DPI is a standard term used to determine the density and thus quality of a print, or the size of a digital image, by the number of dots it contains per inch. It’s an important value to printers as it affects a variety of factors they measure their business by.
DPI AND PPI
Although the terms DPI and PPI (pixels per inch) are used interchangeably in reference to digital images, the value it represents is not interchangeable between printed and digital images, as they measure quite different things.
Digital Images are fixed in their size, and the number of dots (or pixels) present in the image doesn’t alter when you change the DPI/PPI – only the physical measurements of the image does. You can crop images or artificially alter them, but then it ceases to be the same image.
For example if your image is 2000 pixels across in total, when you change the PPI from 500 to 250 you are simply increasing the physical size of the images width from 4 inches (2000/500 = 4) to 8 inches (2000/250 = 8), and decreasing the resolution and quality of the image at the same time.
For printing it works differently, as it is a direct measure of a prints density and quality - and is not connected to the physical size of the image. In opposition to how it works for digital images, as you change the DPI the numbers of dots used to create the printed image most certainly change too.
But while lowering the DPI for a physical print can reduce quality in relation to the PPI of the digital file it’s being printed from, it cannot improve the quality beyond the digital files original PPI.
Continuing on from the previous example, the best quality a print can hope to achieve from the 8inch image is the equivalent of the digital 250 PPI. It could still potentially use a higher DPI to achieve the same quality but it would be at a waste, as all it would do is more accurately produce the flaws in the digital image. You could also reduce the quality further and print at a lower DPI if required.
This is why higher quality digital images are needed when printing, as it retains the ability to vary the quality up or down as needed – a low quality digital image always results in a low quality print.
DENSER NOT BETTER
A fact not known to everyone is the DPI value only refers to the number of dots along one side of an inch and not the total number of dots within it - so the total of dots contained is actually the square of the stated value. I.e. 300dpi = 90,000 dots inside one square inch.
This is quite staggering when you consider it means the density grows exponentially as the DPI increases, and shows comparisons between different qualities of print in a different light. 600dpi isn’t twice as sharp as 300dpi it’s four times as sharp – 300x300 = 90,000, 600x600 = 360,000.
But simply cranking up the DPI is not always beneficial or advisable, as the higher density costs more to produce – both in time and money.
Printing hundreds of thousands of dots per square inch instead of tens of thousands naturally takes the printer itself a lot longer to do, and uses up greater quantities of ink to do it. You may even need to use special inks capable of achieving those levels of detail which adds another costs – and more time spent by someone carrying out the longer task.
This is why most printing tasks don’t use DPI’s much higher than a few hundred despite the potential for it to be in the thousands - and typically the further away something is going to be viewed from, the less DPI it needs.
Billboards run as low as 15dpi, as people only view them properly from a considerable distance away. Computer screens are somewhere between 70-100ppi and glossy magazines are printed at 150dpi. 300+dpi is classed as a high quality print image, and used when the print quality is paramount.
It’s been suggested the human eye can’t even distinguish the difference past a certain value – somewhere between 300-450dpi, and yet we continue to use higher DPI.
One of the reasons for this is it acts as insurance against future developments.
As screen and scanning resolutions continue to improve and develop in new and unforeseen ways, producing higher quality images ensures they will stand the test of time and still look as intended - and most importantly remain useable.
There’s still much to be said preserving physical prints. If properly stored they are probably more durable that the soft copies stored in hard drives given technologies propensity for obsolescence, and technology current lacks the ability to reproduce a copy equal to the original.
The accepted method is to create and preserve at the higher end of what’s possible, but only produce at the quality necessary depending on the task. Any more is currently a waste.