Forum: Pentax SLR Lens Discussion
05-19-2015, 05:49 PM
|
|
Color cast with a particular lens is not unusual and is a signature characteristic of some lenses. For many years, Sigma lenses had a characteristic warm cast. Other lenses tended to be "cool". There are implications in regards to color balance and color response, but for the most part, a mild cast does not affect general photography.
Steve
|
Forum: Pentax SLR Lens Discussion
05-19-2015, 05:40 PM
|
|
There are no thorium-element K-mount lenses. Use of radioactive rare earth glass was phased out in the late 1960s and early 1970s with the last of the M42 Takumars. In addition, the color cast in the earlier Super Takumar lenses is not due to coating deterioration. Initial thought was degradation of balsam cement between the elements, but current thinking is a change at the molecular level as a direct result of the radiation. Why the UV bleaching works is also poorly understood.
Steve
|