Digital Infra-Red

but the AF assist lamp does use IR! When I use my flash unit in dim light I can clearly see a red circle projected on my subject (or is that not infra-enough light?)

Truth is, if you can see it, it ain't infrared. The human eye, which is what most of us are equipped with, can't 'see' wavelengths that fall into the infrared part of the light spectrum as it is outside our range of visual sensitivity at wavelengths longer than about 800nm.

The red light that you can see is within our visual range and has a wavelength of about 640 - 700'ish nm.

That is not to say that there is no infrared component in a focus assist beam, just that you can't see it if there is.

Second point is, how much light other than wavelengths in the infrared band does an IR filter let through? IR filters appear dark because they block light having wavelengths shorter than around 700nm (anything from a very deep red through to the blue end and ultraviolet), but that is still within our visual sensitivity range, but only just.

For everything you ever wanted to know about light, colour, and vision, but were too afraid to ask, take a look at http://www.handprint.com/HP/WCL/wcolor.html

Curiously, rattlesnakes and other members of the viper family can detect infrared radiation, not with their eyes but with special heat (not light)receptors in a 'pit' beneath the eye (only one eye I think, but not sure). Hence the name pit viper. They use this pit to detect a potential meal by being able to 'feel' the heat, rather than see the light, eminating from their prey animals body at night and in poorly lit burrows, caves etc.

Tomorrow . . . quantum physics . . . not.

My cat often gets excited and stares at something I can't see.

Cats are nocturnal sight-hunters. It would be an evolutionary advantage to them if their eyes could not only get double use out of every photon/light wave (reflective surface behind the retina), but "see" the heat-signature of prey. In other words, infared.

Bees see ultraviolet, there are ultraviolet patterns on flowers that humans can't see. They aren't there for us, they are for the bees!

Makes you wonder......

Police helicopters throughout the world are now equipped with 'night-vision' infrared video cameras. I guess these must employ an AF system that does work in infrared to keep track of their target. I seriously doubt if these are focussed manually, but I guess they could be. After all their 'hot' subject, translated to white in the recordings, would be in steep contrast to their 'cold' surroundings shown as grey to black.