adjust the camera aperture to obtain the depth of field, but how to display best image quality

Development Rendering
1

When I use cinecamera adjusting aperture to obtain the depth of field, the quality of the image display is very poor, some noise, It’s not smooth.
It is no problem,when the aperture change to 22, but I don’t have the depth of field.
How to set up, make the image display is normal, including rendering eventually is normal,the best quality

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Cmd: r.DepthOfFieldQuality ?
HELP for ‘r.DepthOfFieldQuality’:
Allows to adjust the depth of field quality. Currently only fully affects BokehDOF. GaussianDOF is either 0 for off, otherwise on.
0: Off
1: Low
2: high quality (default, adaptive, can be 4x slower)
3: very high quality, intended for non realtime cutscenes, CircleDOF only (slow)
4: extremely high quality, intended for non realtime cutscenes, CircleDOF only (very slow)
r.DepthOfFieldQuality = “2” LastSetBy: Scalability