Print technique

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Digital photography diverges sharply from film photography in the area of print technique.

Digital print technique

The key issues in successful digital prints are:

1. Preparation of the image
   1. Colour correction
   2. Image size
   3. Output sharpening
   4. Colour model
2. Choice of technology
3. Choice of stock
4. Maintenance of the equipment
5. Mounting

Preparation of the image

A digital image must be correctly prepared for a successful print

First, it must be colour corrected using a monitor or workflow calibrated for that particular print device. Different types of printer are able to display different gamuts of colours. A CMYK image from a laser printer will inevitably be duller than a gloss image from a contone printer.

Second, it must be scaled to be the exact size required by the print technology. For half-tone printing, the size of the image in pixels per inch PPI should be the size of the final image in lines per inch LPI multiplied by a factor of between 1.25 and 2 for oversampling. For a contone image, oversampling is not required, and the resolution of the image should be the same in PPI as the DPI resolution of the printer. For most inkjet printers, the resolution of the image should be between 240 PPI and 360 PPI at the size of the finished print.

Third, the image must then be correctly sharpened for the exact output size, resolution and printer technology.

Finally, the image should be converted to the correct colour model. For all half-tone printers, this should be CMYK. For other printers, this may be CMYK or RGB, and it is important to check which.

Choice of technology

In general, contone printers give the most photographic reproduction. However, photo-quality inkjet printers can approach this quality, and are cheaper and available in a much wider variety of formats, as well as offering more types of stock. Laser printers and offset litho produce the lowest quality print, although the difference between a cheap office laser printer and a digital press is remarkable.

Choice of stock

The stock is the type of paper on which you print. For contone, this may be limited to gloss or matte, but will be photographic paper which gives optimal colour rendering. For inkjet, there is a very wide range of stock available, ranging from photographic archival papers through to office copier paper. Choosing photographic papers, especially coated papers, will greatly enhance the image. For laser printers, there are still a range of stocks to choose from. Offset litho, the commercial printing process, has a bewildering range of stocks. Essentially, for photography, coated stock is always preferable to uncoated, and gloss paper will rending images better than matte. However, gloss gives exceptionally poor legibility, so most documents will benefit from a half-gloss paper generally referred to as silk. The rendering of an offset litho image is dramatically enhanced if it is varnished, or given a spot UV coat, which, when carefully prepared, can create an impression similar to a contone print on photographic paper.

For all types of printing, heavier papers will hold colour better and give a better final image, but this is most important in offset litho, where any paper below 130 gsm will feel flimsy.

Maintenance of the equipment

All digital printing involves the use of machines. Correct, effective maintenance will keep them running in factory condition. Allowing dust to gather on stock, on printheads, or on other components, however, can result in a rapid degradation of the image. It is always wise to print a test image first, and then to examine it carefully for flaws. Inkjet heads should be calibrated monthly, and contone stock should be kept absolutely dust-free.

Mounting

Fine prints should be mounted using archive quality materials. In the UK, museums and galleries often mount art behind glass, but this introduces reflections which substantially limit the enjoyment of the image. Where possible, it is best to mount photographic images under a matte, which is an angled cutout frame, on a rigid back board, but hung so that the weight of the paper keeps it flat. This is best done with heavy weights of paper. When mounting under glass, a matte of some kind is essential to prevent the glass touching the image. With an inkjet image, this will degrade the image rapidly, but will have the same effect in time on a contone image. Laser images are largely immune from this, but as the quality of a laser print is comparatively low anyway, this is unlikely to be a solution.

• This page was last modified on 11 February 2009, at 11:33.
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