I'm researching the world of photography outside of the visible spectrum and I'm really curious to get my hands on some filters, the Baader-U for example, to try shooting in these spectra. However, I noticed there's a problem -- these filters completely obscure visible light, making the viewfinder and liveview useless. Moreover, it's not simply possible to focus the image then apply the filter, since IR/UV converges at a different distance than visible light. The image will go out of focus when the filter is applied.
There are basically two ways to do this:
Autofocus works properly with an IR-pass filter in place, as does auto-assisted manual focus (you turn the focus ring, the camera tells you if things are in focus). You are effectively focusing in a low-light situation, however, as the autofocus sensor has an IR-blocking filter just like the image sensor does.
Many lenses, especially older ones, have an "IR focus" mark: a red line slightly offset from the visible-light focus mark. You can either focus using visible light, attach the IR-pass filter, and then turn the focus ring so the point that was lined up with the visible-light mark is now lined up with the IR-focus mark, or you can estimate the distance to the subject and set the focus ring directly.
If your camera has a live preview drawing from the image sensor, you can try focusing using that, but as noted above, it's a low-light situation and the display may not be clear enough to use.