Hello Bruce Sir, some aspheric lens elements are made by depositing optical plastic on the glass. Unlike a spherical lens element which is perfectly smooth and roundish, asperical lenses will have a number of jagged edges forming the lens curvature. Also Dupleix lenses (two lenses glued together) no longer use Canada Balsam due to separation and white patches over time. But they use modern and durable man made glue which is basically plastic compounds.
I have had this lens separation and white patches (not fungus) on the Pentax FA 28-70 f4 and a Tokina 20-35. Pentax DSLR lenses do not use optical plastic elements in the lenses, though some manufacturers do. The fresnel lens under the penta prism is made from plastic. Its purpose is to spread the light so that the corners in the viewfinder are not dark. Some lenses in front of the metering light sensors are also plastic. Regards. Bipin -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
