Mystery Scale on Hanimex Zoom Lens

A job of the lens is to alter the direction of incoming light rays in a way that caused these rays trace out a cone of light. The length of this cone, when imaging a far distance object, is called the focal length. We focus our camera by adjusting the distance, lens-to-film (or digital sensor). When the apex of this cone of light rays just kisses the surface of film or sensor, the image will be in sharp focus.

No lens is perfect meaning not all the light rays come to a focus at the desired location. There is a long list of lens defects that we call aberrations. Two such aberrations are called chromatic meaning some rays stray due to their color. Chromatic aberration induces a rainbow or colors that surround image of objects. The lens maker attempts to mitigate by constructing the lens using several individual lens elements (lenses). The idea is, some elements disperse light rays with opposite color errors. A well made compound camera lens well corrects chromatic aberrations.

The thrust of this effort handles the visual range of colors. Infrared rays are essentially ignored except when the lens must be optimized to handle IR. As a result, IR rays come to a focus further away from the lens then their visual cousins. The red index and red curved reference stripe is an offset that allows you to achieve focus when imaging in the IR spectrum.