Thanks Gordo. I think our issue leans more towards the optical bump (which is a term I am not familiar with but I have witnessed the phenomenom). If it were shadestepping I would typically up the resolution or add noise or both. What perplexes me is we can provide the pressroom with baby smooth gradients and we still see a "rattiness" on press with certain gradients-usually shorter ones.
"Optical bump" "tone jump" "intensity jump" "density jump" is a pretty typical artifact with certain halftone dot shapes. It is most commonly associated with the "transforming Euclidean" dot where the dots transform from a round shape (1%-49%) to square at 50% (with linear output) then go back to round for 51%-99%. This dot shape is arguably the most common shape in use. In Esko's case I believe it's called the Paragon Euclidean (short name: PAREUC).
Several strategies have been developed by vendors to mitigate the issue.
One is Esko's "Fogra-Round dots" (short name: R). This is a modified Euclidean Round/Square/Round dot. The optical bump at 50% is mitigated by elongating the dot shape at that tone so that the dots first touch around 45 % form a chain and touch for the second time around 55%. You end up with two optical bumps that are half the intensity and hence are less visible.
The most common strategy is to use an "eccentric" dot shape, typically Elliptical.
In Esko's case the Elliptical (short name: E) dot uses a more elliptical version of the Round Fogra dots. The first touching point is around 35%. Between 35% and 65%, a chain that looks like lines is formed. If you are using a low lpi screen then the chain may be visible and objectionable.
Esko's Rugby (short name: D) screening uses a modified Diamond dot shape with rounded edges. As with Elliptical dots the first touching point is around 35% and between 35% and 65%, a chain that looks like lines is formed. If you are using a low lpi screen then the chain may be visible and objectionable.
It looks like this:
Another alternative is using a non-transforming Round dot. Esko refers to this as Circular dots (short name: C). This screen design places the optical bump in the shadows at about 75%. Because it's in the shadows it is usually not visible. This dot shape may require a dot gain compensation curve because the small dots in the shadows can fill in without it.
Hope this helps a bit.