I must apologize for two things. I was busy today, so this response is delayed. Second apology is that it was arrogant to think you do not know what binoculars are. Truth is, I did not notice the thing that looks like a large lens, on the railing of the U-Boat's conning tower. I still do not know what that thing is. I looked through lots of Google photos and three books that detail U-Boats in World War II. I still have not found anything that looks like a large magnifying glass, mounted at eye level on a U-Boat.
In 1942 and 1943, there were large metal rings installed on some U-Boat conning towers. However, the ring is larger, about one meter in diameter and does not contain a glass lens. These rings are also mounted nearly three meters above the deck on the top of the conning tower. They were an aerial, that could detect meter length radio waves, used on the earliest RAF Coastal Command air search radar. It was useless in detecting centimeteric length radar waves, so U-Boats built after the summer of 1943 did not have these radar detectors.
I also found out, U-Boats had two periscopes. One was the standard periscope, used to plot a submerged torpedo attack. The second periscope was smaller and had a fish eyed lens and was used to scan the skies for airplanes, when the U-Boat was on the surface. These second, air-search periscopes were only installed, after the Germans decided to augment the flak battery on their U-Boats, in late 1942.
I also noticed the German Navy did install one pair of oversized binoculars on their U-boats. They were to big for a sailor to wear around his neck and required a mono-pod to be installed on the conning tower, so the U-Boat could plot a night time surface attack.