If you’ve downloaded this tutorial, it’s probably because at some point you’ve become confused by the terms DPI and PPI, and the way they are often used interchangeably. We’ll explain what each term means, and describe why they are not interchangeable.
While both pixels and bytes refer to the sizes of digital objects, they measure completely different things. MegaPixels measure the size of digital images, sensors and displays. MegaBytes measure the size of digital files (such as photos, documents, etc).
Digital images are made up of thousands of tiny, block-like picture elements (pixels). Similarly, computer monitors can display pictures because the screen is divided into millions of pixels arranged in rows and columns. Each pixel is assigned a tonal value (black, white, shades of gray, or color).
A pixel is the smallest unit of an image that can be controlled, as well as being the smallest screen element.
A pixel does not have a fixed size, because its size depends on the dimensions of the image. With two files of the same dimensions, a file with a resolution of 150 pixels-per-inch will have larger pixels than a file of 300 pixels-per-inch.
The greater the number of pixels per inch (PPI), the clearer the image or resolution will be. Greater numbers of pixels-per-inch allow for more refinement of the image, which results in higher, truer image replication.
Pixels Determine Image Resolution
The term "resolution" refers to the amount of information a digital or printed image file contains, typically measured in pixels. Generally speaking, the higher an image's resolution (i.e. the more pixels it contains), the more detailed and sharp it will appear when printed.
Image resolution is determined by multiplying the number of horizontal pixels by the number of vertical pixels in an image. Thus, an image that is 3456 x 2304 pixels is said to have a resolution of 8 megapixels (7,962,624 pixels).
The larger an image is (i.e. the more pixels it contains), the more space it takes up on your hard-drive or memory card. It will also show up larger on your monitor, and when you print it out. For example, a 10-megapixel image contains more horizontal and vertical pixels than a 6-megapixel image, so it will take up more space when viewed at its full size on your monitor, and will print out larger.
A megapixel refers to one million pixels, and is commonly used in reference to digital cameras as an indication of resolution capability.
For digital cameras, the term megapixel expresses the number of image sensor elements of a digital camera. For example, a camera with an array of 2048×1536 sensor elements is commonly said to be a "3.1 megapixels camera" (2048 pixels × 1536 pixels = 3,145,728 pixels, or “3.1 megapixels”).
For some unknown reason, when referring to a monitor or digital display, manufacturers still list a pair of numbers, such as a "640 by 480 display” (which means it has 640 pixels horizontally, and 480 vertically), rather than calling it a *0.3 megapixel display” (640 × 480 = 307,200 pixels or 0.3 megapixels).
Bytes, KiloBytes, and MegaBytes
Computer storage and memory are measured in units called kilobytes (KB), megabytes (MB) and gigabytes (GB). These units measure the amount of information, or data, stored on these devices. They also refer to the amount of information, or data, contained in a digital file.
At present, the most commonly used unit of measurement for digital image file sizes is ‘megabytes’.
Resolution (megapixels) and File Size (megabytes)
Understanding how pixels translate into bytes is easiest if you imagine you are saving a file in a format with no compression. Each pixel is expressed as three 8-bit values (in jpeg or jpg images called 24 bit color depth, raw images use 30 bit color depths or better). There is one of these 8-bit values each for Red, Green, and Blue. So, a 6 megapixel (MP) image would occupy 6,000,000 pixels x 3 bytes or 18,000,000 bytes uncompressed (i.e. 18 MB).
Generally speaking, the more megapixels a digital image contains, the more megabytes (MB) it will consume on a hard drive or memory card. However, the file format (JPEG, TIFF, etc) you choose to save the image as will also affect its size.
For example, a 3.1-megapixel digital photo saved as an uncompressed TIFF file would consume about 9 MB of hard drive space. By contrast, that exact same image saved in the JPEG format with slight image compression would consume around 600 KB of hard drive space.