What is PPI? How more PPI can improve your display?

PPI stands for Pixels Per Inch. It measures the display resolution, or pixel density, of a monitor or screen. The measure is also used to indicate the resolution of a digital image, as well as the resolution capacity of a camera or scanner capturing an image. A pixel is a single point of data in a digital image or on a monitor, and the PPI measurement indicates the number of pixels contained within the image or screen. Pixels per inch is expressed by indicating the number of pixels available horizontally by the number available vertically. A digital image containing a large number of pixels will hold more detailed visual information and be able to be reproduced in larger formats without pixelation, a form of image distortion in which individual pixels become visible to the naked eye. A monitor or screen with a high number of PPI will show a greater level of detail.

An image 200 pixels across and 200 pixels down will be expressed as a 200x200 PPI image. Although there are exceptions, most devices rely on square pixels in capturing and displaying images.

Similarly, Digital cameras will frequently express resolution in terms of megapixels. The megapixel measurement is determined by multiplying the horizontal PPI measure by the vertical. For instance, a camera which captures images at 1600x1200 would be a 1.9-megapixel camera.

The PPI of an image helps to determine the maximum size that can be printed without pixelation. For example, a 1.9-megapixel camera is capable to produce good quality 4x6-inch prints. If print larger than this size, the image will begin to look blurry or fuzzy.

In the early 2000s, the most common computer display resolution was 1024x768. But, PPI in computer monitors, televisions, scanners and cameras have improved rapidly in recent years. The higher this is, the better the pictures will look in terms of contrast and quality, in particular, the text will look more like the sharpness of text on a mobile or tablet. If your new 1920x1080 screen is the same physical size as the older 1366x768, then the number of Pixels per Inch will be higher on the new screen.

If you have an image that is 900 pixels wide, it will get smaller as you increase the PPI. At 100 pixels per inch, it would print at nine inches wide. At 300 pixels it would print at three inches wide. At 600 ppi you would get a printed image of only 1.5 inches wide.

Does pixel density really matter?

Yes. It does, and it matters quite a lot. When you go buy a new smartphone, or computer monitor, or TV, or any other type of device that has a display, you might be tempted to choose the one that has the largest diagonal size.

You do generally want higher PPI values for output when you can get them (up to a limit - if the resolution is higher than the output device can support, you'll actually lose quality), but that means printing larger image files at smaller physical sizes, or displaying the image on a screen with more tightly-packed pixels which amount to the same thing. You may need to calculate the appropriate number of pixels for output, which does involve PPI, but that is usually to match the output device's resolution, the desired print size, and the image file.

When you want to print, you'll get the best quality if the pixels in your image match the printer's pixels. If the printer prints at 400 ppi, and you want your image to print at 16x20", it ought to be a 6400x8000 pixel image, otherwise, the printer will have to scale it for you, and that doesn't always go well. The PPI value embedded in the file is used mostly in pre-print processing applications to determine the image size.

Not all devices that can be used to view photos have the same pixel density. Computer screens tend to range from 72ppi to 109 ppi in terms of screen pixel density. Relatively speaking, this is fairly low. Newer handheld devices have screen densities anywhere from 135 ppi to as high as 360 ppi. Such devices would be the likes of smartphones, tablets like the iPad, Nexis Android tablets, and forthcoming Windows 8 tablets. Printers can range from as little as 100 ppi to as high as 720ppi for inkjet printers.

Well, how do you determine the pixel density?

The better way to understand the value of PPI or pixel density is knowing to how large to export your processed photos for viewing on various devices. If you intend to produce images for viewing on a standard computer screen, you can generally assume around 72-100 ppi, and exporting at 4x6 to 5x7 size from a physical screen space standpoint will usually produce photos that are easiest for potential online viewers to observe. If you intend to produce images for viewing on portable hand-held devices, you should target a higher resolution, around 150 ppi would be better.

When it comes to print, knowing what PPI to print at can be extremely important, it can greatly affect how large you can print. If you intend to print your photos at 8x10, you are probably interested in printing at least 300 ppi or higher; even as high as 600 ppi in Canon and HP printers, or 720 ppi in Epson printers.

Most photos, excepting some form such as architectural photography, contain more detail than can be resolved in a 300ppi print at 8x10 for normal viewing distances, so printing at higher resolutions is often necessary to fully realize the detail you may have in a photograph. When printing at "native" print size or the size relatively close to the native image dimensions of your photographs which tends to fall in the range of 11x16" to 17x22", you will probably want to print around 300ppi. At normal viewing distances, this should capture all the fine detail in your photo well enough to be observed by most viewers, and will usually require little to no scaling of your image to print, means, you'll get the best results at the largest native size.

If you intend to enlarge your photos, you will probably want to print at a lower resolution. Average enlargements, which might be up to around 24x36" or so, can often be done at around 150 ppi, however, 200 ppi tends to generate slightly better results for average viewing distances in normal papers, and in many cases even canvas. For significant enlargements, up to 40x60, you will want to print at 100 ppi to 150 ppi, with the expectation that such photos will be viewed at a distance of at least several feet if not more.

To calculate the pixel density of your smartphone, start by taking its resolution. You can look at the resolution in either the horizontal or vertical directions. Once you have the resolution, measure the length of the display. This should be recorded in inches (2.54cm = 1 inch). By dividing the two numbers, you’ll end up with the “pixels per inch” (ppi).

For example, a display that measures 4.35x2.45 inches, its size is usually stated as 4.99-inches: this is the length along the diagonal. You can use Pythagoras' theorem to convert this to a horizontal & vertical length.

Does more PPI can improve your display?

Although higher PPI is always theoretically better, before making any purchase, you should also know that the eyes of most humans are probably not able to perceive any differences in a PPI that’s higher than a certain limit.

But, what that limit is when the human eye stops being able to see more details on a screen and the pixel density stops being important, is a matter of debate. For now, neither researchers nor ordinary people can give you a straight answer and say that “pixel density becomes meaningless after, say, a value of 570 PPI.” Generally, a higher pixel density is better than a lower one.