why 300dpi? and what’s a dpi anyway?
January 20th, 2009 | Published in bits and bytes
“We need the pictures at 300dpi.”
This is the standard request from clients, but it doesn’t actually mean anything.
dpi is an acronym for Dots Per Inch, so 300dpi means 300 dots per inch. So how many dots is that exactly? Well, that would depend on the number of inches…
Digital cameras shoot pictures at a fixed size in pixels. For example, my Canon 5D is an approx. 12 megapixel camera and produces files which are 4372 pixels on the longest side and 2906 pixels on the shorter side. I can’t change that, its the size that the sensor is manufactured to produce. But, I can deliver that file at any dpi I choose. If I deliver a full size file, then the picture, when it is printed at 300dpi, will be approx. 14.6×9.7″. (4372/300=14.6 and 2906/300=9.7). This size is absolutely fine for most uses.
DPI is only meaningful when it is quoted in relation to the final print size. So, if you need a picture to be printed as a full page of a magazine (say 12×8″) at 300dpi then you need a file which is 3600×2400 pixels.
If you only want to publish a picture on your website then you only need 72dpi, the standard screen resolution for web design (although screens do vary). So if you want your picture to appear roughly 4×3″ on screen you need a file which is 288×216 pixels. Easy.
A newspaper generally only needs about 200dpi because of the low quality paper and printing process, so a typical half-page picture in a tabloid paper, say around 9×6″ would need a file of 9×200=1800 by 6×200=1200. This works out to be a 1800×1200=2.16 megapixel image. That’s how news photographers got away with two megapixel cameras for the first few years of the digital revolution.
So what does all this mean in practice?
Generally, for pictures destined for press releases, the web and newspaper use, I supply files at around 10×8″ at 300dpi. For magazines or other high quality print, I supply files at their original camera size. For web only use, a 640×480 file is usually plenty.
It all comes down to this – if the photographer knows how the pictures are going to be used, he can supply the files at a suitable size. Otherwise its just guesswork.
