Picture size, resolution

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There are two meanings for the picture size: size in pixels (pixels width x pixels height = total pixels), or, on printed outputs, the size in dimensions (pixels per cm or inches). For our digital media needs, we use pixel count as parameter for the size of a picture.

That’s why web designers and web picture-editors use pixel units in Photoshop and Illustrator (Preferences → Units and Rulers → Pixels for all parameters!)

The resolution is determined by the count of pixels per one inch, called ppi (pixels per inch), or mostly dpi (dots per inch).

The standard resolution for digital displays is 72 dpi, while it varies from 100 to 400+ for print: 100-200 dpi (standard for newspapers, large format posters), 200-300 dpi Color magazines, brochures, 400-… dpi for high quality prints.

Note the 72 dpi standard value for electronic screens does not refer to real inch measurements. It is a relative, not absolute, method, since it’s based in relationship with pixels that do not have a defined size, see “pixel density” below.

On printed images, the dpi value refers to real inch mesurements. A picture printed with 300 dpi will count 300 pixels on one physical inch (in one row) .

Sample – print a  picture with 3000 x 2000 pixels

  • With 100 dpi, a print of this picture is roughly 76cm x 52 cm,
  • With 200 dpi, a print of this picture is roughly 38 cm x 26 cm,
  • With 400 dpi, a print of this picture is roughly 19 cm x 13 cm

This sample should explain how resolution and pixel count relate together.

Pixel density

Electronic screens do have very different pixel densities. While, for instance, this projected image here, made with a older beamer, contains only 1600×1200 pixel, on a surface of approximately 2m x 1.5m, a mobile phone with Ultra-HD screen displays 3840×2160 pixels  on a surface of 12.8cm x 7.2cm approximately. The pixel density on that mobile device is around 40x higher than on the beamer’s screen.

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