Book Cover Design: Images and Resolution For Print – How to Get Pictures That Pop
So you've taken some lovely pictures with your digital camera and there is one area of an image that you would like cropped out and used for your book cover. Sadly the designer has said that it is too small so can't be used. What happened?
As far as publishing goes this is perhaps the least understood aspect. At one time what you needed was a photograph which you sent to the printer or publisher who did their magic and voila; the picture appeared in your book. Now in the computer era the picture can be in a variety of digital formats and resolutions, some usable and some not.
An original photograph is referred to as a continuous tone image. The tones move seamlessly from white through grey to black and through the colour spectrum. A photograph does have a grain structure but it is microscopic and contains orders of magnitude more information than a digital image. Digital images also have a grain structure called pixels. Each pixel can have many values (in the millions for a colour image) but the entire pixel has the same value. When we take a picture with a digital camera (or scan a photograph) we do it at a particular resolution. This refers to the number of pixels in the image and is usually given as number of pixels width and pixels height. This is referred to as a raster image due to the way it is imaged on screen -- from left to right and top to bottom. This is also the order in which the pixels values are given in the image file.
To get the illusion of continuous tone when a photograph is printed we use ink dots. Don't confuse ink dots with pixels, they are not the same. These dots are usually printed in a rectangular grid called a screen, measured in ink dots per inch and the dots themselves vary in size. (Screens are often described as lines, of ink dots, per inch.) So as the tone in an image gets darker the ink dots get larger but the screen, dots (lines) per inch, remain the same. You would think that the higher the number of dots per inch the better the resulting picture and you would be mostly correct, but this is dependent on the printing press, the type of paper and the chemistry of the ink. It turns out that we need about 300 pixels per inch to give a good quality image at 130 line screen (130 ink dots per inch).
Printing houses today also use laser technology but they call them image setters not laser printers and they produce much higher resolution images than a laser printer you might use in your office or at home. To produce the 130 line screen, the image setter needs to be capable of over 2000 pixels per inch where your laser printer is only capable of 600. A very thorough explanation of this can be found in the book Real World Scanning and Halftones.
 So the digital image you sent was perhaps 3,456 pixels x 2,304 pixels. This is the top resolution you might get from an 8 megapixel camera. This image could print a cover about 12 inches across. The salesman may have told you that you could print a poster with these images but he is talking about printing on an ink jet printer not a printing press -- different technologies with different requirements. The 12 inches sounds good as far as printing your cover but you only want a small crop of the image. It turns out that the crop you want is only 1,000 pixels wide and 1,600 pixels high. The minimum you need for your cover is 1,800 pixels wide by 2700 high (6 inches multiplied by 300 pixels per inch, and 9 inches multiplied by 300 pixels per inch). So the designer tells you the image is too small or more correctly, isn't high enough resolution.
If you provide original photographs or negatives the designer or printer will scan them at the required resolution (providing of course that the pictures are of reasonable size, don't have to be cropped too much and are in focus). If you provide digital images they must be of a high enough resolution.
Just a quick word about original photographs: They do need to be reasonably good quality. I have tried to scan 8" x 10" portraits that were so poorly done that it was almost impossible to get a good image. I have also scanned an 1880 era carte-de-visite photo that is only about 2.5" x 3.5" that has fabulous detail.
At the risk of confusing you, lets discus a different technology: stochastic screening. Stochastic is simply a fancy word meaning random. (If you are a mathematician you are now jumping up and down, incensed that I would use such a simplistic definition. It's a little like a fashion designer telling you that aubergine isn’t purple. If this doesn't mean anything to you believe me when I say that it doesn't matter, so please don't loose any sleep over it. ;-) Conventional printing uses lines of varying sizes of ink dots, stochastic printing uses random patterns of small ink dots that don't change in size but vary in number or density. This is roughly the technology your ink jet printer uses. The benefit is you might be able to get away with a smaller (lower resolution) image using stochastic printing. Don't take this as a licence to crop your digital images to death, there is still a great benefit to using high resolution images and stochastic printing will benefit more from very high resolution than conventional printing will.
Although this technology isn't really new its adoption has been slow in the print industry. If your printer happens to use this type of screening it can be a plus and your designer will make any necessary adjustments to take advantage of it.
I hope this has made the concept of resolution a little clearer as far as the print industry is concerned.
Michael Dyer
Graphic Designer
http://www.mocabookdesign.com/
Michael has been a full time graphic designer for over 15 years. He has designed all manner of communication but his first love is a good book finely bound.
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