Cinematography-Theory-And-Practice

Figure 10.14. The Hurter andDriffield D Log E curve for negativedensity. with light can be reduced to pure silver if placed in developingfluids. The activity of the developer and time of developmentwill determine how much of the sensitized halide will be converted.The chemistry for color film is different from the solutionsused for black-and-white films, but the basic concepts arethe same.DENSITOMETRYTo understand film we must look at its response curve. This classicalapproach to densitometry (the scientific analysis of exposure) wasH&Dcurve or sometimes the D log E curve. This is sometimes shortened toLog E curve (Figure 10.14). It plots the amount of exposure (E) inlogarithmic units along the horizontal axis and the amount of densitychange in the negative “D” along the vertical axis.In theory, it makes sense that we would want the film to changein density in exact proportion to change in the amount of lightreflected by different parts of the scene. After all, we are trying tomake an image that accurately portrays the real scene, right?Let’s look at a theoretical linear film (Figure 10.15). For every additionalunit of exposure, the density of the negative changes exactlyone unit. That is, there is an exact correspondence between theamount of light in the scene and the change in the density of thenegative. Sounds perfect, doesn’t it? The slope of the line for thisfilm would be 45 degrees exactly.The slope of this line is a measure of the contrastiness of the film.In a film where large changes in exposure only change the negativedensity a little (low contrast reproduction), the slope is very shallow.Where a film is very contrasty, the slope is very high; in other words,small changes in the amount of light cause the film density to changedrastically. The extreme is something called litho film, which is usedin the printing industry. Everything in litho film is either black orwhite — there are no shades of gray. In other words, if the light isabove a certain level, the image is completely white. If it is below acertain level, it is completely black. This is as contrasty as a film canget. The slope for litho film (which is strictly black-and-white withno grays) would be a vertical line.No film acts in the perfectly linear manner of this first example(i.e., the changes in the film exactly correspond to the change in theamount of light). In this diagram, we see a film that only changes1/2 unit of density for each additional unit of light. This is a low-exposure191