Point the camera at a white background, and observe this phenomenon with a magnifying glass.
将相机对准白色背景，然后用放大镜观察这种现象。
2
The opacity can depend on the distance to the camera, meaning that the images in the background can be a little transparent to give the feeling of depth.
不透明度可以根据离摄像机的远近来确定，也就说，背景中的图形要更加不透明一些，给人以纵深感。
3
Basically, it is the ability to use your camera and lenses to control the relationship between the background and foreground of your pictures.