Each halftone dot is rendered by a PostScript RIP from the pixel data and to an output device called an imagesetter. The halftone dot shown here is plotted using a 16 x 16 dot matrix. This matrix can therefore reproduce a total of 256 shades of grey 16x16=256 The dpi resolution of the imagesetter, divided by 16, will equal the best line screen resolution. 2400dpi divided by 16 = 150lpi (others can be used ie 133 and 100 etc but 150 will display the most levels of grey.)

Repro Considerations

The structure of the final print output appearance bears no relationship to the pixel structure of a digital image. A pixel in a digital image does not equal a cell of halftone dots on the page. To explain this, the black cell in a CMYK set is normally at 45 degress. If the Black Screen is at a 45 degree angle (which it normally is) the narrowest horizontal width of the black dot is 1.41 (square root of 2).

If we extend the width of the data creating the halftone cell, then multiplying the the pixel sample by factor of 1.41 would mean that there would be at least a pixel width of information with which to generate the black plate. 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, ie multiples of x1.41, x1.5 or x2. This multiplication is also known as ‘the halftone factor’, but which is best? Some will say the 1.41 or 1.5 multipliction produces crisper detail than the higher ratio of x2. At No Duff Stuff we use the x2 factor (133 line screen x 2= 266 pixels per inch). The reason for this is there is enough data in the supplied file to survive any extra unforeseen adjustments (ie enlargement) without compromising the print quality. At the same time the file will not be unnecessarily large

Conclusion:
Minimum resolution for High Res would be using a 133 line srceen,
133 x 1.41 = 188ppi. At No Duff Stuff our standard is to use a factor of 2 ie 133 x 2 = 266ppi