What is Infrared Index?

Going through the manual for my newly acquired EF70-200mm f/4L, what is Infrared Index? Manual mentions it relates to IR film, but does it have any significance when using a IR filter?

The IR index is the mark at which objects focus when shooting infrared using an IR filter. Ordinarily, if you have an object sitting, say, 20 feet away from the film (sensor) plane, you would align the 20 ft. mark on the lens to the focus index mark. However, if you are shooting IR, then you have to align the 20 ft. mark to the IR index mark.

Thanks for your clarification, Leo. But I am still kind of confused. Ideally, I would be turning the focussing ring to get the best possible focus. Now, if I rotate the ring to set this distance to IR mark, won't the image be OOF?

Light refracts differently depending on wavelength and the focal length of your lens (and therefore the proper focus setting) depends on it. That's why you need to shift focusing point to IR index marked on the lens. Image in the viewfinder, which is result of visible light, will look OOF, but recorded image produced by IR light will be sharp. That was procedure back in film camera days. I guess it is the same for digital.

It might appear that way, but visible light and infrared light focus at different points. Since you can't see infrared light, you really can't tell if the infrared image is in focus or not. Ideally, you would focus on something with no filter, then move the focus ring to the IR mark, attach your filter and take the picture (its very hard to see, much less focus, through the viewfinder when an IR filter is attached).

As a practical matter, you can usually ignore this whole thing altogether. Most infrared shots are wide-angle landscapes with small apertures, where the slight difference in focus will be taken care of with DOF.

Ah... that makes sense. Thanks for clarifying.