The internal construction of all zoom lenses is very complex and contains a wide variety of large and small parts. In fact, zoom lenses are usually made up of well over one hundred individual parts.
A conventional zoom lens is normally made up of three specific moving optical groups plus one stationary optical group, with each optical group containing two or more lens elements but many actually contain substantially more.
Within a zoom lens there are generally three independently moving groups of optical elements and one stationary group of optical elements. Some zoom lenses actually move all four optical groups when the lens is zoomed. These four optical groups are the focus, variator, compensator and master group.
The focus group moves forward or backward as necessary for focusing and zooming the lens. The variator group changes the magnification power of the focus group and is responsible for changing the focal length of a zoom lens when it is zoomed. The main purpose of the compensator group is to maintain correct focus whenever the zoom lens is zoomed. The variator and compensator groups work in tandem to create the change in focal length when the lens is zoomed, and the compensator group also ensures that the lens remains in focus while being zoomed. The master group relays the optical path to the camera’s focal plane and also enables the zoom lens to be more compact.
However certain zoom lenses, particularly very wide angle to moderate telephoto zooms, actually move all four optical groups as the lens is zoomed, meaning that every optical group in these is part of the overall zooming function and may perform more than one function.
Zoom lenses feature extremely complicated internal construction, as not only must the various optical groups have the correct rate of movement, but this must also be done precisely while maintaining optical alignment of the optical elements. This movement is achieved via a series of nested barrels or sleeves, which feature machined cam-slots and cam-followers.
The challenge for optical engineers is to optimise the lens by combining optical elements using a wide range of options to create lenses that perform exceptionally well.
