The use of some terms in digital imaging has become a matter of great confusion for many people. It is really a cumbersome task for a layman to differentiate between Dots per Inch (DPI) and Pixels per Image (PPI). Well, both DPI and PPI are the measure of resolution for both onscreen and print images. It is pertinent to note that DPI is relatively an old term. People have been using this very term to relate everything about the digital images and their size and resolution before the appearance of PPI in recent years. Currently, PPI is most commonly used to manage resolution and size of digital images yet DPI is still used for some software and documents.
The following lines explain what Dots per Inch (DPI) means and how it affects the resolution of your image.
The DPI specifically refers to the printers. In some words, it is only used in printers nowadays. Everyone should be aware that all the printers consist of different colored inks which combine to create every single pixel output. There are usually 4-6 differently colored inks in a printer. The printer works on the principle of mixing all these colors to create an image.
The printer has to mix the inks because of a small number of colors available to it. It has to create an image using only these 4-6 colors. Actually, a series of tiny dots create each pixel of an image. You can consider these tiny dots as sub pixels. It is important to note that the tonality of the printer will be higher if there are more dots per inch in an image. That is higher the DPI, better will be the quality of the image.
Colors in the image will also look better if the DPI is higher. Most importantly, the colors will blend with each other naturally and smoothly. There is also a reverse side of using high DPI. That is the printing job will be slower and the printer will use more ink. In order to overcome this problem, you must try to lower the DPI setting without distorting the image. The lowest DPI setting which does not reduce the image quality is the one you should be using.
Against common perception, DPI has nothing to do with the video quality. Almost all the modern video formats, as well as rasterized images, store their data solely as pixels. The do not depend upon DPI or real world measurements such as inches for their quality. On the other hand, monitors also display video only in units of pixels. If you are displaying a video or rasterized image, the monitor maps them pixels by pixels instead of dots per inch.
You may have to use DPI in videos and rasterized images when you want to map real pixels into real word measurements. For example, 20 points are roughly 0.28 inches. Therefore, a 20 point Font will use dots per image settings to determine how many pixels it should use.
You might be surprised to learn that most pros recommend using 300-400 DPI resolution for digital images. This raises a very important question. Why not use higher 1000 or 2000 DPI for an image or for that matter, lower 120 DPI? Is there a threshold for lower and higher image resolution beyond which you should not venture.
It is also pertinent to note that an image with 72 DPI might look amazing on your computer screen. However, you may be shocked to test the image on your 720 HDTV. Although, 400 DPI is always better than 120 DPI, the actual image resolution you want to use depends upon the type of image and your requirements.
For instance, most old photo labs would develop your photos using only 300 DPI physically. As a matter of fact, 300-400 DPI is still good enough for most of the photos. For example, if you scan an old photo at 300 DPI, you are very likely to get an exact copy of your photo. On the contrary, if you scan it using 600, 900 or higher DPI, it will distort your photo instead to provide you with more details. 300 DPI is the limit for such photos and you should not go higher or lower.
Again, the quality of the image depends upon the device on which you are viewing your image. You might totally distort your image if you want to get the bigger print of your 300 DPI photo. This is where you must scan your image at higher DPI. It will help you retain the image quality without stretching the image.
The DPI can also have an effect on the physical size of the image. You might have to increase or decrease the DPI in order to set the image to the default size of the printer you are using to print the image. For example, the DPI of the photo should be 400 if the default print dimension is 5 inch and you want to set the image width to 2000 pixels. The formula is simply. You will divide 2000 with 5 to get 400 DPI.
You should try not to resample or resize your image while changing its DPI. It is advisable to retain the original pixel dimensions while changing your DPI. These pixels normally determine the real digital resolution of an image. This way, you will be able to retain the size and resolution of the original image without distorting its quality. The only thing you will be changing is its inner dots per inch (DPI) dimensions.
There are many programs such as Photoshop and Illustrator which help you change the DPI without disturbing the physical size of the image. The only thing you need to keep in mind is that you should never resample or resize your image while changing the DPI. Your new photo will be the carbon copy of the original as long as its pixels remain unchanged.