To turn on focal blur, you must specify the aperture keyword followed by a float value which determines the depth of the sharpness zone. Large apertures give a lot of blurring, while narrow apertures will give a wide zone of sharpness. Note that, while this behaves as a real camera does, the values for aperture are purely arbitrary and are not related to f -stops.
More rays give a smoother appearance but is slower.
Objects close to this point are in focus and those farther from that point are more blurred. At that point, shooting more rays would not result in a significant change. The confidence and variance keywords are followed by float values to control the adaptive function. The confidence value is used to determine when the samples seem to be close enough to the correct color. The variance value specifies an acceptable tolerance on the variance of the samples taken so far.