Dots per inch (DPI) is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch (2.54 cm).
DPI is not really used to describe the resolution number of dots per inch in a digital print and the printing resolution of a hard copy print dot gain; the increase in the size of the halftone dots during printing. This is caused by the spreading of ink on the surface of the media.
Up to a point, printers with higher DPI produce clearer and more detailed output. A printer does not necessarily have a single DPI measurement; it is dependent on print mode, which is usually influenced by driver settings. The range of DPI supported by a printer is most dependent on the print head technology it uses. A dot matrix printer, for example, applies ink via tiny rods striking an ink ribbon, and has a relatively low resolution, typically in the range of 60 to 90 DPI. An inkjet printer sprays ink through tiny nozzles, and is typically capable of 300-600 DPI. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 1,800 DPI.
The DPI measurement of a printer often needs to be considerably higher than the pixels per inch (PPI) measurement of a video display in order to produce similar-quality output. This is due to the limited range of colours for each dot typically available on a printer. At each dot position, the simplest type of colour printer can either print no dot, or print a dot consisting of a fixed volume of ink in each of four colour channels (typically CMYK with cyan, magenta, yellow and black ink) or 24 = 16 colours on laser, wax and most inkjet printers.
Higher-end inkjet printers can offer 5, 6 or 7 ink colours giving 32, 64 or 128 possible tones per dot location. Contrast this to a standard sRGB monitor where each pixel produces 256 intensities of light in each of three channels (RGB).
While some colour printers can produce variable drop volumes at each dot position, and may use additional ink-colour channels, the number of colours is still typically less than on a monitor. Most printers must therefore produce additional colours through a halftone or dithering process. The exception to this rule is a dye-sublimation printer that utilizes a printing method more akin to pixels per inch.
The printing process could require a region of four to six dots (measured across each side) in order to faithfully reproduce the colour contained in a single pixel. An image that is 100 pixels wide may need to be 400 to 600 dots in width in the printed output; if a 100×100-pixel image is to be printed inside a one-inch square, the printer must be capable of 400 to 600 dots per inch in order to accurately reproduce the image.
PHOTO RESOLUTION:
People often ask me how big a digital image needs to be for printing. This is a very relative question, based on variables such a print quality and viewing distance. But I’ll try and answer it first on the basic level of resolution.
First off, I want to make it clear that we are not talking about file byte size. We cannot judge an image based off the file byte size, since most image files are often compressed. But we can judge an image based on the total number of pixels that make up an image, which can be determined by the height multiplied by the width of the image in pixels. These pixel dimensions are usually measured in megapixels.
So how do we measure height and width? When you hear people talking about photo “resolution” they often use terms like DPI (Dots Per Inch) and PPI (Pixel Per Inch). DPI is a physical measurement for photographs, because dots have an actual size. You can see the size of the dots if you look carefully at photo printed out, or an image generated on a computer screen. PPI is a virtual measurement used for digital images. This is because pixels are points in space we define mathematically, adding informational characteristics to such as color and contrast. They have no actual size or physical characteristics until converted to something real, such as a dot of ink or colored light on a computer screen.
Now that we have made this clear, it’s easy to understand why most people do not talk about digital images in terms of PPI. They usually only refer to pixel dimensions. This is because you can’t mathematically assume there will be a specific number of pixels in an inch. You simply can’t. They are relative points in space! The concept of “per inch” can really only apply to a physical entity, such as a dot. Thus DPI resolution.
At 300 DPI the eye at an average small photo viewing distance stops distinguishing between dots and creates a resulting shape. Computer monitors were found to work well at a minimum of 72 DPI, Newspaper text at 125 DPI, and more recently 600 DPI has become more of a photography printing standard. But for over a decade we have thought of 300 DPI to be a visual minimum for printing a standard photo.
DPI & PPI EXPLAINED
There seems to be a great deal of confusion among many people regarding the use of some terms in digital imaging. One of the more common sources of confusion is the difference between DPI and PPI. The main problem with this is that DPI (dots per inch) is an old term that has been applied to everything relating to resolution and the size of a digital image. This is very confusing because different situations work with resolution in very different ways, and having a single term for all of them just makes things more confusing. More recently, the term PPI (pixels per inch) has appeared in common usage and is far more specific for what the term entails. DPI is still used in some documents and software when PPI is really what they mean, but this is changing. This article is an attempt to explain what the 2 terms mean and how they should be used.
PPI
Let’s start with PPI, it’s easy to understand. This is the number of pixels per inch in your image. This will affect the print size of your photo and will affect the quality of the output. The way that it will affect the quality of the output is that if there are too few pixels per inch, then the pixels will be very large and you will get a very pixelated image (jagged edges, you will actually see individual pixels, not good). You’ll hear various different numbers thrown around as to what an acceptable PPI for a print-out is. A lot of this will depend on the size of the print. This is because you look at large prints from a further distance than a small print, so you can get away with a lower PPI and still have the image look fine.
All that PPI does is affect the print size of the image. There are 2 ways that you can change the print size, by resampling or by not resampling. Not resampling is what you normally want to do, this will only change the size of the print. Using resampling will actually change the number of pixels (and thus the file size) in order to match the print size. So for instance, if you don’t resample, changing the PPI setting will increase or decrease the print size (it will increase if you drop the PPI, it will decrease if you increase the PPI). With resampling, if you change the PPI, you will loose pixels (if you set the PPI to a lower value) or you will have pixels created (if you increase the PPI). Creating pixels is a bad idea, they get generated by the computer and the results aren’t usually that good. Throwing away pixels is fine as long as you won’t need the bigger size later (that’s why it’s usually a good idea to save the original large file).
An Example
Suppose you have a 100 x 100 pixel image, it could be printed at many different sizes. If you set the image to print at 10 PPI, then you’d have a 10″ x 10″ image. If you set the image to print at 100 PPI, you’d have a 1″ x 1″ image. Note that adjusting this value doesn’t effect the number of pixels in the image at all, it just changes how big the print will be.
Take our 100 x 100 pixel image again. Suppose it’s set at 100 PPI (producing the same 1″ x 1″ printed image). With re-sampling off, when you adjust the PPI the dimensions adjust as well, this is how things worked in the example above. With re-sampling on, the dimensions won’t change. So, if you changed the PPI to 10 with re-sampling on, you would still keep a 1″ x 1″ image and the computer would throw out pixels to make the image stay that size. So in this case, you’d end up with a 10 x 10 pixel image in the end. If you went the other way, and changed the PPI to 300, then the computer would generate pixels to make a 300 x 300 pixel image that’s still 1″ x 1″ when printed.
Usually, the only reason you want to use re-sampling is for reducing the size of your image. For example, my scanner produces 3888 x 2592 images. These images are too big to use online (both for display and because of file size). By using re-sampling, I can adjust the size of the images to something more appropriate for online use.
DPI
Now let’s talk about DPI. DPI only refers to the printer. Every pixel output is made up of different coloured inks (usually 4-6 colours, although many printers use more now). Because of the small number of colours, the printer needs to be able to mix these inks to make up all the colours of the image. So each pixel of the image is created by a series of tiny dots (you could think of them as sub-pixels). Generally, the higher the DPI, the better the tonality of the image, colours should look better and blends between colours should be smoother. You’ll also use more ink and the print job will be slower. You might want to try setting your printer to a lower DPI to save ink and speed up the job, see if you notice any difference in quality. The lowest setting where you don’t see any loss in quality should be the best one to use.
So a 1200 dpi printer uses 1200 dots of ink in every inch to make up the colours. If you were printing a 300 PPI image, then every pixel would be made up of 16 smaller ink dots (1200 DPI x 1200 DPI / 300 PPI x 300 PPI). A lower DPI would have fewer ink dots making up each pixel, which would make the colour look worse. A higher DPI would have more ink dots for each pixel and should give more accurate colour (especially under close examination).
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