Sensitometry is the science of measuring the response of photographic emulsions to light. "Image-structure" refers to the properties that determine how well the film can faithfully record detail. The appearance and utility of a photographic record are closely associated with the sensitometric and image-structure characteristics of the film used to make that record. The ways in which a film is exposed, processed, and viewed affect the degree to which the film's sensitometric and image-structure potential is realized. The age of unexposed film and the conditions under which it was stored also affect the sensitivity of the emulsion. Indeed, measurements of film characteristics made by particular processors using particular equipment and those reported on data sheets may differ slightly. Still, the information on the data sheet provides a useful basis for comparing films. When cinematographers need a high degree of control over the outcome, they should have the laboratory test the film they have chosen under conditions that match as nearly as possible those expected in practice.
Understanding Sensitometric Information
Transmission density (D) is a rneasure of the light-controlling power of the
silver or dye deposit in a film emulsion. In color films, the density of she
cyan dye represents its controlling power to red light, that of magenta dye to
green light, and that of yellow dye to blue light. Transmission density may
be mathematically defined as the common logarithm (Log base 10) of the
ratio of the light incident on processed film (Po) to the light transmitted by
the film (Pt).
The measured value of the density depends on the spectral distribution of the exposing light, the spectral absorption of the film image, and the special sensitivity of the receptor. When the spectral sensitivity of the receptor approximates that of the human eye, the density is called visual density. When it approximates that of a duplicating or print stock, the condition is called printing density.
For practical purposes, transmission density is measured in two ways:
| SAMPLE DENSITY |  | INTEGRATING SHPERE |
| TOTALLY DIFFUSE DENSITY |
| SPECULAR DENSITY |
To simulate actual conditions of film use, totally diffuse density readings are routinely used when motion-picture films are to be contact printed onto positive print stock. Specular density readings are appropriate when a film is to be optically printed or directly projected. However, totally diffuse density measurements are accepted in the trade for routine control in both contact and optical printing of color films. Totally diffuse density and specular density are almost equivalent for color films because the scattering effect of the dyes is slight, unlike the effect of silver in black-and-white emulsions.
Characteristic Curves(11)
A characteristic curve is a graph of the relationship between the amount of
exposure given a film and its corresponding density after processing. The
density values that produce the curve are measured on a film test strip that is
exposed in a sensitometer under carefully controlled conditions and
processed under equally controlled conditions. When a particular
application requires precise information about the reactions of an emulsion
to unusual light-filming action in a parking lot illuminated by sodium vapor
lights, for example, you can filter the exposing light in the sensitometer can
be filtered to simulate that to which the film will actually be exposed. A
specially constructed step tablet, consisting of a strip of film or glass
containing a graduated series of neutral densities differing by a conslant
factor, is placed on the surface of the test strip to control the amount of
exposure, the exposure time being held constant. The resulting range of
densities in the test strip simulates most picture-taking situations, in which
an object modulates the light over a wide range of illuminance, causing a
range of exposures (different densities) on the film.
After processing, the graduated densities on die processed test strip are measured with a densitometer. The amount of exposure (measured in lux1) received by each step on the test strip is multiplied by the exposure time (measured in seconds) to produce exposure values in units of lux-seconds. T'he logarithms (base 10) of the exposure values (log H) are plotted on the horizontal scale of the graph and the corresponding densities are plotted on the vertical scale to produce the characteristic curve. This curve is also known as the sensitometric curve, the D Log H (or E) curve, or the H&D Hurter and Driffield) curve2.
In the following table, the lux-sec values are shown below the log
exposure values. The equivalent transmittance and opacity values are shown
to the left of the density values.
The characteristic curve for a test film exposed and processed as described in the table is an absolute or real characteristic curve of a particular film processed in a particular manner.
Sometimes it is necessary to establish that the values produced by one densitometer are comparable to those produced by another one. Status densitometry is used for this. Status densitometry refers to measurements made on a densitometer that conforms to a specified unfiltered spectral response (Dawson and Voglesong, Response Functions for Color Densitometry, PS&E Journal, Volume 17, No. 5 Sept/Oct 1973). When a set of carefully matched filters is used with such a densitometer, the term Status A densitometry is used. The densities of color positive materials (reversal, duplicating, and print) are measured by Status A densitometry. When a different set of carefully matched filters is incorporated in the densitometer, the term Status M densitometry is used. The densities of color preprint films (color negative, intemegative, intermediate, low-contrast reversal original, and reversal intermediate) are measured by Status M densitometry. (DAK Densitometer Filter Sets are purchased directly from the manufacturers of densitometers. For further information, contact the densitometer manufacturer.)
 |  |
| Figure 4 | |
| These illustrations show the relationship between subject luminance, negative den- sity, and the characteristic curve. There is one stop difference in luminance between each of the points 2 to 10. Point 1 is a specular highlight which photographs as if it were about 2 stops brighter than point 2, which is a diffuse highlight. Point 9 is the tone to be reproduced just lighter than black. There are 7 stops difference between points 2 and 9, which is the typical range for normal luminance range subjects. Point 10 is about one stop darker than point 9, and reproduces as black. The graph shows where points of these brightness differ- ences generally fall on a characteristic curve. Point 9 is exposed on the speed point of the film, which develops to a density of about 0.10 above the base plus fog density (the density of the clear film base after developing). The density range from point 9 to point 2 is about 1.05. |
 |
Representative characteristic curves are those that are typical of a product and are made by averaging the results from a number of tests made on a number of production batches of film. The curves shown in the data sheets are representative curves.
Relative characteristic curves are formed by plotting the densities of the test film against the densities of a specific uncalibrated sensitometric-step scale used to produce the test film. These are commonly used in laboratories as process control tools.
Black-and-white films usually have one characteristic curve (see Figures 5 and 6). A color film, on the other hand, has three characteristic curves, one each for the red-modulating (cyan-colored) dye layer, the green- modulating (magenta-colored ) dye layer, and the blue-modulating (yellow- colored) dye layer (see Figures 7 and 8). Because reversal films yield a positive image after processing, their characteristic curves are inverse to those of negative films (compare Figures 5 and 6).
| BLACK-AND-WHITE NEGATIVE FILM | BLACK-AND-WHITE REVERSAL FILM |
 |  |
| LOG EXPOSURE (lux-sec) | LOG EXPOSURE (lux-sec) |
| Figure 5 | Figure 6 |
COLOR NEGATIVE FILM | COLOR REVERSAL FILM |
 |  |
| LOG EXPOSURE (lux-sec) | LOG EXPOSURE (lux-sec) |
| Figure 7 | Figure 8 |
General Curve Regions
Regardless of film type, all characteristic curves are composed of five
regions: D-min, the toe, the straight-line portion, the shoulder and D-max.
Exposures less than at A on negative film or greater than at A on reversal film will not be recorded as changes in density. This constant density area of a black-and-white film curve is called base plus fog. In a color film, it is termed minimum density or D-min.
The toe (A to B), as shown in Figure 9, is the portion of the characteristic curve where the slope (or gradient) increases gradually with constant changes in exposure (log H).
The straight-line (B to C), Figure 10, is the portion of the curve where the slope does not change; the density change for a given log-exposure change remains constant or linear. For optimum results, all significant picture information is placed on the straight-line portion.
The shoulder (C to D), Figure 11, is the portion of the curve where the slope decreases. Further changes in exposure (log H) will produce no increase in density because the maximum density (D-max) of the film has been reached.
Base density is the density of fixed-out (all silver removed) negative- positive film that is unexposed and undeveloped. Net densities produced by exposure and development are measured from the base density. For reversal films, the analogous term of D-min describes the area receiving total exposure and complete processing. The resulting density is that of the film base with any residual dyes.
Fog refers to the net density produced during development of negative-
positive films in areas that have had no exposure. Fog caused by
development may be increased with extended development time or increased
developer temperatures. The type of developing agent and the pH value of
the developer can also affect the degree of fog. The net fog value for a given
development time is obtained by subtracting the base density from the
density of the unexposed but processed film. When such values are
determined for a series of development times, a time-fog curve (Figure 12)
showing the rate of fog growth with development can be plotted.
 |  |
| LOG EXPOSURE (lux-sec) | LOG EXPOSURE (lux-sec) |
| Figure 9 | Figure 10 |
 | |
| LOG EXPOSURE (lux-sec) | |
| Figure 11 |
Curve Values
You can derive additional values from the characteristic curve that not only
illustrate properties of the film but also aid in predicting results and solving
problems that may occur during picture-taking or during the developing and
printing processes.
Speed describes the inherent sensitivity of an emulsion to light under specified conditions of exposure and development. The speed of a film is represented by a number derived from the film's characteristic curve.
Contrast refers to the separation of lightness and darkness (called "tones") in a film or print and is broadly represented by the slope of the characteristic curve. Adjectives such as flat or soft and contrasty or hard are often used to describe contrast. In general, the steeper the slope of the characteristic curve, the higher the contrast. The terms gamma and average gradient refer to numerical means for indicating the contrast of the photographic image.
Gamma is the slope of the straight-line portion of the characteristic curve or the tangent of the angle (a) formed by the straight line with the horizontal. In Figure 5, the tangent of the angle (a) is obtained by dividing the density increase by the log exposure change. The resulting numerical value is referred to as gamma.
Gamma does not describe contrast characteristics of the toe or the shoulder. Camera negative films record some parts of scenes, such as shadow areas, on the top portion of the characteristic curve. Gamma does not account for this aspect of contrast.
Average gradient is the slope of the line connecting two points bordering a specified log-exposure interval on the characteristic curve. The location of the two points includes portions of the curve beyond the straight-line portion. Thus, the average gradient can describe contrast characteristics in areas of the scene not rendered on the straight-line portion of the curve. Measurement of an average gradient extending beyond the straight-line portion is shown in Figure 13.
| CURVES FOR A DEVELOPMENT TIME SERIES ON A TYPICAL BALCK-AND-WHITE NEGATIVE FILM |
AVERAGE GRADIENT DETERMINATION |
 |  |
| LOG EXPOSURE (lux-sec) | |
| Figure 12 | Figure 13 |
The particular gamma or average gradient value to which a specific black-and-white film is developed differs according to the properties and uses of the film. Suggested control gamma values are given on the data sheets for black-and-white negative and positive films.
If characteristic curves for a black-and-white negative or positive film are determined for a series of development times and the gamma or average gradient of each curve is plotted against the time of development, a curve showing the change of gamma or average gradient with increase development is obtained. You can use the time-gamma curve (Figure 14) to find the optimum developing time to produce the control gamma values recommended in the data sheet (or any other gamma desired).
Black-and-white reversal and all color film processes are not controlled by using gamma values.
Flashing camera films to lower contrast is a technique3 that involves uniformly exposing film before processing to lower its overall contrast. It's used with some color films. It is actually an intentional light fogging of the film. You can make the flashing exposure before or after the subject exposure, either in a camera or in a printer. The required amount of exposure and the color of the exposing light depends on the effect desired, the point at which the flashing exposure is applied, the subject of the main exposure, and the film processing. Because of potential latent image changes, a flashing exposure just prior to processing is the preferred method.
 |  |
| DEVELOPMENT TIME (IN MINUTES) | LOG EXPOSURE (lux-sec) |
| Figure 14 | Figure 15 |
This fairly common practice is often used to create a closer match of two films' contrast characteristics when they are intercut. The hypothetical characteristic curves in Figure 15 show what occurs when one film is flashed to approximately match another film's characteristic curve. The illustration has been simplified to show an ideal matching of the two films. In practice, results will depend on the tests run using the specific films intended for a production.
Some film productions use flashing (called "creative flashing") to alter the contrast of the original camera negative of a particular scene to create a specific effect-making pastels from more saturated colors, enhancing shadow detail, and the like. Further discussion of this type of flashing is presented in "Creative Post-Flashing Technique for the The Long Goodbye," American Cinematographer Magazine, March 1973.
Color Sensitivity and Spectral Sensitivity(12)
The term color sensitivity is used on data sheets for some black-and-white
films to describe the portion of the visual spectrum to which the film is
sensitive. All black-and-white camera films are panchromatic (sensitive to
the entire visible spectrum). Some laboratory films are also panchromatic:
EASTMAN Fine Grain Duplicating Panchromatic Negative Film, EASTMAN
Panchromatic Separation Film, and EASTMAN High Contrast Panchromatic
Film.
Some films, called orthochromatic, are sensitive mainly to the blue-and- green portions of Lhe visible spectrum. EASTMAN Direct MP, EASTMAN Reversal BW Print, and EASTMAN Sound Recording II Films are all orthochromatic laboratory or print films.
Films used exclusively to receive images from black-and-white materials are blue-sensitive: EASTMAN Fine Grain Release Positive Film, EASTMAN High Contrast Positive Film, and EASTMAN Fine Grain Duplicating Positive Film.
One film is sensitive to blue light and ultraviolet radiation: EASTMAN Television Recording Film. The extended sensitivity in the ultraviolet region of the spectrum permits the film to respond to the output of cathode- ray tubes.
While color films and panchromatic black-and-white films are sensitive to all wavelengths of visible light, rarely are two films equally sensitive to all wavelengths. Spectral sensitivity describes the relative sensitivity of the emulsion to the spectrum within the film's sensitivity range. The photographic emulsion has inherently the sensitivity of photosensitive silver halide crystals. Itese crystals are sensitive to high-energy radiation, such as X-rays, gamma rays, ultraviolet radiation and blue-light wavelengths (blue- sensitive black-and-white films). In conventional photographic emulsions, sensitivity is limited at the short (ultraviolet) wavelength end to about 250 nanometers (nm) because the gelatin used in the photographic emulsion absorbs much ultraviolet radiation. The sensitivity of an emulsion to the longer wavelengths can be extended by the addition of suitably chosen dyes.
By this means, the emulsion can be made sensitive through the green region (orthochromatic black-and-white films), through the green and red regions (color and panchromatic black-and-white films), and into the near- infrared region of the spectrum (infrared-sensitive film). See Figure 16.
Three spectral sensitivity curves are shown for color films-one each for the red-sensitive (cyan-dye forming), the green-sensitive (magenta-dye forming), and the blue-sensitive (yellow-dye forming) emulsion layers. One curve is shown for black-and-white films. The data are derived by exposing the film to calibrated bands of radiation 10 nanometers wide throughout the spectrum, and the sensitivity is expressed as the reciprocal of the exposure (ergs/cm2) required to produce a specified density. The radiation expressed in nanometers is plotted on the horizontal axis, and the logarithm of sensitivity is plotted on the vertical axis to produce a spectral-sensitivity curve, as shown in Figure 17.
| Figure 16 |  |
Equivalent neutral density (END)-When the amounts of the components of an image are expressed in this unit, each of the density figures tells how dense a gray that component can form.
Because each emulsion layer of a color film has its own speed and
contrast characteristics, equivalent neutral density (END) is derived as a
standard basis for comparison of densities represented by the spectral-
sensitivity curve. For color films, the standard density used to specify
spectral sensitivity is as follows:
For reversal films, END = 1.0
For negative films, direct duplicating, and print films,
END= 1.0 above D-min.
Spectral-Dye-Density Curves(13)
Proessing exposed color film produces cyan, magenta, and yellow dye
images in the three separate layers of the film. The spectral-dye-density
curves (illustrated in Figure 18) indicate the total absorption by each color
dye measured at a particular wavelength of light and the visual neutral
density (at 1.0) of the combined layers measured at the same wavelengths.
Spectral-dye-density curves for reversal and print films represent dyes normalized to form a visual neutral density of 1.0 for a specified viewing and measuring illuminant. Films which are generally viewed by projection are measured with light having a color temperature of 5400 K. Color-masked films have a curve that represents typical dye densities for a mid-scale neutral subject.
The wavelengths of light, expressed in nanometers (nm), are plotted on the horizontal axis, and the corresponding diffuse spectral densities are plotted on the vertical axis. Ideally, a color dye should absorb only in its own region of the spectrum. All color dyes in use absorb some wavelengths in other regions of the spectrum. This unwanted absorption, which could prevent satisfactory color reproduction when the dyes are printed, is corrected in the film's manufacture.
In color negative films, some of the dye-forming couplers incorporated in the emulsion layers at the time of manufacture are colored and are evident in the D-min of the film after development. These residual couplers provide automatic masking to compensate for the effects of unwanted dye absorption when the negative is printed. This explains why negative color films look orange.
Since color reversal films and print films are usually designed for direct projection, the dye-forming couplers must be colorless. In this case, the couplers are selected to produce dyes that will, as closely as possible, absorb in only their respective regions in the spectrum. If these films are printed, they require no printing mask.
Image Structure
The sharpness of image detail that a particular film type can produce cannot
be measured by a single test or expressed by one number. For example,
resolving-power-test data gives a reasonably good indication of image
quality. However, because these values describe the maximum resolving
power a photographic system or component is capable of, they do not
indicate the capacity of the system (or component) to reproduce detail at
other levels. For more complete analyses of detail quality, other evaluating
methods, such as the modulation-transfer function and film granularity, are
often used. An examination of the modulation-transfer curve, RMS
granularity, and both the high- and low-contrast resolving power listings will
provide a good basis for comparison of the detail-imaging qualities of
different films.
Modulation-Transfer Curve(14)
Modulation transfer relates to the ability of a film to reproduce images of
different sizes. The modulation-transfer curve describes a film's capacity to
reproduce the complex spatial frequencies of detail in an object. In physical
terms, the measurements evaluate the effect on the image of light diffusion
within the emulsion. First, film is exposed under carefully controlled
conditions to a series of special test pattems, similar to that illustrated in (a)
of Figure 19. After development, the image (b) is scanned in a
microdensitometer to produce trace (c).
The resulting measurements show the degree of loss in image contrast at
increasingly higher frequencies as the detail becomes finer. These losses in
contrast are compared mathematically with the contrast of the portion of the
image unaffected by detail size. The rate of change or "modulation" (M) of
each pattern can be expressed by this formula in which E represents
exposure:
When the microdensitometer scans the test film, the densities of the trace
are interpreted in terms of exposure, and the effective modulation of the
image (Mi) is calculated. The modulation-transfer factor is the ratio of the
modulation of the developed image to the modulation of the exposing pattern
(Mo), or Mi/Mo. This ratio is plotted on the vertical axis (logarithmic scale)
as a percentage of response. The spatial frequency of the patterns is plotted
on the horizontal axis as cycles per millimeter. Figure 20 shows two such
curves. At lower magnifications, the test film represented by curve A
appears sharper than that represented by curve B; at very high
magnifications, the test film represented by curve B appears sharper.
All of the photographic modulation-transfer curves in the data sheets were determined using a method similar to that specified by ANSI Standard PH2.39-1977. The films were exposed with the specified illuminant to spatially varying sinusoidal test patterns having an aerial-image modulation of a nominal 35 percent at the image plane, with processing as indicated. In practice, most photographic modulation-transfer values are influenced by development adjacency effects and are not exactly equivalent to the true optical modulation-transfer curve of a particular photographic product.
Modulation-transfer measurements can also be made for the non-film components in a photographic system such as cameras, lenses, printers, etc, to analyze or predict the sharpness of the entire system. By multiplying the responses for each ordinate of the individual curves, you can combine the modulation-transfer curve for a film with similar curves for an optical system to calculate the modulation-transfer characteristics of the entire system.
Graininess and Granularity(15)
The terms graininess and granularity are often confused or even used as
synonyms in discussions of silver or dye-deposit distributions in
photographic emulsions. The two terms refer to two distinctly different
ways of evaluating the image structure. When a photographic image is
viewed with sufficient magnification, the viewer experiences the visual
sensation of graininess, a subjective impression of nonuniformity in an
image. This nonuniformity in the image structure can also be measured
objectively with a rnicrodensitometer. This objective evaluation measures
film granularity.
Motion picture films consist of silver halide crystals dispersed in gelatin (the emulsion) which is coated in thin layers on a support (the film base). T'he exposure and development of these crystals forms the photographic image, which is, at some stage, made up of discrete particles of silver. In color processes, where the silver is removed after development the dyes form dye clouds centered on the sites of the developed silver crystals. The crystals vary in size, shape, and sensitivity, and generally are randomly distributed within the emulsion. Within an area of uniform exposure, some of the crystals will be made developable by exposure; others will not.
The location of these crystals is also random. Development usually does
not change the position of a grain, so the image of a uniformly exposed area
is the result of a random distribution either of opaque silver particles (black-
and-white film) or dye clouds (color film), separated by transparent gelatin
(Figures 21 and 22).
 |  | Figure 21 | Figure 22 |
| Grains of silver halide are randomly distributed in the emulsion when it is made. This photomicrograph of a raw emulsion shows silver halide crystals. |
Silver is developed or clouds of dye formed at the sites occupied by the exposed silver halide. Contrary to widely held opinion, there is little migration or physical joining of individual grains. Compare the distribution of silver particles in this photomicrograph with the undeveloped silver halide in Figure 21. |
Although the viewer sees a granular pattern, the eye is not necessarily seeing the individual silver particles, which range from about 0.002 mm down to about a tenth of that size.
At magnifications where the eye cannot distinguish individual particles,
it resolves random groupings of these particles into denser and less dense
areas. As magnification decreases, the observer progressively associates
larger groups of spots as new units of graininess. The size of these
compounded groups gets larger as the magnification decreases, but the
amplitude (the difference in density between the darker and the lighter areas)
decreases. At still lower magnifications, the graininess disappears altogether
because no granular structure can be seen (Figure 23).
a |
b |
c |
d |
e |
(a) A 2.5X enlargement of a negative shows no apparent graininess. (b) At 20X, some graininess shows. (c) When a segement of the negative is inspected at 60X, the indi- vidual silver grains strt to become distin- guishable. (d) With 400X magnification, the discrete grains are easily seen. Note that surface grains are in focus while grains deeper in the emulsion are out of focus. The apparent "clumping" of silver grains is actually caused by overlap of grains at different depths when viewed in two-dimensional projection. (e) The make- up of individual grains takes different forms. This filamentary silver, enlarged by an electron microscope, appears as a single opaque grain at low magnification. |
Randomness is a necessary condition for the phenomenon. If the
particles were arranged in a regu;ar pattern like the halftone dot pattem used
in graphic arts, no sensation of graininess would be created. When a halftone is viewed at a magnification sufficient for the dots to be
distinguished, the eye notices the pattern and does not group dots into new
patterns. Even though the dot pattern can be seen, the eye does not perceive
graininess because the pattern is regular, not random (Figure 24). At lower
magnifications-at which the dots can no longer be resolved-the awareness of
pattern ceases, and the image areas appear uniform.
a | b |
| Figure 24 | |
| If the uniform dot pattern of a conventional halftone is used to reproduce a scene, the eye accepts the image as a smooth, continuous-tone rendition (a). This hap- pens because the dots are regularly spaced. However, when the halftone dots are dis- |
tributed randomly in an area to reproduce a scene (b) the image looks "grainy." Graininess in the image is due, in part, to the random distribution of the individual elements which make up that image. |
When you view a random pattem of small dots magnified enough to resolve the individual dots, you do not perceive an orderly or intelligible pattem. When the magnification is decreased so the dots cannot be resolved, they appear to blend together to form an image whose surface is nonuniform or grainy.
Measuring RMS Granularity
The attributes of the photographic image which cause the human eye to
perceive graininess can also be measured (and simulated) by an electro-
optical system in a microdensitometer. These measurements are analyzed
statstically to provide numerical values that correlate with the visual
impression of graininess. The two major advantages of objective
measurement are that instruments can be devised to make rapid and precise
measurements and that these measurements can be manipulated readily by
mathematical means.
Ordinary densitometers measure density over areas much larger than those of individual silver particles. Since there are so many particles in the aperture of an ordinary densitometer, small variations in the number of particles measured will not affect the reading.
Just as higher magnification increases the apparent graininess, a decrease in the aperture produces higher granularity values. When the aperture of the densitometer is considerably reduced, fewer particles are included and a small change in their number is recorded as a variation in density. Analysis of the magnitude of these variations gives a statistical measure of the granularity of a sample.
In practice, an area of apparently uniform density is continuously
scanned by the small aperture usually 48 nanometers in diameter (see
Figure 25). The transmitted light registers on a photo-sensitive pickup, and
the current produced is then fed to a meter calibrated to read the standard
deviation of the random-density fluctuations (see Figure 26).
 |  |
| Figure 25 | Figure 26 |
| A large aperture "sees" a vast number of individual silver grains. Therefore, small local fluctuations have practically no effect on the density it records. Small apertures (about one twentieth of the larger aperture diameter) detect random differences in grain distribution when they sample the large "uniform" area. |
The signal from a continuous density scan of a grainy emulsion appears the same as random electrical noise when displayed on an oscilloscope. The rms voltmeter gives a direct readout of "noise level." |
Standard deviation describes the distribution of a group of values (in this case, variations in density) about their average. The square root (R) of the arithmetic mean (M) of the squares (S) of the density variations is calculated-hence, the term RMS granularity. For ease of comparison, this small decimal number is multiplied by a factor of 1,000, yielding a small whole number, typically between 5 and 50.
The RMS granularity instrument used at Kodak is calibrated to measure American National Standard (PH2.19-1976) diffuse visual density. The granularity values for Kodak black-and-white and color negative films are determined at a net visual density of 1.00. KODAK and EASTMAN Motion Picture Films are read with a circular aperture 48 micrometers (0.048 mm) in diameter. This aperture gives meaningful readings over the widest range of film samples.
Factors That Affect Graininess
Different developers and different amounts of development affect the
graininess of black-and-white films. The amount of exposure, which
determines the densities of various areas, also affects the graininess of all
films. Because the development processes of color films are rigidly fixed,
the effect of development is rarely a factor in their graininess, although force
processing does cause an increase. Because many color films are made with
emulsion layers of varying graininess levels, increasing the exposure (up to a
point) places more of the density in the finer grained layers, which actually
reduces the overall graininess of the observed images.
Granularity and Color Materials
One might expect a photographic image made up of cyan, magenta, and
yellow dye clouds to appear more grainy than the corresponding silver image
because of color contrast. In fact, close to its resolution limit, the eyes sees
only brightness differences and does not distinguish color in very small
detail.
When color films are projected, the dye-cloud clusters form groups similar to silver-grain clusters in black-and-white films. At high magnifications, these clusters cause the appearance of graininess in the projected screen image.
Some Practical Effects of Graininess and Granularity
With the trend to smaller camera-film formats, has come the need for greater
enlargement of the projected image. At the same time, viewers are
increasingly aware of the granular structure of films.
The photographer wants a fine-grain film but not at the expense of sensitivity or film speed. Faster films usually have larger grains because larger silver halide crystals have a greater probability of being struck by light and made developable. Large silver halide crystals normally develop to larger particles of metallic silver. Thus, the selection of a film is usually a compromise between available speed and tolerable grain.
Photographic scientists are constantly seeking more favorable speed- grain ratios, but the relationship of emulsion speed to the grain structure is also a vital concern to the photographer because the speed-grain relationship indicates whether the emulsion will detect light and, if detected, will form a recognizable image. If a biologist needs to film the life processes of an arnoeba, the amount of allowable light is partly limited by the temperature tolerance of the amoeba. If fast film is used to compensate for limited light, the granularity must be low enough for the film to record the detaiI required by the application. Certainly the viewer should not have to wonder whether the movement on the screen is the amoeba's digestive process or "crawling" grain clusters.
Graininess is most evident in the midtones of a print (i.e., densities of about 0.6 to 0.9). The light tones of the print are on the toe of the characteristic curve where the slope is very much lower than unity. Hence, the contrast with which the graininess is reproduced is very low-decreasing its visibility. In dark tones, the eye is less able to distinguish graininess. The eye easily detects density differences as low as 0.02 in the average highlight density, but can detect density differences only on the order of 0.20 in the average shadow density. In the midtones, where the slope of the curve is constant, the print material has its maximum contrast and the eye can more readily distinguish small density differences; therefore, the granularity can be most easily detected by the eye as graininess.
Another factor in perceiving graininess is the amount of detail in a scene. Graininess is most apparent in large areas with fairly uniform densities and is much less evident in areas full of fine detail or motion.
It is difficult to predict the magnification at which projected print images will be viewed since both the projection magnification and the distance from the observer to the screen can very. Both factors affect the picture magnification, and thus the graininess.
When a motion picture film is seen at great magnification (as from a front-row theater seat), the viewer may be aware of grains "boiling" or "crawling" in uniform areas of the image. This sensation is caused by the frame-to-frame changes of grain positions, which make graininess more noticeable in a motion picture than in a still photograph. Conversely, the moving image tends to distract the viewer's attention away from this sensation, and graininess is, therefore, usually noticed only in static scenes.
Resolving Power(16)
The resolving power of a film emulsion refers to its ability to record fine
detail. It is measured by photographing resolution charts or targets under
exacting test conditions. The parallel lines on resolution charts are separated
from each other by spaces the same width as the lines. The chart contains a
series of graduated parallel-line groups, each group differing from the next
smaller or next larger by a constant factor. The targets are photographed at a
great reduction in size, and the processed image is viewed through a
microscope. The resolution is measured by a visual estimate of the number
of lines per millimeter that can be recognized as separate lines.
The measured resolving power depends on the exposure, the contrast of the test target, and, to a lesser extent, the development of the film. The resolving power of a film is greatest at an intermediate exposure value, falling off greatly at high- and low-exposure values. Obviously, the loss in resolution that accompanies under- or over-exposure is an important reason for observing the constraints of a particular film when making exposures.
Resolution also depends on the contrast of the image, hence, the contrast of the target. Test exposures are usually made with both a high-contrast (luminance ratio 1000: 1) and a low-contrast (1.6:1) target. A film resolves finer detail when the image contrast is higher. Both high- and low-contrast resolving-power values are determined according to a method sirnilar to the one described in ANSI No. PH2.33-1969 1R1976). "Method for Determining the Resolving Power of Photographic Materials," are given on the data sheets. The resolving power reported is based on film exposed and processed as recommended.
The maximum resolution obtainable in practical photographic work is
limited both by the camera lens and by the film. The formula often used to
predict the resolution of a camera original is
RS = Resolution of the system (lens + film)
RF = Resolution of the film
RL = Resolution of the lens
In practice, other external factors, such as camera movement, focus,
aerial haze, etc, also decrease the resolution from the possible maximum.