Lens Spacers, Shims, And Focal Length Extenders

Applications can require a lens to stretch beyond its limits or be precisely dialed-in to its ideal design parameters. Lens spacers, shims, and focal length extenders are simple tools a user can easily use to achieve these requirements. These tools are used in between the lens and lens mount on the camera. Spacers and focal length extenders modify field of view (FOV) or working distance (WD) in fixed focal length lenses, whereas shims can be used to precisely control the WD of telecentric lenses.

Lens Spacers

Most fixed focal length lenses have integrated mechanics to focus on different WDs. The fixed focal length nature of the design means that the elements move throughout a defined range (and do not move relative to one another), which dictates the WDs where the focus is achievable. This predefined range is chosen based on the design of the lens. However, it is often advantageous to stretch a lens beyond its limits to fit an application where smaller FOVs or shorter WDs are required. Augmenting the system with a spacer between the camera and lens changes the range of WDs over which the lens performs optimally.

The main purpose of adding a spacer is to increase the vision system’s magnification or shorten the WD; these two changes occur in tandem and are explained by the Gaussian imaging equations.