A well designed housing back is sculpted around the camera to take advantage of this. However, some divers find that they still have to move their eyes slightly to see into the extreme corners of the screen or that the bulk of their regulator interferes with good viewfinding. An option to overcome this is to use an image reducer. Reducers are accessory optics that screw onto the cameras own viewfinder. They work by reducing the size of the image to include the corners.
Reducers are a compromise solution.
Severe reduction does indeed make it very easy to see into the corners but may impair reading viewfinder displays and focusing manually if a housing offers this facility. Use minimal reduction and you may still have to move your eye to see the screen borders. Subal's reducer is a medium reducer. It is also eyesight adjustable which helps. The use of image reducers is controversial.