AF in infrared light

General question:

Can Canon's AF system on the 5D mk II or any EOS digital camera focus in pure infrared lighting? Or does it need visible light in order to detect contrast and focus? I know the flashes and even the ST-E2 use that projected red pattern, but that is visible, or is some of it infrared also?

The red focus-assist light is mostly infrared, but as you notice some of it is also visible. A deeper IR filter may well permit you to focus in visible darkness (if you see what I mean), but it would also dim the IR portion, probably limiting the range of useful AF assistance. I've never tried it though.

How do you know it's mostly infrared?

Interesting question, i know theres an IR filter on the sensor, but maybe not for the focusing.

It would be mighty helpful to be able to shine a IR light at something and use it for focusing.

I'm thinking like a concert, i could place a small IR Flashlight right near the performers and shoot from across the stage, the AF assist would never reach that far anyways.

Which sensor are you talking about? The image sensor?

Or are you talking about the "Infrared blocking glass" that's a part of the AF sensor optics? That seems to rule out the possibility to focus with light that's IR only.

apersson850 wrote in post #11146493
How do you know it's mostly infrared?

Canon describes it as an infrared AF assist.

OK. Since it's clearly visible it's obvious that they are lying to some extent. But since the whole point with IR is that we can't see it, it's impossible to tell how far into the IR spectra the red light on the flashes go, without a different instrument than just the eye.

Anyway, since the AF optics contains an IR blocking filter, it seems reasonable to believe that you can't focus with light that's IR only.
My thoughts are that the reason they use the red LED focus assist on a flash like the 580 EX II are not exactly related to IR or not, but instead for a couple of other reasons:

• The bursts of "real" flash light other units use are rather annoying, and also consume a bit of the charge level from the capacitor.
  
• Red LEDs were the first type that could be designed, and are still about the most efficient.
  
• The optics in front of the LEDs make them project a pattern which allows focusing even on surfaces where there is no contrast to detect to begin with.