Category Archives: Tech

Why 300 dpi?

And why 300 DPI is only half the story?

Bit mapped images such as JPEG and TIFF files are regular, two dimensional, arrays of single colour picture elements known as pixels; pix for “pictures” and el for “element””.

Storm at South Tyneside

The first commercial digital cameras contained arrays of only 640 by 480 light sensitive chips similarly known as pixels. In terms of modern standards, early cameras could have been described as having only a 0.3 megapixel capability ((640 x 480)/1,000,000). Resulting “low resolution images” were so called because traditional offset litho printing (then and now) specifies a 300 dots per inch requirement in order to produce quality (crisp with no pixels) images when viewed at arms length.

Storm at South Tyneside

In other words a lithographic printing machine uses up 300 pixels of a digital image, both horizontally and vertically, for every inch of image printed. Therefore, the resulting printed image would only be 640/300 = 2.13 inches wide and 480/300 = 1.6 inches high.

For the sake of explanation, let’s imagine that our first digital camera had an array of just 180 by 180 pixels. That’s the size of the pampas grass picture below/left and if printed at 300 dpi the image would only be 180/300 = 0.6 of an inch wide. On your screen it appears bigger because your monitor uses up far fewer pixels per inch of image it is displaying.

If a camera had a sensor made up of only 60 by 60 or 30 by 30 pixels the results, when displayed at the same size, would appear as below.

Although very different whilst being viewed close up to your screen, the images will become more equal the further away your viewpoint. Try walking away from the screen; the further away the more similar the images become.

pampusGrass

180 x 180 pixels

 
pampusGrass

60 x 60 pixels

 
pampusGrass

30 x 30 pixels

 

 

billboard

This is why the iPhone ad above appears sharp when viewed from its intended passenger viewpoint from across the railway line

Billboard

But would appear grainy if it could be viewed closeup, and why 300 dpi is only required for printed images viewed normally at arms length.

If we had arms twice as long, books would have to twice as large but images could be printed at 150 dpi for the same visual experience. Image definition in terms of dpi requirement is therefore directly linked to intended viewpoint.

A modern 10 megapixel camera deploys an array of 10,000,000 pixels which could result an image of 3,870 by 2,580 pixels. If printed in a colour magazine the image could be up to 3,870/300 = 12.9 inches wide. If printed in a fine art catalogue, which is printed at 400 dpi, the image would only be 9.7 inches wide

When you display the same image on a 19 inch (diagonal width), 86.6 dpi, LCD computer monitor which contains 1280 by 1024 light emitting pixels, the image would be 3,870/86.6 = 44.7 inches wide. When printed on an ink jet printer which can produce reasonable quality at 150 dpi the image would be 3,870/150 = 25.8 inches wide (around A2 paper size)

So, next time you’re asked to supply an image at 300 dpi, ask how big the image is to be printed, because only then will you have the full story!
e.g. For a landscape image to be printed 12 by 8 inches at 300 dpi, the image would need to contain at least (300 x 12) x (300 x 8) pixels i.e. 8,640,000 (8.64 megapixels) which is well within the capability of a 10 megapixel camera.