Quote Originally Posted by AllenBaxter View Post
Thanks for the feedback on the pictures. I just want to get my head around understanding the exposure issue better. I have only recently taken up photography and started with digital and now am tyring film. Based on DWThomas and 2F/2F responses it makes sense that one is trying to balance the amount of visible light and IR light onto the film. Am I correct then in understanding that by adjusting the film speed from the box rating of 400 to 25 that I am reducing the visible light sensitivity of the exposure by 4 stops and that by adding the Hoya R72 filter I am again reducing the visible light sensitivity by another 5 stops which would appear to be about 9 stops of less visible light sensitivity. I tried adjusting from ISO 400 to ISO 25 and then compensating by increasing the exposure time and also using ISO 25 and compensating less. My testing involved metering without the filter and noting exposure and taking a shot. I then used this exposure as my basis for making adjustments when I added the R72 filter. Based on my understanding then it would appear that this film likes to be very under exposed for visible light. Is this correct or am I confused? Pictures are posted: http://www.flickr.com/photos/nikonfd...7627083103885/. Thanks for help in better understanding this exposure issue. Next is composition.
Allen:

You are making my head hurt !

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.