using filters

So does film have anything in it to stop IR getting through?

I thought that ordinary stock was not very IR sensitive.

You need to use infrared film if you want to shoot infra red imagery. Older Pentax lenses have an IR focusing index too. You focused by eye in the usual way then moved the point on the focussing ring adjacent to the normal 'UV' index to the IR one.

Jeez, I had forgotten that. The wee red line next to the white one. Ah, that takes me back. So would a digital camera have the same issue as it would try to focus on the visible wavebands? Or would you reset the focus point somehow?

dunno

It's a grey area isn't it. I wondered that too, because for critical focusing it would make AF useless. Quick Shift would work, because a slight tweak after AF would do the trick. But we don't know how well digital lenses are corrected in the IR band and we don't know if the focus point on converted cameras is adjusted as part of the conversion.

Thinking about it, it could mean that the focus adjustment feature could be used for a particular lens in the K20D menu. So you could reserve a lens for IR work and adjust its focus point to compensate. Interesting thought?

Then again, we usually see IR landscapes; so lenses may well be stopped down so far and wide as to make the focus shift academic. I've never shot IR and haven't personally met anyone who has so it is quite curious, I think, that Pentax should have engraved every lens for a small minority of photographers. Or was it in design parlance, "a mug's eyefull'?

In days of yore, so to speak, it was common practice to put an IR correction mark on lenses. It fell by the wayside when zooms became common and masses of markings for a couple of selected focal lengths were perhaps unwieldy.

It focus with the light there is so if only IR light is available it will focus with that, had no problems with focusing but that was with f/5.6 so decently wide might test it with f/2.8 close by focusing.