Image resolution for devices is commonly expressed in one of two forms: PPI or DPI. Two terms that are both similar and different. Both relate to pixel density in different mediums, and although many use them interchangeably, there are differences between the two. Neither one has any direct relation to the content of an image, because regardless of how it is acquired, images are made of pixels, which are dimensionless. Both relate to how an image is either viewed on screen, or printed.
PPI stands for pixels-per-inch, and is the pixel density associated with digital devices typically used to view images. PPI only becomes useful when an image is brought into the real world, for example displaying it on a screen. It refers to the number of pixels that a device (e.g. screen) can display within one inch of space. For example the iMac Pro has 5120×2880 pixels in its display, and its pixel density is 218ppi. The 6.1″ iPhone 12 has a 2532×1170 display with 460ppi. The higher the value for PPI, the greater the pixel density, and the smaller the size of the pixels. Images shown on devices with higher dpi often appear sharper than those displayed on low resolution devices. However this is also dependent on your distance to the device, and the how good your eyes are. The examples in Fig.1 show examples of various pixel densities.
To illustrate how PPI of a device affects the display of an image, consider the example shown in Fig.2. A 280×280 image has been shown simulated on four devices with differing values of PPI: 210, 280, 140, and 70. Higher pixel densities makes the image look sharper – for example at 280ppi, the entire image fits into the 1″×1″ region on the screen. At 70ppi it takes 4″×4″=16in2 to display the same image, which means a much lower resolution where parts of the image may start to look “blocky” and edges appear jagged.
Different sized devices, can have the same resolution, but differing pixel densities. For example consider a series of televisions with 55″, 60″, and 65″ displays. The display resolution for each of these TV’s is the same, at 3840×2160 pixels. The 55″ TV will have 80ppi, the 60″ 73ppi, and the 65″ 68ppi. All have differing PPI because although the number of pixels is the same, the size of the pixels on each device is slightly different. To determine the size of an image on a particular screen, take the dimensions of the image and divide those values by the resolution of the screen. For example, an image that is 3000×2000 pixels in size shown on a device that has a resolution of 218ppi will display as 3000/218=13.76″ wide, and 2000/218=9.17″ high.
DPI implies dots-per-inch, and refers to the resolution value of a physical printing (or scanning) device. Printers reproduce an image by spraying out tiny dots, and the number of dots per inch affects the amount of detail and overall quality of a print. Figure 3 shows the 280×280 image printed out on three devices, one with 280dpi, one with 560dpi, and a third with 140dpi. The lower DPI results in a larger, if somewhat coarser, image. In all cases, the ratio of dot to pixel is 1:1, it’s just the size of the dots that changes. In the first case, 280dpi means that each dot is 1/280″ in size, and the image is printed in an area of 1″×1″. In the second case, the size of each dot is 1/560″, so the image printed in a space of ½”×½”. In the final case, the size of each dot is 1/140″, so the image printed in an area of 2″×2″.
The trick with DPI is how many dots can be packed into an inch? An inkjet printer can produce a resolution of between 720-2880dpi, whereas a laser printer produced images between 600-2400dpi. Theoretically the more dots, the crisper the image, but 300 DPI is already packing 90,000 dots on colour in a square inch, so how much better 1200dpi is, is likely debatable.
Sometimes, the metadata for an image will include the DPI, e.g. DPI=72. This makes no difference to the how an image is viewed on a screen, nor does it impact the size of the image file. Three different copies of the same file can have DPI values of 72, 150, and 300. All three will have the same number of pixels, and the same file size. Only when they are printed out will they appear as different sizes.
DPI and scanning
DPI also relates to scanning. The larger the DPI set on a scan, the more detail the image will contain, however there are limits with respect to the quality of the scanner, and the quality of the image. For example, a 5″×7″ photographed scanned at 150dpi will result in an image of size 750×1050 pixels in size, whereas if it were scanned at 300dpi, the resulting image would be 1500×2100 pixels.
What about LPI?
There is of course a third measure, LPI, or lines-per-inch, which is used in offset printing. This is used to measure resolution in images that are converted into a pattern of dots – halftones. Standard newspapers are printed at 85lpi, and offset-presses are 130-150lpi.
The 72dpi Myth
There is still a lot of talk about the magical 72dpi. This harkens back to the time decades ago when computer screens commonly had 72ppi (the Macintosh 128K had a 512×342 pixel display), as opposed to the denser screens we have now. This had to do with Apple’s attempt to match the size of the text on the screen to the size when it is printed, known as WYSIWYG (What-You-See-Is-What-You-Get). Apple basically matching the 128K to their dot matrix printer, the “ImageWriter”.
Dots and printers
There are different types of printers where dots are perceived a little differently. For example a colour inkjet printer can only create droplets of a few different colours for each dot, so small dots are combined to simulate continuous colour. Modern inkjet printers have high resolutions like 2400×1200, or 2880×1440. A dye-sublimation colour printer on the other hand produce continuous tone colour, meaning that every “dot” in an image can be an arbitrary colour (although there aren’t really any dots). Dye-sub printers normally have a resolution of between 300-500dpi. Many inkjet printers have resolutions like 2880×1440dpi, however this usually describes how many droplets of ink are placed in the 1in2 area, considering many are overlaid to create various colours.