En angliche, sur le site d'Henry Scherer, à propos des "Zeiss bubbles" :
"Why the Bubbles in the Lens Glass?
Modern glass making has changed a lot in modern times. It wasn't so long ago that the bubbles in the glass were a sign the lens was of the highest possible quality. How quickly we have gotten used to photographic lenses that do not have bubbles in them. When optical glass is made all of the ingredients are mixed in a container that can take high temperatures called a crucible. The ingredients are melted at very high temperature. Even when it is melted, glass is very viscous and thick. When the melted ingredients are stirred to mix them together, the mix tends to pick up air which forms bubbles and the ingredients used in the older optical glasses tended to generate gasses as they melted. The bubbles are removed from the glass by allowing the crucible to sit in the high temperature furnace while the bubbles rise to the surface of the melted glass. Up until about 25 years ago the materials of which crucibles for optical glass making were made could not withstand the effects of the extremely high temperature at which glass is made for very long. There was a time limit on how long the crucible could stay in the furnace. In order that the crucible not fall apart it was necessary to remove the crucible from the furnace before all of the bubbles had time to rise through the melted glass and escape from it. Another problem is that the melted glass was corrosive to the inner surface of the crucible. It was necessary to remove the crucible from the furnace before corrosion of the inner surface of the crucible caused impurities to ruin the batch of glass. Another problem is that making the highest quality optical glass required higher temperatures than did lower quality optical glass. So it was possible to make lenses without bubbles, but only if the glass was of a lower quality. Back then, no bubbles in the glass meant it was cheap.
The overall effect of these factors was that for a very long time small bubbles in the glass of a lens were an indicator of the highest quality. It is still such an indicator today for lenses which were made prior to around 1975. Sometime around 1975 crucibles for optical glass making were developed which could withstand the high temperatures required for making the highest quality optical glass long enough so that the molten glass could be left in the furnace until all of the bubbles had risen out. These crucibles were also resistant to the corrosive effects of molten glass on the inner surface of the crucible. Since then high quality optical glass has been virtually bubble free.
So when you are looking deep into a Zeiss lens for your Contax camera, and you see some little bubbles deep inside, rejoice because you have found a rare and special jewel that will take most excellent photographs. The bubbles are photographically insignificant and do not affect the quality of the pictures the lens will take. They are merely an indicator of great quality.
Also keep in mind that when a lens blank contains bubbles it is only natural that during the grinding and polishing process where glass is removed that some of these bubbles will become partially exposed. These exposed bubbles will appear to be small tiny pits on the surface of the glass on both the front and rear elements. These are natural, to be expected and cannot be avoided. They are an artifact of the lens glass being superior. "
