You animate AE's 3D camera properties or apply blur effects to the sliced up photo elements and animate the values.
For that specific shot I would enable depth of field, set the aperture to achieve the softness desired and then animate the camera position and focus distance and/or aperture as needed. These camera controls behave exactly the same as a real camera set with automatic exposure set to Aperture Priority. Changing the focus distance and changing the aperture changes the range of focus, the larger the aperture the more shallow the depth of field, and you don't have to worry about the exposure.
These are all basic skills you should hone to become a competent AE artist.