Infrared focussing assist

Given that:

1. CMOS sensors are sensitive to near-infrared radiation (and have to have a

special filter to filter it out, under normal circumstances)

and

2. The autofocus system in my 5D uses CMOS sensors built into the base of the

camera, distinct from the imaging sensor at the back

I'd like to use an infra-red LED torch as a focus assist for dim light. Now I've

tried this and it doesn't work. I have a cheap infra-red LED torch with 30 LED's

(designed for use with night-vision goggles that also sense near IR) and it

doesn't help the AF. Whereas a little 3 red LED torch does. So that kind-of

imples there's an IR filter over the AF sensors too.

Can anyone confirm or deny that? Could it be removed? Is there any other reason

why the IR torch might not work? It would be really neat if it would.

Auto focus system works with visible light. The sensor is not infrared sensitive or has poor infrared sensitivity. Because the focusing point is different for infrared, if you modify the auto focus system to be sensitive to infrared and use an infrared torch, all normal visible light pictures you shoot will be out of focus!

It also depends of the wavelength of your IR LED. Most of IR LED emit at 1060 nanometers, but if you use IR LED which wavelength is between about 710 and 780 nanometers, the auto focus sensor may work correctly.

AF assist provides a striped light pattern that is designed to ensure that even with a smooth white wall there will be contrast for the sensors to detect. Do you know if your IR torch provides a reasonably detectable intensity pattern?

It's quite probable that there is an IR cut filter in the AF optical train - partly because focus at IR wavelengths is noticeably different to focus at optical wavelengths (indeed, some lenses actually have an IR focus mark that is quite separate from the mark for visible light, allowing IR focus to be offset manually from focus at visible wavelengths when using IR film or an IR sensor). Removing the filter (which might even be on the sensor surface, and thus require chemical etching) is likely to throw your AF off target.

"Do you know if your IR torch provides a reasonably detectable intensity pattern?"

I think this would be a nice-to-have (and the answer's no, it doesn't) but I don't think it's important, after all a plain white flashlight works, as does the dirt-cheap three red LED torch.

The infra-red focusing mark on the lens (and associated focus shift) is probably a bigger issue though, true. I'll see if I can find some LEDs just outside the 700nm range (the torch I have is specified at 850nm) and warm up my soldering iron.

Here you can find 730 nm LED:

http://www.fairchildsemi.com/pf/QE/QEB441.html

Here you can find 700 nm LED:

http://ledmuseum.home.att.net/ledir.htm

And Here you can find a powewfull IR spotlight at 715 nanometers:

http://www.maxmax.com/aIRFlashSpotlight.htm