DAFOS Photo World

When printing a digital image, it is important to know and understand how the size of the print, the viewing distance and the number of pixels in the digital image file interact to determine the perceived quality of the resulting print .

A standard reference is that the human eye can resolve about 300 dots per inch (or DPI – which is 118 dots per centimetre) on a 20×30 cm pages (A4-ish) viewed from a distance of 30 cms. Having more dots per inch than this is wasteful: you can’t see them: having fewer dots per inch and the image may start to appear “soft focus”. To assure quality, a little margin is taken – typically 330 or 360 DPI. For simplicity, like most people, I equate “dots per inch” to “pixels per inch”, though a printer will put down very many more dots in the space of each pixel due to the way they work.

If you do the sums, an A4-sized print needs a little over 10 million pixels to achieve 330 DPI resolution (which is roughly 3800 pixels on its longest side: 330 pixels per inch over 12 inches).

At this same resolution, a print at 40 x 60 cms needs 33 million pixels, and an A0-size (84 x 119 cms) needs 140 million pixels!

Hang on… Such huge files is getting to be a bit silly. Indeed, this observation was mentioned to me in a discussion with an artistic photographer who gets excellent, exhibition quality large prints with less than half the size of file this theory would indicate. Investigating further on the Interwebs I came across this useful little table.

In other words, the further away from the print you stand, the lower the resolution needs to be for the same subjective quality. In fact, looking at the table you can see that the suggested DPI is simply 180 divided by the viewing distance in metres.

Elsewhere on the same Interwebs you can read that the best viewing distance is about 1.5 times the diagonal of the picture. Which makes sense, as you typically view larger things from further away. Put those together and you find that the DPI you need decreases as the print size increases, actually quite exactly, such that the number of pixels you need stays roughly the same, no matter how big a print you want to make!

Now, this is of course a theoretical case. Some people do go up close to exhibition prints to look at fine detail, so this guideline should definitely be regarded as the “minimum case”. In practice, a file with about 20 million pixels is quite big enough for the size of larger prints most of us need to make.

 If the size of print you want to make seems to call for rather more megapixels than your camera gave you in the first place, it is tempting to use an image editor programme to “upscale” the number of pixels. Indeed, most do an excellent job of this. However, and this is again due to the way printers actually work (the difference between “dots per inch” and “pixels per inch”), my preference is to let the printer’s software do this upscaling: it will be better adapted to the exact requirements.

So, bottom line is – at least, if you fully trust the printing service – to get the best possible print, give the printer all of the pixels you have without upsizing.

(A complete table of paper sizes, required DPI and the resulting sizes is given here. Not for the NL, but for an article in “Resources).