Printing and Image Resolution

Printing & Image Resolution

When printing digital images it is important to distinguish image resolution from printer resolution. Unfortunately, scanner manufacturers often measure the resolution of their products in dots per inch (dpi), rather than pixels per inch (ppi) or samples per inch (spi), which can lead to confusion and the misconception that the resolution of a scanned image should always match the resolution of the printer. This is clearly not true for printers that rely on any form of halftone process. For the sake of clarity, therefore, I restrict the use of "dpi" to printers, and "ppi" to scanners and images.

Image Resolution
As I understand it, a digital image consists of a number of picture elements, known as pixels, which carry information about the colour and tonal values required at a particular location in an image. In this sense an image has no physical dimensions (e.g.. 6" x 4" or 240 ppi), but does have a finite number of pixels and a corresponding file size when saved to disk. Thus, the size and resolution of a digitally captured image is indicated by pixel dimensions, e.g., 3000 x 2000 pixels. Pixel dimensions are also used to indicate digital camera resolution and the resolution of different sensor arrays, usually expressed in terms of the total number of pixels captured. However, the idea of image resolution, in the context of printing, relates to the amount of detail that can be reproduced when an image is printed at a particular size. Since pixels are the building blocks of our image, their size depends on output dimensions. In other words, the larger the final print, the fewer the number of pixels per inch (ppi), and the greater their actual size. The quality of an image therefore suffers if the amount of pixel information is insufficient for the reproduction of fine colour and tonal gradations.

These two images give some idea of the difference in quality obtained from the same size print at two different resolutions. The first one represents 360 ppi, the second 60 ppi. The latter is clearly blurred, but it is surprising how low a resolution is possible before differences are perceived by the naked eye. Generally, however, more pixels per inch means more detail and more subtlety of colour.

An image resolution of 200-300 ppi is quite sufficient for good output, although the optimum for most Epson printers is 360 ppi. An image significantly below 150 ppi begins to blur (see above) and will eventually look "pixellated" (see below); and resolutions above 360 ppi usually waste processor time, as the printer has to resample down. If resampling is required it is usually better to do it in Photoshop.

A detail showing "pixellation". Separate pixels, the building blocks of which the image is composed, can be clearly seen. Notice the jagged, or stepped, edges apparent on curves, and the incoherent detail on the bow tie.

I used to scan 35mm film at an optical resolution of 2700 ppi, the maximum available using the Nikon LS20. This generated a file size of about 26MB. When resized to fit A4 paper size the image resolution dropped to about 360 ppi, although the file size remained the same. At A3 size the resolution would drop to around 240 ppi, but still gave an acceptable print. It is possible to maintain the resolution when enlarging, by choosing the resampling option in Photoshop, thereby increasing file size. This allows the insertion of extra pixels - interpolation - by duplicating existing pixels, but can impair quality if overused. Resampling may also be used to downsize pixel dimensions, thus reducing file size - a useful procedure when preparing images for the internet. In contrast to the LS20, the ArtixScan 4000t, can produce 360 ppi at A3 print size without the need for resampling. These scanners may seem dated now, but the same principles apply, whatever the model of scanner used.

It is worth remembering when resizing an image ( assuming no resampling applied ), that the number of pixels present in the original scan remains the same, regardless of changes to the overall output size of the image. For this reason the number of pixels per inch will decrease when an image is enlarged, and increase when image size is reduced. Thus, if an 8" x 6" image with a resolution of 240 ppi is doubled in size to 16" x 12", the resolution would be halved - 120 ppi, but the number of pixels in this image (2.8 million) will have remained unchanged. This simple relationship between image resolution and output dimensions only applies to situations where resizing is performed without the use of pixel resampling - as mentioned in the previous paragraph.

I have tried to graphically illustrate this relationship in the following diagram, where both squares consist of just sixteen pixels. Notice how the second square, representing 2 ppi, has doubled in size. In other words: half the resolution, twice the print dimensions. The background image is not directly related to the drawn pixels in these examples, however, and is there only for effect. In reality, resolutions as low as 4 ppi would not produce a coherent image at this size, but these figures are easy to illustrate and the relationships demonstrated are the same at any resolution.

1" Square: 4 ppi

2" Square: 2 ppi

It is also apparent that the 4 ppi square contains four times as many pixels per square inch than the 2 ppi square: sixteen pixels rather than four. Therefore, a doubling of resolution increases the total number of pixels four fold, for a given print area.

Inexperienced users of image editors such as Photoshop, or Photoshop Elements, may inadvertently fall prey to the unwanted effects of resampling when they prepare an image for printing - usually with dire consequences. My advice to any beginner, initially, is to make sure the Resample checkbox is not ticked when using Photoshop's Image Resize dialogue. This way you will avoid some of the pitfalls of resampling and develop a better understanding of the relationship between image resolution and printer output dimensions. Of course, eventually there will be occasions when the resampling option is indispensable, as mentioned above; but that is another matter.

Printer Resolution
It would be incorrect to think that an image pixel and a printer dot are always equivalent. Although pixels might be thought of as dots of picture information, this is as far as the analogy should go. Whereas a pixel may contain any one of several million colours, a typical printer dot (Epson ink jet) can only avail itself of black and three, possibly five, primary coloured inks. By varying the size of these dots and the space between them, the printer is able to simulate other colour and tonal variations. This process, known as "dithering" or "error diffusion", is a form of halftoning, and is the only way this type of printer can represent the millions of colours found in a continuous tone colour image. The following diagram gives some idea of how the halftone process works.

The diagram shows a series of halftone cells in which dots of black ink have been placed (dithering). As the density of dots increases, various shades of grey, and eventually black, can be achieved, simulating a continuous tone greyscale. Halftone cells are not equivalent to pixels, however, and they are not in themselves printable.

By placing a variable pattern of solid colour dots in a grid of halftone cells, intermediate tonal values (halftones) for each of the primary colours and black can be achieved. The resulting print is really an optical mix of Cyan, Magenta, Yellow, and Black, on a white background. The ink jet is capable of laying down a number of variable sized dots as it renders each image pixel.

For this reason most Epson ink jet printers, with an output of 1440 dpi, are designed to handle a maximum image resolution of 360 ppi. Although this type of printer can be set to output at 360 dpi, the quality of the image would suffer by comparison, producing a more grainy print as a result. Higher printer resolutions allow a less perceptible dither pattern to be used, and a finer rendering of detail can be achieved. Some of Epson's latest printers are now capable of 2880 dpi.

By contrast, a dye sublimation printer can reproduce one of 16 million colours per printed dot. The quality of output at 300 dpi on a dye sub is therefore superior to that which can achieved by an ink jet at the same print resolution, as less dithering is required; in this instance image and printer resolutions could be the same. Hewlett Packard seem to have developed a type of ink jet which can, to a limited extent, overlap dots to produce other colours. This is not dye sublimation as such, but it should reduce the amount of dot dithering that is required.

When outputting photographic images it is always best to choose the "error diffusion" option when it is available on the print driver. The reason for this is that error diffusion produces an irregular pattern of dots, which is better suited to finer colour gradation and detail. On the Photo 1270 this option has now changed to "High Quality Halftoning", but it amounts to the same thing.