I have used my 124G with an 89B (695 nm cutoff) and didn't really worry about it; unlike the earlier IR films, the light spectrum for today's IR films barely extends outside the visible spectrum and as such I don't think compensation is that important. Now I admit that is an opinion, not carefully researched fact.
That said, I think on many lenses the IR adjustment amounts to using approximately the next depth of field mark toward closer focus as the indicator mark.
The 1960 edition of Rollei: The Practical Accessories mentions "ground-in focus compensation." What that means in practice, well, who knows.I managed to find the Rollei IR filter in bayonet 1 size for my Rolleicord. It was new and came from Mr Cad in Croydon England. It was expensive( about 50GBP). It is quite opaque, like an IR720 and gives good results with the Rollei film. I have heard that it is "focus-corrected" but I have never seen this in any Rollei publication. I have used it for landscape, and therefore have not seen the effect with the lens at large apertures. The shots I have are in focus without making any adjustment. In many ways the TLR is the ideal IR camera as you can work with the filter in place and see to compose/focus.
I'm shooting handheld ir portraits and they are focus critical, I'm getting clearly unsharp results at close distances. I'll try the dof trick. I wish ir film wasn't more expensive.
It just occurred to me to wonder what sort of shutter speeds you wind up with? I might suspect some of the focus problem working hand held could be camera motion given what I recall of my exposures; ISO 6 or whatever is not perzactly fast. Of course, I was using a tripod. Maybe a couple of test shots with a solidly anchored camera to isolate variables would be useful. Shoot a yardstick angled at 45º to the lens axis; focus at the middle and see where the results fall.
It does feel sort of perverse that the film that seems to need the most experimentation costs substantially more than the ordinary stuff! :confused:
Leslie Stroebel recommends focusing, and then increasing the lens-to-film distance by 1/400th to correct the focus for infrared film.
This is the reason that the adjustment is much smaller for the current films (with a peak sensitivity near 720nm) then with the old HIE (with a peak sensitivity nearer 850nm).
Guys...
The math is great if i'm shooting still life or landscapes or LF but it won't fly on a model shoot with a TLR.
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