As you learned from the previous section on filters, when you attach an infrared filter to a lens most visible light is blocked. This means that when you look through the viewfinder, you won’t be able to see anything. As well as the obvious compositional difficulties this presents, the infrared filter also renders the camera’s autofocus system useless.
Taking into consideration the fact that wavelengths focus at different points (see ‘Other Basic Theory’), it can be challenging to get correct infrared focus.
The most ideal scenario is to have infrared focus marks on your lens, although most manufacturers now omit these. To use infrared focus marks, you must first set your focus before placing the IR filter over the lens. After attaching the filter to the front of the lens, you then have to ‘shift’ the focus by the amount shown using the infrared focus marks. If you have a lens that moves between two focal lengths (for example a 70-200), the only thing you can do is to estimate where the infrared focus point lies between the two marks for the minimum and maximum focal lengths.
If your lens does not have infrared focus marks, you can add your own to it by continually trying to achieve perfect infrared focus using a trial and error approach, then making small marks on your lens. Alternatively, if you are having difficulty with this, a different option is to set your aperture to f/22 (or the highest you can go) so that you have a large depth of field and hopefully your subject will fall withing the area in focus.