An add-on lens assembly supporting an add-on lens for a camera includes a locking mechanism actuatable by the user in order to increase the attachment force between the lens and the camera. The add-on lens assembly may include a rotatable lock ring having a cam pin that navigates along a surface of a cam such that when the lock ring is rotated, the cam pin causes the cam to translate axially. Axial translation of the cam may compress a spring within the assembly and increase the attachment force between the lens assembly and the camera. The increased force between the add-on lens assembly and the camera may provide additional support for large or heavy assemblies.