In the part where Lydia is taking pictures outside of the house while they are moving their belongings into the house. She looks up at the window and sees the ghosts through her camera, and then looks without her camera. If you notice the model of the professional camera she is using, the image she looks at through the view finder is the reflection from a mirror inside the camera. If they can't have reflections, she shouldn't be able to see them through her camera.