" The iris shot further amplifies the existence of the peephole, invoking the sense of wrongdoing and guilt within them. Fear and suspense stems from within, the sudden revelation that one can be bound to invasion of privacy unknowingly makes one feel vulnerable. At this point, the audiences are led to conjure the possible petrifying consequences of Bate's next move since they are able to identify with his sexual arousal. .
The sense of guilt generated by Bate's peeping is significant here as it awakens the "mother" within him; the ignition of her jealously is the main instigating element of the perturbing shower murder moments after. In fact, when Bates hurries back to the sanctuary of his home, seemingly stumped and lost in a world of guilt, the deep focus and long shot of the corridor leading to the kitchen seems to resemble "Mother's"" continual surveillance and judgment of his actions as well as his state of mind. The existence higher being watching and supervising is thus implicated.
Prior to the stabbing of Arbogast, the birds-eye-view (perhaps coinciding with the bird motif) one gets from the extreme high angle grounds a helpless and shocking effect on the audience in which Hitchcock had intended. The audience would then be able to witness the events happening from both sides of the screen: on the left, they watch in anxiety as Arbogast ascends the stairs, on the right, the anticipation of the murderer bursting through the door, reinforced by the medium shot of the opening door before that scene, elevates the sheer impact of terror. Hitchcock himself assumes that the audience would know what Arbogast's impending fate would be, and thus concluded that "the upward movement would already be established when mother runs out from her bedroom.".
In order to intensify the audience's involvement as voyeurs and to invoke a stronger sense of realism in their witnessing events, it is said that Hitchcock insisted on the use of 50-millimeter lenses through the shooting of the entire movie.