Understanding Printer Jargon

To get the best results when printing digital photos, you should know what the standard printing terms mean. In this Insider, we’ll examine some of them, with the aim of clarifying printing issues that create uncertainty.

ppi vs dpi

The relationship between dots/inch (dpi) and pixels/inch (ppi) probably creates the most confusion in photographers’ minds, especially since some software programs and scanner interfaces seem to use the terms interchangeably. The easiest way to clarify the issue is to say that dpi relates only to the printed output, while ppi delineates the number of pixels per inch in the image itself. In other words, dpi should be used for print resolution, while ppi is for image resolution when describing camera-captured and scanned images and images displayed on monitor screens.

When making prints, photographers usually have little or no control over the output dpi used by the printer, other than selecting between draft and high-quality settings. Consequently, printer dpi specifications have little practical relevance beyond telling you whether the printer is capable of reproducing fine detail – and that capability is wasted unless you have enough data in the original image.

If you shoot with the top resolution and quality settings on a modern, 4-megapixel plus digital camera (in other words, if you have a high ppi figure), there should be plenty of data for the printer to work with at the appropriate size for the image resolution. Some editing software will even warn you that image quality will suffer if you try to print an image larger than an optimum setting.

Input to Output

Unless the manufacturer has decided otherwise, digital camera images have no resolution assigned to them. Some open up at 72ppi, others at 180ppi, a few at 230ppi and several at 314ppi. Consequently, you may need to re-set the resolution of the image before sending it to the printer so the printer driver can ‘render’ it to provide the best possible output. Ideally, this ‘resizing’ should be done in your editing software. (Note: if your digital camera and printer come from the same manufacturer the camera will often provide images with the correct ppi for the printer driver.)

The rendering process determines the size(s) of the dots that make up the printed image, how they are dispersed on the paper (halftoning), the way the colours are produced, and how finely details in the image can be reproduced. These factors also dictate the output dpi for the print. Naturally, the closer the image data matches the requirements of the printing engine, the better the rendering will be.

Printer drivers also control how the ink is deposited on the paper and different printers lay down ink dots in different ways. Don’t assume higher ppi in the image file means better printing dpi. The way the printer driver interprets the image ppi is more important – and that involves complex rendering calculations. Different printer manufacturers have different ways of achieving fine detail reproduction, and different printer drivers require different input data to achieve optimum results.

Most inkjet printers produce tiny, uniformly-sized dots that are deposited stochastically (with varying frequencies) on the paper, where they reproduce the image colours and tones. Canon’s FINE technology is based on microscopic (1 picolitre) ink dots applied with print heads that have thousands of separate nozzles (up to 6144 nozzles in the latest Pixma printers). The drivers of these printers have been designed to work best with image data that ranges from 180-300ppi. In contrast, Epson and Hewlett-Packard printers use variable dot sizes plus a process of overlapping the print lines for stripe-free printing, which Epson calls ‘microweaving’.

For Epson Photo printers, the ideal image resolution range for the printer driver is from 240 to 300ppi for prints up to 20 x 30cm (A4) where the viewer distance is close. Between A3 and A1 (60 x 80cm) sizes, 200ppi is acceptable for photo printing. HP printers start at 150ppi and extend to 300ppi.

However, regardless of the brand, all inkjet printers should produce good prints from files ranging from 240ppi to 300 ppi. This range is generally accepted as optimum for all inkjet printers. Higher resolution settings will be slower to print and output quality may actually be lower because the print head lays down too much ink.

dpi and Printing

Although printer manufacturers highlight dpi resolution in their advertising material, photographers should be aware that there is a limit to how small dots of ink can be made and this can limit actual output resolution. As a rule, higher-specified printers should be able to produce more detailed images with better tonality. Image colours should be more accurate and blends between colours should be smoother. In other words, the higher the printer’s dpi rating, the finer the detail it should be able to reproduce.

But the dpi figure can only tell you so much. It doesn’t tell you how the printer uses the input data to create the print. For example, a 4800dpi printer has the potential to use up to 4800 dots of ink in every square inch to make up the colours. But that may not be necessary – and it may not be possible with many current models, where a pixel can only be Cyan, Magenta, Yellow or Black or any other colour that is ejected from one of the nozzles on the print head. The end result – the print itself – will tell you more about the printer’s capabilities than the dpi figure alone.

Regardless of how the ink dots are deposited, all photo printers attempt to convince viewers they are seeing continuous tones. With some printers, dots are overlaid on each other, causing different ink colours to blend; with others, dot sizes are adjusted to match the image detail. The colours surrounding any one dot will also define what ink colour is used for that specific dot of ink. These factors, plus improvements to dot size, half toning and stochastic screening have contributed more to the output quality from modern photo printers than dpi resolution per se.

‘Optimised’ dpi

Printer manufacturers often provide a figure for ‘optimised resolution’ in their specifications. This normally refers to the fact that the printer uses interpolation to reach the resolution quoted. Depending on the quality of the interpolation algorithms, there may be little or no quality loss at the stated ‘optimised’ dpi setting. Other factors affecting the way the dots are laid down (such as dot sizes and halftoning) can also be involved and will affect the final print. In general, the optimised figure should only be used as a guide to the fineness of the detail the printer can produce, not the size of the prints you can make with it.

If the image file dimensions are larger than the output size you have specified, the printer’s driver will eliminate the excess pixels by interpolation when the print is produced. You are unlikely to notice any loss of output quality when this occurs. However, if the image ppi is less than the optimum ppi for the output size you’ve specified, the printer will add in pixels by interpolation to ‘fill the gaps’. In this case, the more pixels that have to be added (ie, the bigger you print the image) the more likely you are to see some quality loss in the print.