The standard binoculaurs used by crew on watch were Carl Zeiss Mk IV/1T 7x50's. The had a 50mm objective lens, weighed 2 lbs. 8 oz. without case, and were 7 inches long. Special filters could be fitted to the eyepieces to absorb yellow light, helpful at night when communicating with other boats by signal lamp. (See Gannon, Operation Drumbeat, p. 204-205).

U-boats also used a special binocular apparatus for attacking targets on the surface known as the UZO (Uboot-Zieloptic). These binoculars were Zeiss, Jena UDF 7x50 309 torpedo control binoculars and attached to a post on the bridge. They weighed 14 lbs. and had 14 inch barrels featuring swastika and eagle markings. They had sun and spray shades and featured a luminous aiming line for use at night. (Id. at 206). The scene in Das Boot where the tanker is attacked at night shows these binoculars in action.

I hope that helps.

Chris M.
Miami, Florida USA