In recent months, we have seen a significant improvement of the resolution of computer displays. Apple’s ‘Retina’ generation of screens is an example where the number of pixels has been quadrupled to push the quality of the display closer towards the limit of human vision. All of this has been designed for the content to lose its pixelated appearance and start looking more ‘real’ to our eyes.
In print, on the other hand, things haven’t been quite so revolutionary lately. The magic number of 300 dpi has been around for some time, but fear not, as it means the image quality is just high enough.
So, why ‘300’? How the image resolution, in print and in general, relates to human vision?
Our eyes also have a kind of a ‘resolution’ and this resolving power is called visual acuity. It tells how sharp our vision is and it’s measured with a familiar Snellen chart.
Snellen chart
A person whose sight is being tested, stands at a certain distance, usually 6 metres, and reads rows of letters until the smallest of them can be deciphered. The letters in each row are of specific measurements, so the optometrist knows exactly how acute our vision is. The visual acuity is then expressed in form of ‘X/Y’, for example 6/6, 6/12, 6/3. The numerator is the 6 m distance and the smaller the denominator is, the better our vision. 6/6 vision is considered nominal performance of human sight and with this in mind we can try finding more comprehensible value and see how it relates to our magic number.
We will convert visual acuity to more familiar value of DPI. To do this, we’ll need to do some fairly uncomplicated maths. The resolution of our vision is measured in arc minutes. 1 arc minute is 1/60th of a degree and for 6/6 vision it’s exactly 1 arc minute. As it’s the case with any angle, it becomes larger with distance from the point of origin (our eye), so we need to know at what distance we want to measure the angular diameter to complete our calculation.
What’s the average minimal distance we look at print? It varies but let’s assume we look at a brochure or a postcard no closer than 30 cm (12 inch) from our eyes. Seems about right? So, 30 cm it is. Let’s put our values in the formula:
angular diameter = 2 × distance × tan( angle / 2 ) = 2 × 300 mm × tan( 0.017 deg / 2 ) = 0.09 mm
Now, to work out the DPI (dots per inch) value we just need to calculate the number of ‘dots’ in one millimetre and multiply by 25.4 (1 inch = 25.4 mm):
( 1 / 0.09 ) × 25.4 = 282 DPI
The interpretation is that a person with the nominal 6/6 vision and at a distance of 300 mm sees with the ‘resolution’ of 282 DPI. Isn’t it close to our magic number of 300 DPI, commonly used in print?
Obviously it is impossible to use this one value in all print situations. Large posters need much less of image resolution as people typically look at it from far larger distance. But for all ‘handheld’ formats like books, leaflets or catalogues the magic number of 300 dpi seems to make perfect sense.
One more thing!
To make it easier for all designers wanting to know what image resolution to use for their print project, I have added yet another handy tool to our Facebook page that automates the above calculations and gives an exact measurement of visual acuity at several distances, as well as our own recommendation for image resolution in print.
Try our ‘What Image Resolution?‘ tool now and don’t forget to share it with your friends!
