These lens caps are for optical instrument such as binoculars, telescopes and spotting scopes and accessories (eg. eyepieces, Barlow lenses and other accessories).
The sizes of the lens caps are measured as internal diameter of the caps, which is equivalent to the external diameter of the lens including the lens housing.
Cap Size: Internal Diameter 42mm x Internal Height 8mm
Note: The size of the lens cap is measured as Internal Diameter of the cap, which is equivalent to the external diameter of the lens including lens housing (as shown in the picture above). Please allow a small difference in measurement (eg. 0.5-1mm). A rubber cap can normally fit a slightly bigger lens than the specified size (eg. +1mm to 2mm depending on actual size).
If a cap is slightly bigger than the external diameter of the lens housing and there is no matched cap size available for the lens, you may consider putting an adhesive cloth tape around the internal edge/rim of the cap. This can often secure the cap on the lens.