The opacity setting determines what the maximum value is going to be when you leave it on. If you have opacity set to low, then it won’t go any further than that. If you want to change that, you can either set the opacity to 100%, or change the “Painting Mode” setting (under the Airbrush setting) from “Wash” to “Build up” so that it keeps adding up the colour even with low opacity.
When I use the airbrush_pressure tool the paint build-up depends solely on how much I push down on the Wacom, like you describe. With the other airbrush_linear_noisy I can lay down strokes that build up in strength even if I set the opacity to low and do not vary the pressure.
It depends on the tool: hairy tapered and hairy_texture build up strength even if the pressure is light and by continuously pressing down. Others are pressure dependent.
The behaviour can be controlled in the “painting mode” settings in, for example, the hairy brush tool: build up and wash.
*edit I was too slow 
@Tyrant MOnkey: I have noticed the same here.
Have you played with the “Rate” value in the settings for your airbrush (under “airbrush”) ?
If i understand correctly what you say, the behaviour you are searching for is obtained for a value set under 15 it seems…
wish they’d knock off that filter layer preview, since it’s previewing nothing and its freaking hard to choose stuff.
It depends on the tool: hairy tapered and hairy_texture build up strength even if the pressure is light and by continuously pressing down
That’s not what i witness…
If I choose “Build up”, it only works the very next time i use the tool. After that very next time, it dosen’t keep adding up opacity…
I wonder if that’s been corrected in a recent build ?
Maybe I am too much in Photoshop mode but that is not how an airbrush tool is supposed to work. In Photoshop as long as you keep your tip down, it doesn’t matter if you started with a 10% or 40% opacity air brush, it will accumulate until it reaches a 100%. So effectively airbrushes should be build-up by default. I will check that setting on the brushes it see what it does. But airbrushes in most packages paint by accumulation.
@Gwenouille thanks I will also try out your suggestions.
It’s a good program but boy I am struggling with documentation a bit, the manual is worse than Blender’s.
@ Tyrant Monkey:
I just upgraded to 2.8.2 and i still witness the same odd behaviour.
Maybe you can confirm ?
New file.
Select Airbrush pressure as a tool
Try it: it doesn’t work as you want.
Now open the properties for that tool (at the top: Edit Brush Setting)
Go to the " Painting mode" line
Choose “Build up”
Now try it IMMEDIATELY on the canvas : it works !
BUT it only works once doesn’t it ? After you tried it once, it is back to ancient “static” behaviour
Can someone confirm ?
@Gwenouille I tried it out using the airbrush pressure changing from wash to buildup made no difference but than I think I accidentally stumbled on the issue. I am on Windows 7 64bit, 8gb ram, krita 2.8.2, Wacom Bamboo(old one). It’s the tablet that is the issue. I tried the Airbrush Pressure preset and it builds up opacity with the mouse to 1 but when I use my pen then it does one dab and that is it, my opacity and flow are set to 1.
So using a mouse the airbrush preset accumulates to the, I don’t know what you would call this, master or overall opacity but using a wacom pen doesn’t. Seems to be a bug to me.
Well, I have an intuos 3, and it is just the same as you. OK with the mouse, but wrong with the intuos.
I wonder if that has been reported…
I don’t think it has been reported.
I can’t get it to happen here, however, I do have an intuous 3 I lent to someone. I’ll be visiting them this weekend, so I can try to check there.
But it sounds like a really really strange bug though. Airbrush rate should not be affected by tablet code O_O.
Also, airbrush build-up is not disabled on any painting-mode, so it should work on both build-up and wash.
@Tyrant, what manual are you using? Because I am actually currently updating the thing, and you might just be reading my terrible first-draft quality writing 
EDIT: In fact, we’re actually trying to work together to get it filled up here:
http://forum.kde.nl/viewtopic.php?f=288&t=120792 Join us if you’d like :p.
Works fine on my Intuos 1!
@Therahedwig I will keep an eye out on the manual but as far as helping, I would probably do more harm than good I’m slowly learning how to use Krita I only started using it fully about two to three months back.
Yes, rather strange: i can see the green light on the top edge of the intuos when i press down, even slightly, telling me that I am actually exerting pressure down, and thus emitting a “click” signal continuously, just like when i keep pressing LMB down. It really should behave the same.
I’ve tried to register on the forum there (KDE identity) but I still haven’t received any confirmation mail…
In a way, I hope you will experience the same problem while visiting your friend with the intuos 3: that’d give more weight to my/our feedback.
For your information, I use 64 bits Krita 2.8.2 on Windows 7 64 bits.
Let me see for the Intuos drivers: 6.3.4-3
OK, I finally got my registration mail and posted the problem here:
http://forum.kde.org/viewtopic.php?f=281&t=120924
EDIT: same behaviour with latest 6.8.3 drivers…
I find that krita is incredibly unstable with the catalyst driver under linux. When switching to the open source radeon driver seems to get rid a lot of issues. At least for me it did.
I got it, but, I’m so used to Gimp that I have not really tried to use Krita. Krita 2.8 looks AMAZING. I need to just sit down and learn it.
Alright, the airbrush bug has been noted down here:
https://bugs.kde.org/show_bug.cgi?id=334282
Basically, the problem is that the intuous is too precise, and gives too many move-events. The airbrush setting sets the brush to only add on timer when the stylus is still. Because the stylus is never quite still on a intuous, the timer-based dabs are never added.
There’s a proposal to change functionality that must be looked at, I myself am edging toward airbrush meaning timer-based dabs only, because that’s more similar to how real-life airbrushes work. Anyway, posting this here so that those interested are notified. 
I’d favor timer-based too; at least that is what I expect “airbrush” to mean. I rarely use that functionality in any digital painting program though, so I don’t have a strong opinion on it either I suppose.
OK, all right, I understand it better now.
Yes, one would think it has nothing to do with movement. After all, a real airbrush continues to spit out, whether you move or not.
Thanks for your help !
Tried 2.8.2.0 on Win7 x64. After two strokes of calligraphy brush it not only crashed, but took the entire OS with it. I had to shut down the computer with power button. Even the uninstall process partially failed, because it didn’t know how to remove the shortcut (there was some error message). Well, I guess it was good that it didn’t make any real damage.