The larger of the two is for the viewing lens and the red dot on the face has to be positioned at the top after you have clicked it into place. This is the parallax correction. If you have a rubber lens shade you can put it on the bottom before or after you have both parts of the Rolleinar in place. If you have the metal lens shade You have to have it in place over the bottom lens before you put on the top lens if you want to use it.