What does the ft-m data on Canon 17-40 lens mean?

Look at your picture, it's not like the focus ruler of 17-40, but more like the focus ruler of 24- 105.

First question, ft is the abbreviation of feet, m is the abbreviation of meters, 1ft = 0.3048m ... This is from the focal length. The scale of the lower vertical line is the current focal length, that is, the distance between the current focal plane and the camera photosensitive element. Simply put, if the red line below the focus scale points to 5 meters, that is to say, something photographed 5 meters away from the camera is clear (in focus). The ∞ symbol stands for infinity.

The second question, the following 50, 35 and 24 represent the compensation value of infrared photography, which is the current focal length and can be ignored in normal use. But in the film age, there is a kind of film called infrared black-and-white film, which is very sensitive to infrared light invisible to the naked eye and has special advantages in shooting landscapes or night vision photography. However, the imaging principle of infrared photography is slightly different from that of visible light imaging, so compensation is needed in photography. The method is: focus first, remember the distance pointed by the focusing scale after focusing, and then manually twist the focusing ring according to the current focal length to make the infrared marking line point to the focusing distance. For example, at present, when shooting with a focal length of 35mm, we should first focus according to the normal method. If the normal focus scale points to 5m, you need to manually twist the focus ring when shooting, so that the 35 infrared reticle is aimed at 5m of the focus scale, and you can take a clear picture.