Often you will have the following problem with your images: Your photo shows an object, an animal or a person you want to focus on. However, both the object and the background appear sharp. It is a good idea to bring the target object into focus by blurring certain areas of the background while keeping the target object sharp. Thus, the depth of field of the background should be changed. The Background Blur function of the PostMatting menu helps here.
In the example image you can see two flamingos, which are obviously the target object of the photo. But also the background is sharp. It should now be slightly blurred to focus more on the flamingos.  If you select Lens Blur after the matting process from the Post-Matting menu, the Lens Blur menu appears:

(1) Radius Range: Here you can use the slider to define the radius of the blur effect. As you will see, a high value increases the blur effect of the background.

(2) Highlight: Use the highlight slider to define how much highlights of the background, i.e. bright image areas, should be highlighted. In the example image, these are the bright areas in the water where the sky is reflected. A high value makes these areas appear even brighter and thus emphasizes them.

(3) Aperture Shape: Here you can change the lens shape to change the type of blur effect. Try the different shapes until you find one that suits your needs.

(4) Rotate Mask: Use this slider to change the direction of the blur effect between 0 and 360 degrees.

(5) Depth Mode: In depth mode, you can switch between Plain, Horizon and Tunnel. Select Plain to get a smooth blur effect across the entire background. Select Horizontal to obtain a different amount of blur effect between near areas of the background and more distant areas. Select Tunnel to blur certain regions of the background. The grid below allows you to specify the region or, if you select  Horizontal, the position of the horizon from which you want to blur or sharpen. In our example image, we select Horizontal to keep the close areas of the background, i.e. approximately the area in front of the flamingos’ legs, sharp and to blur the more distant areas of the background. With the grid we determine the distance from which the background image should be blurred: Try a little around to become familiar with the features and understand the implications.

(6) Focus (close to far): This slider lets you ultimately reverse the depth-of-field effect. For the example image, in which the horizontal depth mode made the near background areas sharp and the far background areas blurred, a change from focus (close to far) from 0 to 100 would completely reverse the focus area. The close background areas would become blurred and the far background areas would become sharp.
When you are satisfied with your depth of field effect, click Apply and then OK.

In the case of the sample image, the result is as follows: As you can see, the close background areas and the flamingos are sharp, i.e. in focus, whereas the more distant areas of the background are blurred.In addition, the cropping result, i.e. the two flamingos with transparent background, is now available in the layer area.

So you can continue with the extracted object afterwards (edit, save etc.) if you don’t like the soft background. To display the flamingos without background, simply deactivate the background layer by clicking on the “Eye symbol”: