as you can see in the picture, my red ball looks half transparent after render (left one). But why?

I tried to change the lens clip from 0,1 to 0,001 but it was not helful. In the later szenes, the red ball is ok.

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Lens clipping is for screen view, go to camera settings and setup Depth of Field properly. If the object is near set small distance when far set bigger. you can insert key frame.I found something similar about focusing camera maybe it will be helpful.
How to focus on the selected object?
If you turn of Depth of Field is not blurred you focused your camera on other ball.
Make some simple scene and make some tests with simple objects how they will behave.