>>296651Standard digital cameras have an infrared filter attached directly over the sensor. This is basically the opposite of the 'Infrared' filters that you can get to stick on your lens--it actually blocks out infrared light, whereas the others block everything-but-infrared. I'll call the type that generally goes on the lens an R72 so I don't have to keep saying "IR" filter in two different contexts to mean two completely different things.
So if you put an R72 filter on a normal digital camera, the R72 blocks out most of the visible light and the IR filter on the sensor blocks out most of the infrared light, which leaves very little light actually hitting the sensor, which means annoyingly long exposure times wide open with a fast lens at ISO3200 in broad daylight.
You can, however, get your camera modified for IR, which basically involves removing the IR filter over the sensor and replacing it with an R72-type filter. So then the only filtering is of visible light, with lots of IR light hitting the sensor (digital sensors being naturally sensitive to infrared). The advantage of that is that you can actually see through your lens to focus and frame (the R72 blocks most visible light from your eye, too), and exposure times are reasonable again.
Downsides:
1. Focusing gets a little wonky, especially at wide apertures. Infrared light doesn't focus at quite the same spot as visible light, and lenses are optimized for passing visible light. The good places that'll IR-modify your camera will also adjust the AF so it works with your lens...but they can't adjust it so it works with ALL of your lenses (maybe if you have something like the higher-end EOS models that let you specify a per-lens focus adjustment).
2. If you want to take a non-IR photo, you're screwed.
