The TWO FIELD has a semi-circular close-up lens attached to half of the filter. It's used to bring both the foreground and background into the frame, roughly in equal parts, to achieve a pan-focus effect. It's a rotating type, like a polarizing filter, so it can be used vertically or horizontally. How it's used depends on the individual. Since it's an old item, there are scratches, dirt, and coating degradation to the extent that they don't affect practical use.