Through design, these lenses force light waves of different wavelengths to focus on the same plane. As you can imagine this results in a far sharper image. Most lenses, however, can't tackle infrared wavelengths and make them focus on the same plane as visible light. This makes it necessary to shift the focus of our lenses when we attempt to shoot infrared film. To make this easier, lens makers have put an infrared mark on most of their lenses to aid in this shift.
Exceptions are most autofocus lenses and many zoom lenses.
If you are new to infrared you may not have seen this focusing aid so let's look at what and where it is. Check your lens. To focus using this, focus as you usually do, and then look down at your lens. In the middle of your depth of field scale you will see a centering mark. This centering mark will be pointing at a distance on your range or distance scale opposite of the depth of field scale.