is the most common printing technique. It consists of a series of dots, arranged in lines that together make up the image. There is a limit to how many lines you can print. Too many lines per inch would force you to have very thin lines and ultimately too thin to be used on paper. Ultimately, lines would bleed into one another. The practical upper limit is 150 lines per inch. Most magazines use 133 lines per inch. Newspapers use less. They can go as low as 100 lines per inch. And billboards that are printed with offset technology go as far down as 70 lines per inch.

There is a direct relation between lines per inch and dots per inch. You only need twice as many dots as you have lines. We could go into great length explaining why, suffice it to accept that you never need more than twice as many dots per line. You can do it if you want, it will make no difference. It simply cannot. As a result, if your paper is excellent, your printing plates are too, and your ink is top quality, you can probably print as high as 150 lines per inch - therefore 300 dots per inch. That's already quite high. An A4 image at 300 dpi can easily weigh 30 Mb. If you have a thick document, it can quickly roll into the gigabytes.

Another commonly used figure is points. It is mostly used in typographic fonts. 72 points equal one inch. That is the origin. Today, though most people do not know exactly what they mean, they use them knowing that a 10 to 12-point text is commonly used for paragraph text with 18 to 24-point text used for headings. Big posters will obviously use much larger sizes. A good rule to remember is that anything below 10-points is not easily read by older readers or those with slightly less than perfect vision.

An important figure in printing is g/m2. Paper is not defined by its thickness, as many think, but by its weight per square meter (in some imperial measure countries, pounds per square foot are still used, but less and less so). Often, the m2 unit is omitted and just grammes (g) is used. A good reference is photocopy paper, it is 80 g. Inside paper that feels a little thick is usually in the 130 to 150 g range. A good paperback cover is usually 225 to 250 g. Anything thicker is usually considered board, not paper. The trend is towards thinner, lighter papers. Lighter the paper requires less raw materials and therefore less energy. There are currently 60 g papers with the same opacity and strength as 80 g traditional papers.

Pixels, points, dpi, lpi, g/m2... next issue we'll look at bytes, milli amperes per hour and other often confusing units.