The world is unjustly full of misunderstandings about Dots Per Inch, or “dpi”, sometime also referred to as “pixels per inch” or “ppi” (but then my cool title wouldn’t work… 😊 ).

For my work (day-job or as photographer) I often get asked to “send the picture to me in high resolution – 300dpi”: the person asking often doesn’t seem to realise that this is a meaningless request. In order to comply, I then have to ask “how many inches will the picture be printed to?”.


I usually end up just sending the picture with the maximum number of pixels I have in the image, which is often a very big file and certainly “overkill” for most uses (with a file from my Canon 5DmkII, which has 21MPx, a 300dpi print would be about 30×45 cms big – roughly A3 size).

One time, to make a point, I sent a “300dpi” file which had 300px on each side, which is of course the same as 300dpi (ppi) for a 1 inch (2.5cms) square print, but which looks rather poor when printed on an A4 page…

The moral of this sorry tale? There are a few:

  • Digital files don’t really know about “dpi”, as this refers to the physical size of a print. All they know about is pixels, and how many there are. The “dpi” data found in some digital files is calculated from: dpi = (number of pixels along a side of the print) / (length of the side of the print in inches) – literally “dots per inch”.
  • Start thinking about your image in terms of how many pixels it has. Some typical applications have typical numbers of pixels for optimum use. See the table below.
  • Learn how to re-size your digital pictures to give files with a suitable number of pixels. There are some tutorials on the Viewfinders web-site under “Resources” that can help with this.

Typical pixels needed for various uses:

 200 –  400 px highSmall web-page image / thumbnail
 900 – 1200 px highLarger web-page image
1050 x 1980 pxFull HD image (computer monitor / TV)
4200 x 7920 pxQuad HD monitor (but this is getting silly…)
1200 x 1800 px10×15 cms print
2500 x 3500 pxA4 / 20×30 cms print
3500 x 5000 pxA3 / 30 x 45 cms print
 900 px short sideVF newsletter, small illustration
1600 px short sideVF newsletter, large image