log(E) or log(exposure) on the horizontal axis is the amount of exposure that the film received.
D log(E) or Density-log(exposure) on the vertical axis is the density that the exposure on the horizontal axis produced on the film.
Both log(E) and D log(E) values are such that a change of 0.30 is a one f-stop change in exposure or density. 0.10 is 1/3 stop, 0.60 is two stops, 1.2 is 4 stops.
So the graph shows how exposure (x-axis) affects density (y-axis) with a given film/developer combination at a stated developing time, temp, dilution, and agitation method.
Shadow values are on the left, and the curve tends to flatten out there as it approaches the density of the base support. That part of the curve is called the 'toe'. A long-toed film rises slowly and gradually curves into the middle straight line portion of the curve. A short-toed film rises more quickly off the base density and has more shadow contrast, but loses shadow detail rapidly if you underexpose.
Mid-tones are in the middle, straight line portion of the curve. The steeper that is, the more contrast there is in the midtones, and in the film overall.
The upper right of the curve is the 'shoulder' and describes the highlight response of the film/developer combination. Some films, especially the engineered grain, films like delta, t-grain, sigma grain films, tend to stay straight along the same path as the middle of the curve, and some even rise a bit. Other films have a characteristic roll off on the shoulder, compressing highlight values into more similar densities, making them easier to fit on limited range enlarging papers, but giving less contrasty highlights.
With less development, the curve drops a hair lower on the graph and its slope decreases. With less agitation, the curve may also have a shallower slope (less contrast) and you might introduce increased roll-off in the shoulder. Sometimes you can introduce more or less of a shoulder with a choice of developer and/or agitation.
The H&D characteristic curve is a great summary of the general characteristics of a film, and of a film/developer combination. It can also be combined with a similar paper curve to get an end-to-end description of what happens with a particular combination of film, film developer, processing method, paper, and paper developer combination.
Lee
Nice .pdf explanation in the previous post. Hadn't seen it before.
Thanks Ron for the correction.
