WBM has the best set of gradient values for diffusion printing that I've seen. I probably think this because they closely agree with mine. The difference is in how they were determined. Mine incorporates flare into the equation, WBM doesn't.
Originally Posted by Rafal Lukawiecki
The equation to determine the CI for any luminance range is:
CI = NDR / (log subject luminance range - flare)
The luminance range of a scene is reduced at the film plane by flare. The range you are developing for is the one that actually strikes the film. With a one stop flare factor, a scene will a 7 stop luminance range effectively becomes a 6 stop range at the film plane. If you develop for a 7 stop range, you would be under developing. One of the ways the various methods that don't factor in flare use to still produce good CI values or accurate processing is to use higher negative density range values. The average log exposure range (which equates to the aim negative density range) for a grade two paper printed with a diffusion enlarger is 1.05. WBM uses a 1.20 and the Zone System uses a 1.25 range. This effectively compensates for the lack of a flare adjustment to the luminance range. And while this approach will produce a good negative, it doesn't correctly reflect the process.
One consideration to maintain when factoring in flare into the equation is that the degree of flare isn't a constant. Flare varies with different luminance ranges. It's virtually impossible to determine the exact flare value for any given scene but flare tends to change by 1/3 stop per stop change in the luminance range. If the value of flare is kept constant, the CI values for shorter luminances will be too high producing contrasty negative and with longer luminance ranges, a fixed flare value will produce CI values that are too low. The developmental model that best reflects the shooting conditions would use a variable flare model.
Another factor that needs to be considered is that there isn't a perfect relationship between the negative density range and the paper LER. Loyd Jones found that “for the soft papers, the density scales of the negative (NDR) should in most cases exceed the sensitometric exposure scale of the paper (LER), whereas, for the hard papers, the density scales of the negatives should in most cases be less than the sensitometric exposure scale of the paper (LER).” In other words, it’s not only acceptable, but preferable to have a slightly contrastier negative for contrasty scenes and a slightly flatter negative for flatter scenes.
This is why my developmental model averages the fixed flare method and the variable flare method. In the following graph, all the models are based on a 1.05 NDR. An increase in the aim NDR with the no flare curve will shift it upward to be even with the others. The similar shape of the no flare and practical flare model curves is why the WBM with its higher NDR and no flare approach and my practical flare model agree so closely.
Fix, no, variable flare graph copy.jpg
In practice with the use of variable contrast papers, personal taste, the uncertainty of the flare, etc, how critical are these distinctions? In most situations, not very. There's always been a large range of acceptability in photography. Afterall, it's an art form that uses science. I just think it's important to understand how it really works.
Last edited by Stephen Benskin; 09-01-2012 at 12:04 PM. Click to view previous post history.