Objective diameter is outside (housing) diameter of front side (objective) on rifle scope.
On most rifle scopes objective diameter is bigger than tube diameter. Keep in mind, the last number in the description of rifle scope tells you the size of the objective lens diameter (example: 2 – 7 x 32). Objective diameter is always bigger then lens diameter.
Objective diameter is important to know, when buying ring mounts. The bigger the objective diameter the higher mount you will need.
This information is also important for all users of clip-on devices.
Source: Revija Lovec