A value of 100 corresponds to an aperture radius of 1 meter. We are assuming that most renderings need a much lower value than that. Maybe the size of your scene is wrong, if it is too big then the depth of field becomes larger.
hello roeland, in fact it is arch scenes, and I want to make shift tilt effect without changing scale of my scene (kind of lazyness).... if it is not a problem to increase this max value, I would be happy