What does the red R in old manual lenses mean? [duplicate]

Question

This lens for example:

It has a R between the hyperfocal markings. What does that mean and how can I use it?

Note: To those who said this link is the same question as mine, the link questions about Red values and that's for modern lenses while there is no Red value in vintage lenses as you can see in the picture above, there is just a single R

Answer 1

So far the answers have not correctly explained just how to use this red mark. They talk about correcting the focus point by moving the focus setting. But they do not explain the need to make the original focus setting without the IR filter in place, and then replacing the filter and moving the distance number of the original focus point to the IR focus line. Reasons are at least two-fold: 1) an IR filter is very dark and difficult to focus through in the first place, and 2) if you do manage to focus through the filter, you are still using visible light to do the focusing since you cannot see IR light. So, you focus using visible light and then move the setting and the filter to accommodate IR.

Answer 2

This sign is used when you focus for infrared light (using IR film or filter). It can be in form of red dot or line. This is need because the IR light wave length is longer than visible light and you need to change the focal plane accordingly.

Here is photo from Wikipedia where you can see why you need to change the focal plane:

Answer 3

All lenses suffer from chromatic aberration. Its a fact that each color of light comes to a focus at different distances from the lens. If your camera lens is simple as to design, your images would be marred by a terrible rainbow fringe surrounding the outline of objects. This lens is not simple;it has been designed to mitigate chromatic aberration. This is accomplished by pairing a convex and concave lens. The convex one has a shorter focal length than needed. The concave one lengthens the overall focal length to specification. Each has opposite chromatic aberrations, thus they nearly cancel this menace. Thus, red, green and blue come to a focus at nearly the same distance downstream from the lens. Sorry to report that infrared is not corrected; it comes to a focus much further downstream. If you are imaging under IR light, you must apply a focus position correction. The red point on the lens barrel is a revised index point. In other words, you manually shift and set the distance to this index location.

Answer 4

That's used for Infrared focusing when using infrared film and red filter. Since infrared focuses at a different point than visible light you'd first focus normally then move the distance indicated at the normal focus point (red line at 5.6) to the IR focus point.