I know that the little red numbers under the distance scale are Infrared Index. According to the lens's manual, you should use these numbers when you do infrared photography. However I don't understand why I need a different distance scale numbers. Also, how exactly I'm supposed to use them when shooting?

Canon EF 24-105mm F/4 L IS USM


IR light is not bent quite like visible light when it passes through the lenses. This is why you need a separate IR focusing mark in the first place. You have one IR mark for shooting at 24mm, one for 35mm and one for 50mm on this lens. Obviously, IR does not bend consistently at the various focal lengths of the lens either. Neither, strictly speaking, does visible light (the 24-105 is not a "parfocal" lens) but this is much less noticeable.

As for usage: Zoom the lens to your desired focal length, let us say 35mm. Now, measure or guesstimate the distance to your subject. Let us say 5 meters. Manually focus the lens so that the 5m indicator in the focus distance window on the lens is aligned with the red "35" IR index mark. Take your photo.

Of course, you will need a film camera with special IR-sensitive film to do this, or a specially modified digital body, plus an IR filter on the lens which blocks out everything except IR light. (The image sensor in a normal digital camera is quite insensitive to infrared.)

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