The BTZS incident metering system was something that Phil Davis taught some time ago when he created some software for the Palm Pilot.
I have the fourth edition titled BTZS Beyond The Zone System. The book goes pretty deep on sensitometry whch is aimed at the serious darkroom user but it also covers his method of using the incident light meter as an alternative to the Zone System.
I have and do use the zone system placement method for my large format black and white film work but because my workflow is of the hybrid type, I am not sure just how effective it really is when the scanner is addded to the mix.
I was particular interested in what Phil Davis had to say about the BTZS incident method so after reading the book several times and conducting some online research, I thought it was about time to do some testing of my own.
As I work in a hybrid workflow, I decided not to do all the Sensitometry testing that he explains in the book, mainly because I felt that it was not required and to be honest, I was more interested in producing a negative for which the scanner could read rather than conducting testing for graded paper and enlargers.
BTZS Incident Metering.
Without quoting what Phil describes in the book, the basic idea is that we take two incident light readings, one for the shadow area and one of the bright area of the scene in which we want to expose.
The shadow or Low reading is what we base our exposure on and the bright or High reading is what we use to calculate the subject brightness range (SBR) for which we can determine our development time.
This Is How I use It
Set the ISO in the meter to twice the value of what you normaly use for a particular film, For example, if I am using FomaPan 200, I would normaly rate this at ISO 100 because I feel I get better shadow detail by exposing it at 1/2 box speed so ion the case of this BTZS metering, I would set the meter to ISO 200.
I would then take a reading in open shade which is what I would use for the exposure and then take a second reading in the brightest part of the scene. Subtract one from the other and then add the difference to 5, This is the SBR of the scene. If the value is 7 then I would develop at my normal time, if it was 9 I would class this as N-2 and reduce the development time by roughly 30%.
- Low reading = EV 10
- High Reading = EV 12
- The difference is 2 and then added to 5 = 7 (7 been considered as a normal scene)
By using a black card and a white card, you can get more creative by matching the tonality with the strength of the light source. Moving the light meter dome more towards the light source will change the SBR which in turn is going to possibly alter the development time.