A convertible lens is a lens consisting of at least two separatable parts, each containing a part of the lense's elements. 100 years ago these lenses were a common option for photo amateurs to give a double or triple extension folding camera more versatility. The common lenses of that era consisted of a pair of lens tubes, one to screw into the back of an aperture/shutter-unit, the other to be screwed into its front side. Mounted that way the lens element groups of both lens tubes gave the complete standard lens, usable with single extension of the camera bellows. Mounting only one of the lens tubes meant to switch to a tele focal length which needed double or triple bellows extension.
More sophisticated convertibles made it possible to exchange the front tube while leaving the back tube mounted without need to extend the bellows. Therefore several different front lens tubes had to be used to achieve different focal lengthes. The most successful related camera conception was a rigid body SLR, the Contaflex.
Some modern compact cameras have a double-focus lens instead of a single-focus or zoom lens. In most cases the focus switch of such cameras shifts one lens element into (or over), or vice versa out of (or away from) the lens tube.