Help with infrared focussing please :)

[xkaes]

As others have mentioned, modern 'Infrared' film is not the same as the true infrared film that was available in the past. Don't worry too much about focus. The photo attached was taken with a Bronica S2 on Rollei Infrared and a generic 720nm filter. I hand metered using an ASA/ISO of 6. I don't remember the exposure settings but it was something in the 1/8 or 1/4 second area and shot on a tripod.

If you shot on a tripod and stop down, no problem. I don't, so focusing is something for me to consider.

 

[250swb]

Sure, a tripod is always an option, but if you don't want to use a tripod, you have to open up. I shoot lots of slow film, not just IR, without a tripod -- at a wide aperture. Try it, you'll like it.

My lenses have seven or eight f/stops on average, some lots more, and I paid for every f/stop the company gave for me to choose from, not just for the creamy bokeh limit!

 

[xkaes]

My lenses have seven or eight f/stops on average, some lots more, and I paid for every f/stop the company gave for me to choose from, not just for the creamy bokeh limit!

For those of you who always lug around a tripod, you can skip this post.

I use all of my f-stops and shutters as well -- when I can. But when I'm shooting IR, things change. I either have to use such a slow speed that I need a tripod or I need to open up the lens -- a lot. When I don't want the limitations of a tripod, I have no choice but to open up. Opening up is not a problem with normal film, but it is for IR.

Of course, since I always use a tripod for 4x5, this issue is only relevant to me with smaller formats -- which I use to avoid a tripod (most of the time).

 

[skahde]

Shoving all theory, assumptions, and considerations aside I did what any photographer should do in my world: I tried it out!

With Ilford SFX and a mild 89B filter critical focus wide open at infinity - about 3 km away subject - shifted on a Bronica PG 5.6/250 mm to the first depth-field marking. I used the markings as guideline as they are about 1 mm apart, a good measure for focus-bracketing.
With the PG 4.5/200 mm critical focus was acquired on between the first and second marking. In both cases focus landed about halfway between infinity for white light and the classic IR marking we all know was for HIE and 850 nm.

So if you think, ignoring the issue and stopping down is sufficient, go ahead and more power to you. It will propably work just fine in most cases. I rather use the corrections I found to work even wide open and stop down in addition.