In this tutorial I'll explain how I control my blur effects. I use Krita 2.9 and G'MIC filter, you can also use krita's default blur filters too, but G'MIC produces a cleaner results and has many more options. Similar blur filters are also available in photoshop (I'm not certain if it has all of them, but at least the basics (gaussian, linear, depth of field).
Blur effects can enchance your drawings if you want to create a focus point in your image, increase depth of field effect or display motion. Angular blur can give a nice focus point, depth-of-field is self explainery, gaussian blur gives an unfocused effect and linear and radial is useful for motion effects.
First of all, never create your blur effect on the same layer as the one with the sharp picture, that usually looks awful. Instead dublicate your layers and make sure the bottom one is the entire sharp picture. When you draw you can also put to-be-blurred objects on their own layer with transparent background! Blurring them will reveal some of the layer below so make sure there's some content!
In this tutorial I'll use angular blur on an entire picture. I've dublicated the piece into three layers, bottom will stay untouched and the ones above will recieve a blur effect of different strenght! Then I'll erase bits of the blurred layer to reveal the sharper layers below.
The Blur Filter
Here's what G'MIC looks like and where you'll find the blur effect in question. Amplitude is how strong your effect is, on my middle layer I'll only make a very weak effect. X and Y will define your center point of the blur. On my third (top) layer I'll give the same settings but higher amplitude only.
Control the Effect
After having the two layers blurred, take airbrush and turn it into an eraser (or use "airbrush eraser"). It's good to use a big airbrush for this, so that the areas you erase don't display ugly edges but transist smoothly into the layer below (the less blurred layer!)
1. Untouched picture, with all layers visible, you should only see the top heavily blurred layer.
2. Erase to reveal middle layer, the green area defines the area I erased to reveal the lightly blurred middle layer.
3. Erase middle layer to reveal sharp spots, the green area defines what I erased from the middle lightly blurred layer to reveal bits of the sharp layer below all.
Sometimes blur effect can ruin a piece too, always compare an coinsider if a lighter effect looks better. People generally love to look at details in a piece and a blurred piece can be sad to look at.