Fast lenses let you shoot at a very opened aperture but make sure that you focus at just the right spot as there is extremely shallow depth of field.
Although the maximun aperture of 85mm lens is 1.8 but the camera has been set to f 4 to ensure enough range of focus.
Although the 50mm lens could be opened to 1.7 its been opened to f2 only and the focus has been bang on the subject.
85mm f1.8 lens with exposure at 1/60 at f 2.5
Especially when shooting groups who are not standing at exactly the same distance from the camera it is preferred to close down a bit so as to increase the depth of field.