By analyzing a characteristic curve, you can determine the effective speed of the emulsion, the contrast, the latitude, and the useful exposure range. (Figure 2-6) at the end of the chapter shows a typical characteristic curve with the various parts and their names. Refer to figure 2-6 frequently as you proceed through this chapter.

The speed of the material being evaluated determines the position of the curve regarding log exposure (log H) or horizontal axis. The length and slope of the straight-line section are the main variations to the shape of the curve. All densities on the straight-line section of the D-log H curve increase proportionally with an increase in development time. The slope or gradient of the straight-line section of the curve also increases, as development is increased; that is, it gets steeper.

Toe Section

The top section fig. 2-6 is a concave, rising portion of the curve that gradually increases in

Figure 2-4.-Plotting points for the characteristic curve.

density. This section is defined as a region of unequal rise, because the density does not increase equally for equal increases in exposure. Subject tones exposed in this section are reproduced with small, unequal density differences. Exposures made in this section of the curve lack detail. For ground pictorial photography, satisfactory exposures are made when the shadow areas of the subject fall in the toe area.

Figure 2-5.-Drawing the characteristic curve.

Notice in the toe section of the illustration (fig. 2-6) that steps 1 through 4 do not show a change in density. This indicates that the film has not been affected by light, and the density is not a result of exposure, but of the emulsion base and the emulsion fog (gross fog) present in all emulsions. Step 5 shows a slight increase in density. This is called the threshold and indicates the least amount of exposure that will produce a noticeable change in density.

 Straight-Line Section

Further to the right (fig 2-6) is a section of the curve that appears to be a straight line. Note that in many cases there will not be a well-defined straight line. This section of the curve has a constant slope, and in addition, the gradient of the slope here is greater than in any other part of the curve. In the straight-line section of the characteristic curve, there is an equal increase in density for an equal increase in exposure. This is the most important part of the curve. Subject exposures that fall on the straight line produce constant density differences.

Shoulder Section

The upper section of the curve fig. 2-6 is a convex, curved line that gradually decreases in slope. This section of the curve is called the shoulder. Like the toe area, equal changes in exposure do not produce equal differences in density. Tones of the subject falling very far up in this section are blocked; that is, reproduced with densities too heavy for printing or maximum detail. For normal exposures, bright highlight tones of the subject tend to be reproduced in the shoulder section of the curve.

Exposure

Although sensitometry is a tool of the lab technician, it also is significant to the photographer. Notice how this applies to exposing film. For example, when a uniformly lighted gray card is photographed, there is a single exposure, corresponding to a single point somewhere on the log-H axis. When the light on the surface of the gray card is increased and another photograph is made (maintaining the same camera settings), the exposure and the log H also increase. This causes a shift to the right on the log-H axis. There should be a corresponding increase in density, and the two factors again should plot on the characteristic curve.

Extending this to a scene with a large number of luminances (reflectances) or high-luminance ratio, the tonal differences in the subject, the lighting, and the camera settings determine the film exposure that produces varying amounts of densities. These density differences must be related to the log-H differences that produce them; that is, density differences in the negative must be considered in their relationship with the tones of the subject.

The density differences in a negative can be partially controlled by placing the exposures corresponding to the subject tones in the correct section of the characteristic curve. This is done by adjusting the camera settings correctly, providing that the range of tones in the subject is not too great.

Emulsion Latitude

The emulsion latitude is the exposure range where there is a proportional relationship between density differences and log-H differences. In other words, it is simply the range of exposure covered by the straight-line section of the characteristic curve fig 2-6. The latitude of an emulsion, therefore, determines the brightness range of the subject that can be reproduced proportionally. Latitude may be expressed either as the difference in log-H values between the two extreme points of the straight line or as the exposure ratio between these same two points.

The emulsion latitude of light-sensitive materials varies according to the purpose for which it was designed-from 1:400 or more for long-scale panchromatic film, and from 1:20 or less for process film. For any given emulsion, the emulsion latitude varies according to the contrast. The emulsion latitude decreases as the contrast increases.

Log Exposure Range

The useful exposure range includes part of both extremities (toe and shoulder) as well as the straight-line section of the curve. For ground pictorial film the useful exposure range of a sensitized material is greater than the emulsion latitude, since portions of the toe and shoulder regions of the curve are used without sacrificing print quality.

The approximate lower limit of the useful exposure range is the density point on the characteristic curve that is not less than 0.10 above the gross fog of the film. This point is referred to as minimum useful density. The upper limit is generally located at 90 percent of maximum density on the shoulder of the curve and is referred to as maximum useful density. In practice, many photographers use a much lower upper limit because of the high densities involved

Exposure Latitude

Exposure latitude is the allowable range of exposures for a given photographic emulsion. It varies with the brightness range of a scene. Film with a wide exposure latitude permits a greater variance of exposure and still produces an acceptable negative. Exposure latitude is associated with the useful log-exposure range and can be thought of as the margin of camera exposure error. Scenes that have a relatively low-luminance ratio and are photographed using a long-scale film have more exposure latitude than a scene with a high-luminance ratio using the same film. For example, you are photographing a subject with a luminance ratio of 60:1. This subject requires a log-exposure range of 1.78 (log of 60 = 1.78). The useful log-exposure range of the film/development combination is 2.70. This leaves a difference in logs of 0.92 (2.70 - 1.78 = 0.92). In this example, the exposure latitude is about three f/stops (remember, one f/stop = 0.30). Normally, the lower the contrast of the scene and the faster the film, the greater is the exposure latitude.

Modem high-speed films have an overall exposure latitude of several stops for an average subject. However, regardless of the brightness range of the scene, color reversal and very slow black-and-white and color negative films have very little exposure latitude because of their increased inherent contrast. Thus the range of exposure lies within a narrow limit that may be less than one-half to one f/stop.

Gamma

In technical terms, gamma (signified by the Greek letter 7) is a sensitometric quantity that indicates the slope or gradient of the straight-line section of the characteristic curve. It is interpreted as a measure of the contrast reproduced in a negative image; that is, the ratio of negative contrast to original subject contrast for a given range of tonal values. It measures the degree of development of photographic materials, since changes in development affect contrast or affect the slope of the curve. Exposure changes, as explained previously, shift the position of the points right or left on the log-H axis without altering the slope of the curve. Thus the tendency is for exposure to control the density and development to control the contrast of the image reproduced. Remember the expression, "Expose for shadow density-develop for contrast."

Mathematically, gamma is the ratio of height gained to distance traveled in a horizontal direction. In determining gamma, the height is density (D), and the horizontal base is log exposure (log H). An ideal film and processing might produce an increase of .3 density for each .3 increase of exposure. This ratio is 0.3:0.3, or 1.0.

Most ground pictorial subjects call for film with a gamma value around 0.75, varying from 0.65 to 0.90. Such emulsions record the wide range of tones present in outdoor scenes. In practice, each of the main groups of negative materials has its own individual characteristics. Gamma is useful to you, because it indicates how the photographic material responds to changes in exposure and processing.

From the previous discussion, you may have noticed that gamma is definable in different ways. Some more useful definitions include the following:

Gamma is the numerical measure of the contrast reproduced in an image.

Gamma is a numerical measure of the degree of development (for a given material).

Technically, gamma is the slope of the straight-line section of the characteristic curve. Once a characteristic curve has been plotted, gamma can be determined in a number of ways. Two of the most common methods are as follows:

Basic method-This method shown in figure 2-7 involves the ratio between densities and the exposures that produced them. Any two points on the straight line are chosen. (More reliability results when the points are widely separated.) Gamma is the result of dividing the change, or difference in density, by the difference in log H between the two points. The formula is as follows:

where

(Delta) = Symbol for change or difference

Figure 2-7-Computing Gamma.

The values of D1, D2, log Hl, and log H2 are read directly from the graph. The mathematical computation for this method is shown at the top of figure 2-7

Quick method-This method uses a gamma gauge fig. 2-8 . The arrow of the gauge is placed on the straight-line section of the curve with its base line parallel to the log-H axis. Gamma is indicated where

Figure 2-8--Using a gamma gauge.

the straight line intersects the scale. As you can see, the gamma for the plot is 0.82.