Understanding Image Resolution
TechsPlace | Using an invalid image resolution is one of the most popular mistakes that graphic designers make when creating a print. The result is an inaccurate print or a rejection by the printer. If you do not keep an eye on image resolution from the start, you may have to re-create the whole file. So here is the article that will show you the care you must have to get your image right.
What is an image resolution?
The resolution of an image refers to the density of pixels (or printed points) that are part of that image or graphic. The higher the resolution, the greater the definition and detail of the image. An image with low resolution will be blurred and less detailed. But to improve the low-resolution image we can use photo enhancement software.
The resolution of an image is calculated in DPI (Dots Per Inch) and PPI (Pixels Per Inch). There are differences between these two: DPI refers to printed documents and amount and spacing between cyan, magenta, yellow and black dots, while PPI refers to pixels on the screen. They are not the same thing, but graphic designers and professionals tend to use the terms in turn. Often, for example, when talking about an image of 72 DPI that is on the screen, one is wanting to say “72 PPI”.
300 PPI vs 150 PPI vs 72 PPI
If you create a 10 by 10 inch document (let’s use this measure for easy calculation) with a resolution of 72 PPI and another document with the same size but with 300 PPI in Photoshop, you may notice that they have different sizes on the screen. This happens because of the different number of pixels used in each inch. In the 72 PPI file, you can only place 720 pixels across the document. At 300 PPI, there are 3000 pixels.
When we create images for the web, usually 72 PPI is enough. If we calculate the total resolution of a “relatively old” 15-inch monitor with a resolution of 1024 x 768 pixels, it will be the equivalent of 68 PPI. However, a 15-inch monitor of a 1440 x 900-pixel MacBook Pro will have a total resolution of 96 PPI. When the screen sizes and resolutions increase, we can also increase the resolution of our images, but for now, 72 PPI is acceptable and the most common.
Already when creating a document that will be printed, we will have to use a resolution of 300 PPI. If the graphics people say that their images or graphics need to be created with “a resolution of 300 DPI”, they actually mean 300 PPI (remember that DPI is for the document that is already printed, and PPI is for the which is still on the screen). If you create an image with 72 PPI, it will have to be redone because you can’t magically create the additional pixels out of nowhere. If you simply changed the resolution of the file from 72 to 300, Photoshop will guess the colors of the new pixels, but I do not even have to mention that the final product will not look good. This is called resampling.
You even can resize your image from 72 PPI to 300 PPI. Then Photoshop will divide the pixels so that there are 300 in each inch, instead of 72. However, Photoshop will eventually run out of pixels to split and the document will be reduced in size. This does not help much because the image that was 297 x 210mm in size will be 72 x 50mm, which may not help much when you are working with photos or posters that need to have a definite size. Sometimes they will ask you to create documents at 150 PPI. This is usually done in cases of large prints that will be seen only a certain distance, so closely the quality does not matter so much (billboards, for example). The lower resolution helps your computer process the information and results in a file with lesser weight.