Most thorium glass lenses from this era will have a slight yellowing. To reverse it, stick the lens under a UV lamp for a week or so and it will be good as new. The yellowing will affect overall light transmission and reduced contrast. With color films--It was crate a color cast. Here you can see I also polished the brass aperture ring. Its my lens--Im an eccentric shooter--Ill make it my own. If I ever want to spend the $700 for a perfect lens, I will keep it all stock and pristine.