lens width
lens socket width

The second number is the objective lens width, measured in millimeters.

Camera lens is directed straight down giving a bird eye view (see lens width of vision at bottom of page) Power Pack supplied.

He scaled down the proportions to fit the lens width available to him; nevertheless, the finished set was still too large for his studio, and was therefore set up outdoors, against the wall of his own garden in Montreuil.