Comparing different design approaches to zoom lenses.

Notable benefits of zoom lenses include versatility, offering a range of focal lengths in a single lens, and the convenience of portability and the ability to capture images without changing lenses. The most popular approaches to designing a zoom lens include manual focus, motorised / focus, tracking and more recently optically compensated While all these design approaches provide the ability to change the focal length within a range, each type of zoom lens has its place for different applications.
Wide angle lenses – advantages, disadvantages and applications

Manual Zoom
A manual zoom design allows the magnification / focal length to be changed manually and then the image can also be focused manually. This is an inherently simple approach to creating a zoom lens. An example of this type of zoom lens is the manual SLR zoom as used by many photographers. Such a design provides user with the control to quickly frame the shot and achieve a sharp focused image.

Motorised Zoom / Focus
The key advantage of this design approach is that when designed in conjunction with the camera the lens can be kept very compact. Typically, the optical cells will be mounted on rails and stepper motors are used to accurately position the cells according to a look up table and feedback from the camera. An example of this design type is the Sony Block camera. The motorised zoom / focus approach achieves large zoom ranges (typically X40) and can utilise digital and optical zoom together to further increase the zoom range. A shortfall of this zoom lens design approach is that it is very difficult to ruggedise.

Tracking Zoom
The key benefit of a tracking zoom lens is that it holds focus throughout the zoom range. This approach is useful if the subject needs to be kept in focus when zooming. A tracking zoom is designed independently of a camera and uses linked cam tubes to drive the zoom and focus cells precisely to maintain the image quality. This type of zoom lens does need to be set up on the camera to achieve the accurate tracking. Tracking zoom lenses are typically used in industrial applications where the object distance may change but focus must be maintained. Tracking zoom lenses are also popular in CCTV applications. Resolve Optic’s largest market for tracking zooms is the nuclear industry where these versatile lenses are used for CCTV monitoring and inspection in areas where access is restricted and the zoom can be used to view objects further away. Beneficially tracking zooms designs are also relatively easy to ruggedise due to their construction.

Optically Compensated Zoom
The above zoom lens designs rely on a cam mechanism to control the individual movement of the different optical cells within the lens to maintain focus throughout the zoom range. To ensure smooth zoom movement these moving parts greased and this can be a problem in harsh environments or in space applications. By comparison, optically corrected zoom lenses, also known as optically compensated zoom lenses) are simpler in design relying on two optical cells moving together each side of a fixed cell. Advantageously this movement is linear so there is no need for complex cam mechanisms. The movement in an optically corrected zoom lens is effectively a push-pull movement. The benefits of an optically corrected zoom lens design include simplified construction. They are more rugged than traditional zoom lenses and no greases are required for smooth zoom operation making them ideal for space applications or harsh environments. However, optically corrected zoom lenses do have limitations – they are not suited for applications requiring a large field-of view or where a zoom range greater than x4 is required.

To learn more about application optimised zoom lenses please visit https://www.resolveoptics.com/application-optimised-zoom-lenses/