Why can't I dither?

[Chacaboy]

I am in CS 5 creating graphics without any images using the gradient tool in 40" X 30" X 300 px 8bit files on a Mac 10.7. The gradient tool is always set on "Dither", but I still get significant banding throughout the gradient but worse in the darker grays. The most popular fix I could find on-line was a downloadable action, which converts the file to 16 bit, creates a 50% gray layer on top, adds noise to the gray layer, puts the gray layer in linear light mode, reduces the opacity of the gray layer to 1% and the fill opacity to 19%, and names the layer "dither gradient". All the respondents on the list (mostly graphic designers) who tried it were successful, but it did nothing for me at all.

I've posted on this list with the same issue in blue skies, and have never completely resolved it to my satisfaction. I'm beginning to wonder if I should re-install my program but would rather not do that until I've checked with the experts on this list. Any suggestions for new fixes, or reasons this one won't work for me would be appreciated.

[Peter McLennan]

Are you working in 16 bit?  Try adding a very small amount of noise.

[kirkt]

Do you actually zoom into your document to see if there is dithering present in the gradient?  I set up a document to your specs (in sRGB) and made a black to white gradient with dithering enabled and added a curve adjustment layer to add contrast - there was dithering but it is not apparent (as it should be) until you zoom to 200-500%. 

Is your display capable of rendering a smooth gradient?

kirk

[Chacaboy]

Now I'm working in 16 bit which is much better, but the eight bit files cannot be re-built and needs a fix. Using less grain/noise in the overlay didn't seem to do it. In 32 bit, there is no banding at all, but those files are too big for my set-up. What I don't get is why the fix that worked for everyone else, doesn't work for me.

[Chacaboy]

The magnification and the display question is a good one. I can see banding at 25%. I'm using an Apple monitor. I made an 8 X 10 print and can see less banding than in the display, but I'll have to run a mag strip to see what it looks like at full size. One gradient black to white doesn't produce much banding. It's the gradients with say five white points on black with tight slopes that produce it. I'll have to run mag strips to see what this is in reality. Thanks for the tip.

[Chacaboy]

I've printed mag strips and found banding in both eight and sixteen bit file, even in a file that was created in sixteen bit. It's not as bad in 16 bit as in 8 bit but it's still there, and I have not found a fix. The grain overlay doesn't work for me. Any ideas out there would be appreciated.