A previous post on zoom and focus movement methods described threaded assemblies. In this post we will discuss how a cam system works. There are three main components: a front lens group, a rear lens group and a cam mechanism.
The front lens group is attached into a front carriage and the rear lens group is attached into a rear carriage.
Both carriages move along rods fitted into a lens housing and parallel to the optical axis.
A typical distance for slider bearings and the guide is just 6 to 10 microns (0.00025 to 0.0004 in.). The reason is that with larger clearance the image can jump causing it to go out of focus when the zoom motion reverses.
The smaller the angle of ascent of cam slots the bigger will be the magnitude of the backlash of the focus ring when the zoom motion reverses.
The advantage of the cam system is that it allows you to move more than one lens group by a nonlinear law of a motion at the same time.
A front lens group is moved according to one law of motion and a rear lens group is moved on another one.
The disadvantage of such a design is the high manufacturing precision required because manufacturing errors in the helical cam and backlashes in conjugations between cam tracks and cam followers.