English: Some older photographic lenses were produced with glass elements containing small amounts of thorium. If stored in the dark for prolonged periods of time, these lenses can undergo what is commonly called “yellowing”. While some photographers enjoy the effect the yellowed glass has on their photographs (it acts similar to a weak yellow filter), others prefer to reverse it by exposing the lens to a UV light source.

• left: a yellowed Super-Multi-Coated Takumar 50mm F1.4
• center: an identical lens that had the same amount of yellowing, but was treated with a LED-based UV-emitting flashlight for several days until the yellowing almost disappeared.
• right: an Auto Yashinon-DS 50mm f1.9 that showed no signs of yellowing for comparison

Please note: Not all lenses that appear yellow do so because of “yellowing” – many lens coatings do look golden/yellow/brown as well.