The Characteristic Curve
The convention used throughout photography is to plot the D Log E curve on the same x and y scales. This ensures that it is easy to compare one film stock with another by visually comparing the response curves. These are called Characteristic Curves or H & D Curves after the originators, Hurter and Driffield.
On the left you can see an example of characteristic curve, as provided by
a film stock manufacturer (in this case, Eastman Kodak for Duplicating Negative
5234)
This being a negative stock, the contrast is rather low, and you can see the
variations in contrast you can obtain by reducing processing time.
The average photographic subject will not use the full range of the camera negative material's scale.
The section that is used depends upon the Image Brightness Range, which, in turn, is dependent upon the Subject Brightness Range and the Flare Factor [the degree of light scatter of the camera system that results in stray non-image light striking the film. The "average" subject in sunlight has a subject brightness range of about 160:1. The average camera has a flare factor of 2.5, which means that the image brightness range is reduced to 64:1. In terms of Log Exposure 1.8 log E units of the characteristic curve [Log10 64] will be used by that subject, extending approximately from a point on the toe to a point on the straight line.
If the film material in question is capable of producing a change in density over a total log E range of 3.0 units, then obviously 1.2 units are unused by this subject. This represents the latitude of the material and would in this case be equivalent to 4 stops of camera exposure (1 stop = 0.3 Log E). In the photographic positive, the best visual effect occurs when the whole of the density range is used as this gives the maximum tonal range in the reproduction.
A negative processed film, whether it is a negative or positive image, is said to have a negative curve [i.e. low density at low exposure]; a reversal processed film, whether it is a negative or positive image is said to have a positive curve [i.e. low density at high exposure].
Parts of a curve
It is common to talk about the various parts of a characteristic curve using rather anatomical analogies!
The Toe
The Toe of the characteristic curve is the region where the density is slowly building up its response to light. Some of the shadow detail of the subject will normally record on the toe of a film.
The Straight Line
The Straight Line is the region where density is increases in a linear uniform way with increasing log exposure. Both the length and steepness of the straight line are largely determined during emulsion manufacture, although the angle of the slope is also dependent upon the level of development. Older materials were designed with as long a straight section as possible [especially negatives] but it has been recognised over the last 20 or 30 years that this is not a prerequisite of a good quality picture. Some modern materials are markedly "dog-legged" or "hunched".
The Shoulder of the Curve
The Shoulder of the curve is reached when the rate of increase in density with exposure begins to diminish until no further density increase occurs - the value at this point being termed the D.Max, which is dependent upon the emulsion and its level of development. If the development has been taken to its limit, then the D.Max obtained will solely depend upon the quantity of silver salts employed in making the product. A variety of relevant parameters can be calculated from characteristic curves. Parameters are numerical values for certain characteristics of photographic emulsions and it is important that these parameters are relevant to the way in which a film is used. Speed, contrast, D.Max and Colour Balance are all parameters that are useful for describing films and are all calculated from the characteristic curve.