Adobe Photoshop CS5 for Photographers - Martin Evening [134]
2.
In the case of a CMYK print output, four plates are used, of which only the yellow plate is actually angled at zero degrees. The black plate is normally angled at 45 degrees and the cyan and magenta plates at less sharp angles. If you overlay the same pixel resolution of ×2 the line screen, you will notice that there is no direct relationship between the pixel and line screen resolutions.
3.
Let's be clear, there is no single empirical formula that can be used to determine the ideal ‘half toning factor’. Should it be ×2 or ×1.5? The black plate is the widest at 45 degrees and the black plate information is usually more prominent than the three color plates. If a half toning factor of ×1.41 (the square root of 2) is used, the pixel resolution will be more synchronized with this angled halftone screen. There is no right or wrong half toning factor – the RIP will process pixel data at any resolution. What we can say though is if there are too few pixels, the print quality will be poor, and having more than the optimum number does not necessarily equate to better output, it just means more pixels. What this shows is the logic behind the ×1.41 minimum halftone factor recommendation.
It is for this reason you will find that the image output resolution asked for by printers is usually at least 1.41 times the halftone screen frequency used. This multiplication is also known as the ‘halftone factor’, but you will also find that multiples of ×1.41, ×1.5 or ×2 are commonly used, so which is best? You can always ask the printer what they prefer you to supply them with. Some will say that the 1.41:1 or 1.5:1 multiplication produces the sharpest detail, while others may request a ratio of 2:1. I usually reckon that a halftone factor of 1.5:1 should be fine for general image reproduction, but photographic subjects with fine image detail do benefit from using a higher halftone factor. You also have to take into account the screening method used. It is claimed that Stochastic or FM screening permits a more flexible choice of ratios ranging from 1:1 to 2:1.
Determining output image size
Image size is determined by the final output requirements and, at the beginning of a digital job, the most important information you need to know is:
• How large will the picture appear on the page, poster, etc.?
• What is the screen frequency being used by the printer – how many lpi?
• What is the preferred halftone factor used to determine the output resolution?
• Will the designer need to allow for page bleed, or want to crop the image?
Let me give you a practical example here. If the line screen used by the printer is, say, 175 lpi, the pixel resolution will therefore need to be around at least 260 pixels per inch (if you use the ×1.5 halftone factor), but probably no more than 350 pixels per inch (if you use the ×2 halftone factor). Ultimately it always helps to know in advance the intended line screen, the ideal halftone factor and final print output size. Using this information you can then calculate the ideal image size for the files you are about to supply to the printer. There is no point supplying files that are bigger than necessary as you will simply waste time uploading unnecessarily big files. On the other hand, designers do like having the freedom to take a supplied image and scale it in the design layout program to suit the requirements of a layout. For this reason it may be worth using a ×2 halftone factor. There will then always be enough data in the supplied image to crop or scale up a photo without adversely compromising the final print quality.
But we always use 300 ppi!
There is a common misconception in the design industry that everything must be supplied at 300 pixels per inch. This crops up all the time when you are contacting clients to ask what resolution you should supply your image files at. Somehow the idea has got around that everything from a picture in a newspaper to a 48-sheet poster