Copy quality poor when copying halftones

[fishleg]

I don't know if anyone else has noticed this but if you photocopy a halftone image the machine adds a lot of noise into the image. It's as if the copier tries to sharpen text that isn't there its very hard to describe but the best I can say is it's like noise in the image.

The only mode that doesn't seem to add it is if you tell the machine the original is printed on photo paper.

I've noticed it on multiple machines so it's just the way they are but the image quality is so bad when copying halftones in text/photo mode you'd think the scanner was faulty. It's worse through the dp but still bad on the glass maybe half as bad as through dp.

I've seen it on 8052, 6052, 8353 machines. I'll try and get a sample and upload it tomorrow.

[PrintWhisperer]

Re: Copy quality poor when copying halftones

TL : DR - Try the 'Printed Original' setting for 'copies of copies'

If you take a nice magazine cover (offset ink) and get good results versus copying/scanning things that have been cmyk printed, then this setting is designed to compensate for that.

Because printed originals are made up of dots, and when you scan you are taking tiny 'pictures' of those dots (pixels) you can run into some uneven dot overlap scenarios (harmonics), and halftones magnify the problem because they are not strong enough to be seen clearly for their pure components (which are scanned in RGB, so there's that too).
PDF compression can further amplify the problem, introducing moire patterns from the harmonics.

Think of how a green patch is made on a color printer. There is no green, it's entirely CYK dots. So how does the scanner decide to make it green? If it's printed at 600dpi and I scan it at 300 DPI, which dots do I wind up taking that tiny picture of? In a perfect world, each scanned pixel would see 2 CY/K print pixels and be and exact shade of green.

BUT at .5x dpi scan rate I have a repeating harmonic and even the slightest magnification creates an uneven overlay of scanned pixels on printed pixels. So even though they all should be the same, they are not actually seeing the same thing.

It's all averaged out via various algorithms (dithering). Fiery training has a class on the various profiles and how they are used for reprinting the scanned image and changing the way it figures out how to render the dots in the 'uneven' areas.

Print, scan, displaying images..all of it comes down to 'What to do with dots'

[fishleg]

Re: Copy quality poor when copying halftones

TL : DR - Try the 'Printed Original' setting for 'copies of copies'

If you take a nice magazine cover (offset ink) and get good results versus copying/scanning things that have been cmyk printed, then this setting is designed to compensate for that.

Because printed originals are made up of dots, and when you scan you are taking tiny 'pictures' of those dots (pixels) you can run into some uneven dot overlap scenarios (harmonics), and halftones magnify the problem because they are not strong enough to be seen clearly for their pure components (which are scanned in RGB, so there's that too).
PDF compression can further amplify the problem, introducing moire patterns from the harmonics.

Think of how a green patch is made on a color printer. There is no green, it's entirely CYK dots. So how does the scanner decide to make it green? If it's printed at 600dpi and I scan it at 300 DPI, which dots do I wind up taking that tiny picture of? In a perfect world, each scanned pixel would see 2 CY/K print pixels and be and exact shade of green.

BUT at .5x dpi scan rate I have a repeating harmonic and even the slightest magnification creates an uneven overlay of scanned pixels on printed pixels. So even though they all should be the same, they are not actually seeing the same thing.

It's all averaged out via various algorithms (dithering). Fiery training has a class on the various profiles and how they are used for reprinting the scanned image and changing the way it figures out how to render the dots in the 'uneven' areas.

Print, scan, displaying images..all of it comes down to 'What to do with dots'

Yeah I figured it is something like that but didn't really understand why so appreciate the explanation.

We have a lot of schools who do booklets and they copy originals with a colour cover with a large picture on the front and they complained about the copy quality. It was almost impossible to get the image to a level they would accept as changing the mode would blur the text but give a good front cover.

In the modes it had 2 custom modes but I couldn't work out what they are for or what they do?

[davidmtupper]

Re: Copy quality poor when copying halftones

I think the problem often starts when the user prints a proof of their output and then uses that to make copies for the final output. If they would learn to print instead of copy whenever possible a lot of this type of problem would not happen.

[PrintWhisperer]

Re: Copy quality poor when copying halftones

... It was almost impossible to get the image to a level they would accept as changing the mode would blur the text but give a good front cover.

In the modes it had 2 custom modes but I couldn't work out what they are for or what they do?

For copying, IDK what custom you are referring to. The Imaging->Color reproduction may have a 'custom' option which is used to engage the alt. color profiles from MM485. This generally changes the 'hue' and not the density (to match Sharp/Ricoh/Fuji-xerox, gamuts)

Recommended available settngs would be:
Original Image -> Graphic/Map-> Printer Output
One Touch Image Adjust ->Vivid
Sharpness -> (Increase)
Saturation -> (Increase)
Contrast -> (Increase)
Trapping-> On (Heavy)

Copies of copies (of copies) will always degrade as scanning is NEVER 100% accurate.(Replicative fading) I've seen some amazing levels of this, where the color they wind up seeing is nothing like the original PC image. Oranges drifting all the way to purple.

If they cannot find the PC image which was originally printed, they generally have no idea what it is supposed to look like.