Mystery Scale on Hanimex Zoom Lens
I’m a bit of a newbie, and I’ve just bought a zoom lens for my OM10. It's a Hanimex One-Touch Zoom Lens 80-200mm F4.5 Macro
It’s arrived and I’m trying to understand the scales on the lens.
I can understand the usual focusing distance scale in green and white shown in the first photo, as well as the aperture ring nearest the camera body, and the focal length scale along the yellow line down the lens centre.
The scale I don't understand is the curved red arc that peels away from the centre line, denoted "R" nearest the body. I believe it has something to do with the Macro function of the lens, as it's in the same colour as the ratios and focal length shown in picture 2.
I'd like to try and better understand the red markings on the lens. I understand that zoom lenses aren't truly macro. However, I don't know why it states f = 200mm in red, nor do I understand the curved red line and the ratios it gives in red.
The behaviour of the lens is as follows:
A: sliding the focusing ring towards and away from the body zooms in and out respectively
B: rotating the focusing ring focuses as usual
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.
The red line is the adjustment in focus for IR light, as evidenced by the fact that it starts at the red R mark at the 200mm zoom mark. It changes depending on the focal length, so it would need to be adjusted differently as you change focal length on a zoom.