Re: Standard Line Screen
Don,
RE:
"So I used the formula, and 2400/150 = 16, and 16 squared is 256, so for my uncoated (where I use 150 l.s.) has 256 levels of gray according to the formula provided.
But when I use the same formula for my coated (which uses 175 l.s.), I get 2400/175 = 13.714285, and that squared is 188.08161, so for my coated I have 188 levels of gray according to the formula provided. This means I can expect banding because I don't have 256 levels of gray. I'm asking is this correct? I use round dot screens, Nexus' CQS Nominal screening 165,45,90,105 (aka 75,45,0,15). Does the formula still apply, and should expect banding? And if so, what could I do to fix it?"
Don, that gray level limitation formula is outdated for modern AM/XM screens. I'm not familiar enough with Nexus screening - you will need to check with them directly to find out whether this is an issue for you. Vendors such as Agfa (ABS), Heidelberg (HQS & IS), Kodak (Maxtone) use a technique called dithering to increase the number of tones without the need to increase the resolution and compromising performance. For example, let's say that a combination of resolution and screen frequency (according to the formula) gives us only 100 tones and we want 200. By alternating 70% halftone dots with 71% halftone dots, we can simulate a tone area of 70.5% - an average of the values of the adjacent halftone dots. Doing this for all values from 0% to 100% effectively doubles the number of tones that can be represented. Some screening implementations allow you to set the parameters of how and how many gray levels will be maintained as you increase LPI (to avoid shadestepping/banding). In general, FM screens eliminate shadestepping.
Just a few more thoughts regarding points raised in this thread.
1) You do not necessarily need to go to a lower screen ruling when printing on uncoated paper.
2) Changing screen ruling is not an optimal method of managing dot gain
3) AM/XM screening dot shape will not impact number of possible gray levels
4) The actual screen frequency you get at any specific request will depend on the the particular screening algorithm being used (varies by vendor) and device resolution, and sometimes color). for example, on a 2400 dpi device - a request for 175 lpi may actually deliver a 169.706 lpi ruling
best, gordo