In printing , image resolution refers to the amount of detail that there is in an image or photo and hence how clear or sharp it appears in a printed document. If the image resolution is too low it will look blocky or pixelated when printed; if it is too high it will make the file size of the finished document very large and take longer than necessary to download or process for no added quality when printed.
Image resolution is controlled by the number of pixels that make up the image in its electronic format. When these pixels are processed for printing they are converted into dots and the measurement is given as DPI (dots per inch). The more pixels used to make up the image, the higher the resolution and the higher the DPI in the printed image resulting in greater detail in the printed document.
This can be expressed as a formula in order for you to determine how many pixels you need for an image to reproduce at an acceptable resolution when printed.
[Height (in inches) x dpi (desired) = number of pixels] x [Width (in inches) x dpi (desired) = number of pixels] = Total number of pixels in the image
For example, a 2″ x 2″ image at 300 dpi will have 360,000 pixels
(2 x 300) x (2 x 300) = 600 x 600 = 360,000 pixels
If you now increase the size of this image to 4″ x 4″ we only have 360,000 pixels to play with over a larger area so the resolution or dpi will go down
360,000 pix ÷ (4″ x 4″) = 22,500
√ 22,500 = 150dpi
A4 = 2480 x 3508 pix at 300dpi
A5= 2480 x 1748 pix at 300dpi
A6 = 1240 x 1748 pix at 300dpi
A picture or image may look fine on a computer screen but appear pixelated when printed. This is because most monitors only have a screen resolution of 72dpi and therefore only need this resolution to produce a clear detailed image when viewed, whereas in traditional litho printing you require 300dpi.
For digital and large format printing you can use 150dpi depending on how and where it will be viewed.
For example, an image on your computer screen that measures 1″ x 1″ and is 72dpi will only be of sufficient print quality (300dpi), if reproduced at 1/4” x 1/4” in a printed document.
(1″ x 72dpi) x (1″x72dpi) = 5184 pix
5184 pix ÷ (300 dpi x 300 dpi) = 0.0576
√0.0576 = 0.24″
Image resolution is affected by 2 things, the number of dots that make it up and the physical size that the image is reproduced at. As size increases, resolution decreases and visa versa. If you have an image that is 6″ x 4″ at 300dpi it will only be 200dpi if enlarged to 6″ x 9″ and hence can start to look blocky. This can potentially happen if you use part of an image and blow it up or increase the size of an overall picture. The converse can also be an issue although less in terms of quality and more in processing and handling as the resulting file size can be massive particularly when a lot of images are used. As a general rule of thumb, if you blow up an image on your computer screen by 350 – 400% you can get an idea of how it will look in a printed document.
You can change the dpi of an image using a number of different software packages such as Photoshop, Photopaint or Paintshop Pro.
Remember, you can reduce resolution by removing the number of pixels in an image but you can not increase resolution without loss of quality, without reducing the size of the image.
Resolution only affects raster images, vector images are produced differently and are not affected, see Raster vs Vector.