Posterisation in reprint despite identical image files

[kasra]

Greetings,

I'm sorry in advance if this is not the correct forum to post this question.

I needed to print a black-and-white photobook quickly, so I used an online printing service rather than preparing the entire layout in InDesign for a local print house.

I ordered one proof. The print quality was acceptable. I made minor text edits only (no changes to image files) and ordered the full batch.

The second batch has a clear posterisation issue in the shadows. (Attachment shows proof vs final prints.)
The print provider claims this happened because parts of my images fall below RGB 10,10,10, stating:

“Abrupt transitions in dark areas can occur when subtle gradients fall past the threshold our presses can reproduce, causing dark greys to clip to pure black.”

My concerns:

1. The same images printed correctly in the first proof.
2. The artefacts look like posterisation and loss of midtone transitions, not just shadow clipping.
3. It also resembles tone remapping or forced brightening that exposes retouching artefacts.

My questions:

• Is their explanation (shadows below RGB 10 causing this issue) technically valid, given the results?
• Based on the samples, does this look like shadow clipping, poor tone mapping, digital press posterisation, or something else?

I want to understand the root cause before adjusting files based on their guidance.

Technical details:

• No ICC profile provided by the printer. RGB only uploads (no CMYK).
• Files are sRGB, 16-bit TIFF, high resolution.
• Test prints on my photo inkjet look clean.

Thanks,
Kasra

 

[Ynot_UK]

Why are you generating output for print in the RGB colour spectrum?

 

[kasra]

Why are you generating output for print in the RGB colour spectrum?

Hi. This is because this service provider uses its own proprietary application for photobook page layout, which accepts RGB only. So unfortunately, I don't have any control over the conversion process.

Kind regards,
Kasra

 

[SteveSuffRIT]

For this example, I prefer the 2nd print on the left, it has some shadow detail.
The first print on the right has no shadow detail.
An image with a 10 gray scale pixel would be about a 96% dot size.
A traditional sheetfed offset litho press would probably reproduce this as 100% (plug up, fill in). Maybe the digital term is "clip".
Test prints on "your" printer mean nothing, it has to be "proofed" on their production digital press.

 

[kasra]

For this example, I prefer the 2nd print on the left, it has some shadow detail.
The first print on the right has no shadow detail.
An image with a 10 gray scale pixel would be about a 96% dot size.
A traditional sheetfed offset litho press would probably reproduce this as 100% (plug up, fill in). Maybe the digital term is "clip".
Test prints on "your" printer mean nothing, it has to be "proofed" on their production digital press.

Hi Steve,

Thank you so much for your advice.

Please allow me to give some context:
- The darkness in that part of the photo (this is a small clipping of a larger photo) is intentional; I wanted it to be dark, as it's not the main subject.
- I understand the issue of clipping. But the problem with the left print is that the tone curve appears to have been manipulated. That is why I'm interested in finding out what has happened. I was happy with a "clip".
- I did the first print round with the intention of using it as a proof. I was happy with that, considering what can be expected from a consumer-grade print house - but they ruined it for the second print run.

Kind regards,
K

 

[SteveSuffRIT]

Maybe you can send them a gray scale (continuous or stepped, 0-255 GL or 0-100% dot), have them print it, and then you will see the effects of their tone reproduction curve (compression, bumps, cutback, clips)?

 

[kasra]

Maybe you can send them a gray scale (continuous or stepped, 0-255 GL or 0-100% dot), have them print it, and then you will see the effects of their tone reproduction curve (compression, bumps, cutback, clips)?

Hi Steve,

Thank you very much for the advice. Yes, this can be an option for future jobs. But I'm thinking of switching to a different print house with services better suited for professionals.

Kind regards,
Kasra