300dpi vs 350dpi and 150lpi vs 175 lpi

[Gregg]

Hi.

I've always worked by the age-old rule of 2x the line-screen. So if printing with a 150lpi the optimal resolution of your artwork should be 300dpi, and if printing with a 175lpi the optimal resolution of your artwork should be 350dpi.

The majority of our scanning and printing is handled in the Far East. One of our most used repro-houses has suggested that we have all scans done at a minimum of 350dpi (they actual prefer 400dpi), regardless of desired line-screen. They have stated that the quality will be the same regardless. The reason they suggest the uniformed scanning is for enlargement if we re-issue the book at a larger trim (which we often do).

I have no problem going to a higher resolution, but my question is will there really be no difference in quality if something output at, say, 175lpi has a resolution of 300dpi versus 350dpi? If there is no difference than why was the original notion of 2x the line-screen developed. Was it just to keep file size and rips times down?

Thanks in advance.

 

[gordo]

I have no problem going to a higher resolution, but my question is will there really be no difference in quality if something output at, say, 175lpi has a resolution of 300dpi versus 350dpi? If there is no difference than why was the original notion of 2x the line-screen developed. Was it just to keep file size and rips times down?

Thanks in advance.

The guiding principle for understanding what original image resolution (ppi/dpi) is needed compared to the halftone screen (lpi) that will be used is that the image pixels should always be more densely packed (ppi/dpi) than the detail resolving ability (lpi) of the halftone screen that is used. Also, for process work that you have enough sample data points to build a 4/c separation.

In most cases you can even go 1 to 1 - i.e. 175 dpi for a 175 lpi image and not see a difference compared with 350 dpi for a 175 lpi image.

More detail is explained here: The Print Guide: Image resolution for printing - LPI vs DPI a.k.a. LPI vs PPI a.k.a. LPI vs SPI

best, gordo

 

[Gregg]

Thanks, Gordo. That was a very nice article you put together. Really well thought out.

 

[John Clifford]

Yes! When I ask my design students what resolution they should scan images at, they all answer 300 dpi/ppi. When I ask them WHY? They have no idea. The dpi=2lpi at least gets them in the right range. I agree that you might want to go a little higher in the resolution so that you can enlarge a small amount without losing quality. But I'm amazed that so few printers who do sheetfed work and screen at 200lpi ask for 400 dpi/ppi images. Most of them still ask for 300 dpi/ppi because it's "good enough."

I HATE GOOD ENOUGH!!!

 

[gordo]

But I'm amazed that so few printers who do sheetfed work and screen at 200lpi ask for 400 dpi/ppi images. Most of them still ask for 300 dpi/ppi because it's "good enough."

What's also interesting is that most printers don't realize that most RIPs are set by default to resample incoming files/images to 300 dpi irrespective of what lpi the RIP is set to.

Best, gordo

 

[arossetti]

What's also interesting is that most printers don't realize that most RIPs are set by default to resample incoming files/images to 300 dpi irrespective of what lpi the RIP is set to.

Best, gordo

Gordo, what about a 200dot halftone, same math applies 400dpi/ppi?

 

[gordo]

Gordo, what about a 200dot halftone, same math applies 400dpi/ppi?

Your terminology is not clear so I don't understand your question. :-(

 

[arossetti]

Your terminology is not clear so I don't understand your question. :-(

Ha, clearly I don't either. I'm confused why digital printer DFE's default to a dot halftone pattern. I'm also not exactly clear what a dot halftone pattern is. And finally what is the lowest DPI/LPI one should use with a 200 dot pattern RIP setting.

Let me see if I can find the answers to question 1 & 2 and maybe that will help you answer question 3.

 

[gordo]

Ha, clearly I don't either. I'm confused why digital printer DFE's default to a dot halftone pattern. I'm also not exactly clear what a dot halftone pattern is. And finally what is the lowest DPI/LPI one should use with a 200 dot pattern RIP setting.

Let me see if I can find the answers to question 1 & 2 and maybe that will help you answer question 3.

Some things to think about.

A halftone screen primarily images two things - the image you are reproducing and the imaging characteristics of the imaging device itself.

A coarse halftone screen does not reproduce image details well - nor does it reveal imaging device issues either.

A fine halftone screen reproduces image details - and may reveal imaging device issues.

An AM screen typically has a lower detail resolving capability than an FM/Error Diffusion type of screen so while it does not reproduce image details as well - it doesn't reveal imaging device issues either.

A halftone pattern is just the organization of marks that an imaging device makes. The choice of how those marks are organized is dependent on many factors and may be imposed by the manufacturer of the device for a variety of reasons that they may not want to be made public.