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).

On the left is a normal 8 bit pixel (8 x 2 = 16 - 16 x 16 = 256) which gives us 256 levels of gray.How the cells are filled with black defines the grayscale. Which is why the levels dialogue box in phototoshop goes from 0 to 256. (for more info go to the levels page.)

If we extend the width of the data creating the halftone cell, then multiplying the the pixel sample by factor of 1.41 it 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.So under 188ppi would be low resolution. At No Duff Stuff our standard is to use a factor of 2 ie 133 x 2 = 266ppi

Therefore anything under 1.41 x line screen of the output device is low resolution.

The print-based equivalent of the image’s pixel dimensions is the number or frequency of the halftone spots which controls the amount of detail. This setting is variously called the Screen Frequency, Screen Ruling, Line Frequency, Halftone Frequency and is measured Lines Per Inch (lpi) from the days when the halftone was actually produced with a process camera and a grid mesh.

What is Low Resolution?

60 lines per inch (laser printer)
100 lines per inch (imagesetter) greyscale
133 lines per inch (imagesetter) greyscale
150 lines per inch (imagesetter) grey & colour
175 lines per inch (imagesetter) colour

060 x 1.41 = 085 ppi
100 x 1.41 = 141 ppi
133 x 1.41 = 188 ppi
150 x 1.41 = 212 ppi
175 x 1.41 = 247 ppi