I have seen a question asked in a number of forms (and forums). The most common variant seems to be along the lines of, “Why does my super expensive uber-camera only record 72 DPI?”{{1}}

This can get quite complicated, but I will try to keep it simple. Before we can even start, though, we need some definitions:

DPI or Dots Per Inch is an often misused printing term.{{2}} For ink jets printers it refers to the number of distinct drops of a specific colour of ink placed on the paper.  As a simple example imagine a 3 colour printer that has 300 holes in an inch wide print head, 100 for the cyan ink (C), 100 for the magenta (M), and 100 for the yellow (Y). This printer has 300 dots per inch{{3}}. But remember that each of these dots can only be one colour (we will ignore black and other colour inks used in higher end printers, this is a cheap make-believe printer just to demonstrate a point). Additional colours can be made by mixing different numbers of dots of these inks, along with the white of the paper, in a very small area (look at a print under a magnifying glass sometime). The way a printer does this is to place a number of dots of each required colour (CMY) within a matrix{{4}}.

[gn_spoiler title=”click here to see some simple 4 x 4 dot pixels” open=”0″ style=”2″]

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Now, back to our original 300 dpi printer. Imagine that this print head used a 100 dot matrix to make each pixel. It takes 100 dots to make up one colour. It may take 27 cyan dots, 56 magenta dots, and 7 yellow dots while leaving the rest of the matrix white to make up the required colour . Each colour matrix represents ONE pixel. Therefore, this theoretical print head can print three (3) PIXELS per inch (PPI).

This is why some printers say they have 1200 dpi or more. This is NOT the same as 1200 PPI. The number of pixels, and the actual resolution has nothing (ok, very little) to do with the DPI. A higher dpi may mean a higher ppi, it may  mean a larger range of printable colours, but it could also mean absolutely nothing!

So now we know what printed dots are, but what about resolution? Resolution is the number of pixels in an image, usually measured along the horizontal and vertical axes. 640 pixels wide by 480 pixels tall as an example{{5}}.

So where does ppi come in? Let’s say we want to print an 8 inch by 10 inch picture. We know from experience that we need about 300 ppi to make a photo realistic image{{6}}. We multiply 8 inches times 300 ppi for 2400  pixels and 10 times 300 for 3000 pixels. We need an image of 2400 x 3000 pixels{{7}}.

A lot of the confusion comes when printing programs or graphics programs use dpi when they mean ppi. People also get confused when they are re-sizing images. If you don’t change the number of pixels in an image (which is what affects the file size) you can specify any dpi you want. An example I used in a forum was, ” Which has more resolution, a 10 ppi image, or a 1000 ppi image?” The question is unanswerable unless I also tell you either print size, in which case you can calculate pixel dimensions, or pixel dimensions directly.

So keep an eye on pixel dimensions, and make sure your software isn’t changing the pixel count when all you want to do is print your image.

Note: If you have any questions or comments, please contact me so I can incorporate them in future articles.

[[1]]The 72 number may vary. This post was inspired by someone asking about 200 DPI vs 320 DPI. Another common question is, “What dpi should I print at?” hence the title of the article[[1]]

[[2]]It is also been used to  describe the number of each red, green or blue elements on a monitor. In common usage, dpi and ppi (pixels per inch) are often used interchangeably, which leads to much of the confusion.[[2]]

[[3]]This printer doesn’t exist of course, and most print heads have fewer holes than their DPI rating. This is accomplished by the head moving across the print surface and “firing” multiple times per inch during that pass.[[3]]

[[4]]One of the differences between a good photo printer and a regular colour printer is the way these dots are arranged. Some laser printers put down a very regular and rectangular grid. This can be visibly distracting in a photograph.[[4]]

[[5]]For some reason Great Britain (not sure about the rest of Europe) measures images opposite to North America. What we call 8×10 they call 10×8. I guess they think we also drive on the wrong side of the road…[[5]]

[[6]]The ppi required (often also called resolution – see where all the confusion comes in?) is partly dependant on the printer you are using, and partly on the viewing distance. For a regular print you are going to be looking closely at, use around 300 ppi. I may write on this topic in more depth at a later date…. or maybe not….[[6]]

[[7]]as an aside, note that this is a 7.2 megapixel image. If you are willing to go to 200 ppi, which a lot of people find more than acceptable, you can get away with a 3.2 megapixel camera. Yes you can make decent prints with your cell phone, as long as it has good quality pixels (another topic, and rare in a cell phone) and a good lens (very rare).[[7]]