Archived posting to the Leica Users Group, 2000/04/22
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]> Hi All, > > I was checking out a non-leica LTM 28mm lens. The coating looked odd. Almost > rainbow variations in hue in circular pattern like a bullesye target. > Could this be oil onthe back of the front element, or a blistering of the > coating? What you are seeing is probably the interference pattern caused by the inexact matching of radii of cemented surfaces. If the pattern is concentric, I wouldn't worry too much about it. If you see straight lines (which you don't in this case) it is an indication that centering of the two cemented elements are off. The number of lines tells you the difference between the minimum and maximum separation. It is best observed under a sodium illumination or laser. You can multiply the number of lines by the wavelength. I do not observe this interference patterns in my Leica lenses, but it is quite noticeable on my Mamiya 6 75mm f:3.5 lens. It tends to occur more frequently on Plasmat-type (aka Orthometar) lenses because the cemented elements are the largest in the group.