Originally Posted by AllenBaxter
You are making my head hurt :p!
The film is sensitive to both visible light and near infra-red light. The light available in most scenes includes both types. The visible light in those scenes is much stronger than the near infra-red light, so if you shoot without a filter you need to set exposure based on how much visible light is there - otherwise the shot will be way over-exposed.
You add the R72 filter in order to block out most of the visible light, while letting the near infra-red light pass through. Your resulting negative will show mostly the effect of the near infra-red - assuming you set the exposure correctly!.
The question is, of course, how do you determine the correct exposure? The answer comes mostly from experience. That experience is necessary because the exposure meters we have are not sensitive to just near infra-red, but rather are sensitive (mostly) to visible light. The experience tells us that if we measure the visible light available to be at X level, then the near infra-red light available will be at Y level. This is the variable that, well, varies a lot, and it is difficult to both measure or predict. Thus the need to bracket.
When people say that they recommend shooting a 400 ISO Rollei film at EI 3 (for example), with an R72 filter included in the equation, they are really saying that an R72 has a particular filter factor that results in 7 stops less visible light hitting the film, and that when the visible light exposure is reduced by 7 stops from "normal", there is a good chance that the remaining, near infra-red response will be suitable.
So to put it another way, we can measure how much visible light is available, and from experience we know approximately how much near infra-red accompanies that visible light. We know how much effect the R72 has on the visible light, so when we combine that knowledge with our measurement of the visible light, we can determine what exposure to use so that the R72 eliminates almost all the visible light, and leaves the near infra-red to do it's work.
Steve's approach most likely reflects the fact that he shoots IR film in an SLR, and therefore has to take the filter off to see anyways:).
Originally Posted by Sirius Glass
Whereas I use a TLR, so the R72 filter stays on the taking lens while I compose using the separate viewing lens. I'd use the same approach with a rangefinder camera.
One caution though - the spectral sensitivity of different meters varies. So if two photographers are metering through their filters, even if the light is the same, the meter readings may differ.
As a result, each photographer needs to experiment with their meter and their filter, their work-flow, their experience with lighting conditions and their particular aesthetic preferences to determine what will work best for them.
Oh, and bracket;).
Steve, Rollei IR400 with a 25A filter doesn't look much different than any other panchromatic film with a 25A filter. It is an expensive film (10 bucks a roll last I shot it). I wouldn't bother using it unless it is with heavier filters than a 25A. It is not unique unless you use it with an opaque filter, or a near-opaque one like the R72. Without a heavy filter, it might as well be T-Max in the camera, IMO.
HIE, on the other hand, definitely looked like an IR film when used with a 25 or 29 see-through filter. That was HIE's greatest strong point IMO: it could be used hand held with IR-looking results, and without having to shift focus.
This jibes with my experience with the Rollei material. Somewhere I saw a graph of the HIE spectral sensitivity that showed it not only sensitive way farther into the IR spectrum, but also what looked to be a bit of a notch in the yellow green range. Thinking area-under-the-curve stuff I'd guess almost any red filter would skew HIE pretty well towards IR. The Rollei barely makes it into the IR range; I somewhat preferred the effect with a 760 filter over a 720, but that involves working so far down the spectral cut-off it takes a lot of extra exposure and seems a bit touchy for latitude.
Originally Posted by 2F/2F
Enh, experiment, experiment. And as pointed out, though the Rollei 400 is a nice pan film, the price of a roll would buy about three of Acros 100!
In my experience with infrared films using a Hoya R72 filter, I always meter the visible light and focus, apply the filter, adjust to IR focusing marked on the lens and adjust / bracket +4 to 6 stops. Never had a problem with any infrared film and the results are amazing! As far as ASA/ISO goes, I either follow the retailer's suggestion or set it at 200. I heard with these new IR films in the market today, a Red 25 filter is impracticable but I have never tried it for myself. I am a big fan of EFKE IR820 Aura because it's the closet film to Kodak HIE (R.I.P.).