On the Fifth Drop of Project Zero: Maiden of Black Water, after you’ve found Rui in the Shrine of Dolls and entered the Womb Cavern, you’ll find a locked heavy lattice door at the eastern end of the cavern. You need to get through this door in order to reach the Veiled House, but you don’t have a key.
Fortunately, your Camera Obscura reacts to the door, so switch to photo mode and take a Psychic Photograph of it (here’s a reminder of how to do that, if you need one). The resulting snap will look like this:
That’s the exact same room you’re currently standing in, the only difference being that one of the boxes is open. In order to get that box open in the real world, you need to check each of the boxes in the room. Once you’ve done that, one of the boxes will open and a hostile ghost will come slithering out.
Defeat the ghost, then check the open box to find the Iris Plate Key, the design of which matches the lock on the heavy lattice door. So use this key to open the door, and make your way through to the Veiled House.