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 high||Small web-page image / thumbnail|
|900 – 1200 px high||Larger web-page image|
|1050 x 1980 px||Full HD image (computer monitor / TV)|
|4200 x 7920 px||Quad HD monitor (but this is getting silly…)|
|1200 x 1800 px||10×15 cms print|
|2500 x 3500 px||A4 / 20×30 cms print|
|3500 x 5000 px||A3 / 30 x 45 cms print|
|900 px short side||VF newsletter, small illustration|
|1600 px short side||VF newsletter, large image|