This refers to cameras that take the photograph and allow you to view the subject through the same lens. This is possible by the use of a prism and a mirror. The mirror is located behind the lens and reflects the image up into the prism. It is the prism that corrects the image back to your eye so that the image is right side up. This combination provides an erect image, that is, the image reaching your eye looks the same as the scene as viewed without looking through the camera. What is on your left, is on the left side of the image, what is in your right, is on the right side of the image.