Pixels Per inch (PPI) refers to the number of pixels contained in a linear inch of a digital image file. Dots Per Inch (DPI) Refers to how many dots of ink are dropped down in the printing of a file.
Often the two terminologies are interchanged; in conversation around file creation PPI is still called DPI. However as you will notice in Photoshop the wording used is Pixels Per Inch, and has been for a very long time.
A pixel is the tiny little square of colour forming part of a multifaceted image full of many tiny little squares. Each square is a pixel and pixels are bitmap images. Common bitmap/pixel image file types are JPEG, TIFF and PNG.
Let’s look at an example:
- Image one is 150 PPI and printed at 750 DPI
- Image two is 300 PPI and printed at 750 DPI
So you can see that the physical world of ink being dropped from the printer is not reliant on the amount of pixels you have in an image.
The amount of pixels you have in an image pertain to the individual quality of that image and will dictate its use around being scaled up and maintaining a high quality. We have all heard of pixilation, it occurs when a small image or image with a low PPI (such as web images which are usually 72 PPI) are stretched to print bigger. Each pixel is being stretched causing the image to expand at low quality.
Take Away
- Refer to file/ document set-up resolution as PPI
- Always check the PPI and size of an image before beginning work with it so you know the sort of outcome and scale that can be achieved
- DPI refers to the world of inks and printers.