Here's a simple way you can visualise the shape of any convolution filter, such as a blur or defocus. Easy in Fusion, but the same approach works with most image processing apps.
- In a new comp, start with a black background, about 64 x 64 is fine, and make it float32.
- Add a single white pixel in the middle of it (I used Paint with the pixel brush).
- Now add a tool that does filtering, such as Blur.
- Normalise the image (in Fusion you can use the AutoGain tool, or just click the Show Normalised Image button on the image view toolbar, at the right).