Post by Steaphany on Jul 11, 2013 3:00:02 GMT
Adapted from "Fundamentals of Film Exposure" on Optical Microscopy Primer, Exposure Value, Exposure, and Sensitometry:
Despite the complex array of settings and adjustments found on modern film cameras, exposure of film boils down to a simple relationship between two important variables: the amount of time the film is exposed to light and the intensity of that light. Films are formulated by the manufacturer to respond according to the following formula, E = l x t, where E is the proper exposure, l is the intensity of illuminating light rays, and t is the film emulsion exposure time in seconds or fractions thereof.
Thus, an increase in light intensity, other things being equal, would call for a shorter exposure time. Similarly, a decrease in light intensity would require a longer exposure time. This linear relationship is known as the reciprocity law and applies to both black & white and color films over a wide range of exposure times and illumination intensities. The response of a film emulsion to exposure and development is often plotted as a graph of film density verses the exposure time (or its base 10 logarithm), and is typically referred to as a characteristic curve. Plotting exposure data by this method was originally suggested by Ferdinand Hurter and Vero Driffeld. Each film has a unique characteristic curve when processed or developed under a set of standardized conditions. Exposure is described in terms of lux-seconds, which is a measure of the amount of exposure a section of film would receive under a controlled set of conditions. One lux-second is the amount of light received by film placed one meter from a standard candle for a single second.
Film density is a measure of the light-stopping ability of film and is related to the opacity and transmittance of the film. Transmittance is defined as the ratio of light transmitted by the film divided by the total amount of light incident on the film surface. When 50 percent of the incident light is transmitted through the film, the transmittance for that film is equal to 0.5. Opacity is defined as the reciprocal of transmittance, so a film having a transmittance value of 0.5 would have a corresponding opacity of 2.0. Density is defined as the logarithm of opacity, so that a film having an opacity of 2.0 will have a density of 0.3 and, as discussed above, will transmit 50 percent of light incident on the film surface. Film density is dependent upon the quantity of metallic silver present in the developed image.
The shape of a characteristic curve yields a significant amount of information about a particular film and the conditions used to process or develop the film. For black & white and color negative films, the curve is generally divided into three distinct regions, a toe region to the left, a linear region in the center, and a shoulder region on the right. Shapes and slopes of characteristic curves vary, depending upon exposure and processing conditions and the film emulsion characteristics:
The toe region of the curve is usually crescent-shaped and represents an exposure region where gray tones are compressed, with the separation between shadow densities becoming progressively less when moving from right to left on the curve. The shape and length of the toe region varies from film to film, which is described as being either short-, medium- or long-toed. Short toed films expand shadow tones and long-toed films compress tones and are useful in high-contrast situations. The right-hand border of the toe region represents the practical underexposure limit for a particular film and the position of the toe along the Density scale provides an indication of the film's base line fog.
The linear region of a characteristic curve is also termed the film latitude and represents the useful range of exposure times for a particular film, i.e. the span along the horizontal Exposure axis. Color and Black & White negative films display a substantially greater degree of latitude than do reversal processed films, primarily because many exposure errors can be corrected during printing. Overexposure is tolerated by negative films much better than underexposure and leads to better prints, although the proper exposure will always produce superior results.
The shoulder of a characteristic curve is the region where the slope decreases and the curve tends to level off and become horizontal. The start of the shoulder region is the practical overexposure limit of a film.
The slope of the linear region, termed Gamma is a measure of the film's contrast and is adjustable through variation in developer chemistry and the amount of development time (and/or temperature). When development time is shortened or low-contrast developers are used to process film, gamma values are decreased. Alternatively, when development time is extended or high-contrast developers are used, the slope of the characteristic curve increases, as does the gamma value.
Reversal processed films produce better results when slightly underexposed, yielding more image detail than when they are overexposed. In general, high contrast films (such as Kodak Technical Pan) or films processed in high contrast developers will have the least exposure latitude, whereas lower contrast films will have the most. The selection of film developer and processing conditions is critical in the final outcome of contrast in negative films. Increased development time usually leads to a decrease in exposure latitude due to an increase in contrast.
Every image captured on film is composed of a range of light intensities with highlights positioned on the right side of a characteristic curve and shadows positioned on the left. When a photograph is overexposed, the intensity values all shift towards the right and overall density is increased. Alternatively, when a photograph is underexposed, intensity values shift to the left and resulting film density is decreased.
With extremely short or long exposures, the film's reciprocity relationship no longer holds, and the film requires additional exposure time to yield proper exposure. This phenomenon is called reciprocity failure, and it occurs in all color and black & white photographic emulsions regardless of film speed, dye composition, or silver halide concentration. The term "failure" only indicates that the linear relationship between exposure time and light intensity no longer holds and does not indicate a failure of the film emulsion in terms of performance. With classical film characteristic curves, the point of reciprocity failure can be determined by the position and shape of curved regions (the toe and shoulder) occurring to the left and right of the linear portion of the graph. Changes in film response to illumination levels are sometimes referred to as long-exposure effects and short-exposure effects. Under low-light conditions, exposure times must often be extended to the point of significant film speed loss. Extremely short exposures produce the same effect. Most films also display an increase in scattering of exposed silver halide grains, the formation of smaller latent-image centers, and a lower rate of development at the latent-image centers with exceedingly short exposure times.
The film manufacturers' data sheets and characteristic curves suggest how much additional time is needed for proper exposure. Each individual film emulsion has a response tuned to a particular range of illumination values, outside of which the film's response is compromised and the reciprocity law no longer holds. Reciprocity failure can often be compensated simply by an increase in exposure times or processing conditions for black & white films, but this is not always the case with color negative and transparency films. Most color films have three color-sensitive dye layers, each of which has a slightly different characteristic curve position and slope resulting in varied responses to the reciprocity effect with the potential to cause undesirable color shifts or casts. Often, both exposure times and color balance filters must be adjusted to compensate for very long or short exposures when using color films.
Despite the complex array of settings and adjustments found on modern film cameras, exposure of film boils down to a simple relationship between two important variables: the amount of time the film is exposed to light and the intensity of that light. Films are formulated by the manufacturer to respond according to the following formula, E = l x t, where E is the proper exposure, l is the intensity of illuminating light rays, and t is the film emulsion exposure time in seconds or fractions thereof.
Thus, an increase in light intensity, other things being equal, would call for a shorter exposure time. Similarly, a decrease in light intensity would require a longer exposure time. This linear relationship is known as the reciprocity law and applies to both black & white and color films over a wide range of exposure times and illumination intensities. The response of a film emulsion to exposure and development is often plotted as a graph of film density verses the exposure time (or its base 10 logarithm), and is typically referred to as a characteristic curve. Plotting exposure data by this method was originally suggested by Ferdinand Hurter and Vero Driffeld. Each film has a unique characteristic curve when processed or developed under a set of standardized conditions. Exposure is described in terms of lux-seconds, which is a measure of the amount of exposure a section of film would receive under a controlled set of conditions. One lux-second is the amount of light received by film placed one meter from a standard candle for a single second.
Film density is a measure of the light-stopping ability of film and is related to the opacity and transmittance of the film. Transmittance is defined as the ratio of light transmitted by the film divided by the total amount of light incident on the film surface. When 50 percent of the incident light is transmitted through the film, the transmittance for that film is equal to 0.5. Opacity is defined as the reciprocal of transmittance, so a film having a transmittance value of 0.5 would have a corresponding opacity of 2.0. Density is defined as the logarithm of opacity, so that a film having an opacity of 2.0 will have a density of 0.3 and, as discussed above, will transmit 50 percent of light incident on the film surface. Film density is dependent upon the quantity of metallic silver present in the developed image.
The shape of a characteristic curve yields a significant amount of information about a particular film and the conditions used to process or develop the film. For black & white and color negative films, the curve is generally divided into three distinct regions, a toe region to the left, a linear region in the center, and a shoulder region on the right. Shapes and slopes of characteristic curves vary, depending upon exposure and processing conditions and the film emulsion characteristics:
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The toe region of the curve is usually crescent-shaped and represents an exposure region where gray tones are compressed, with the separation between shadow densities becoming progressively less when moving from right to left on the curve. The shape and length of the toe region varies from film to film, which is described as being either short-, medium- or long-toed. Short toed films expand shadow tones and long-toed films compress tones and are useful in high-contrast situations. The right-hand border of the toe region represents the practical underexposure limit for a particular film and the position of the toe along the Density scale provides an indication of the film's base line fog.
The linear region of a characteristic curve is also termed the film latitude and represents the useful range of exposure times for a particular film, i.e. the span along the horizontal Exposure axis. Color and Black & White negative films display a substantially greater degree of latitude than do reversal processed films, primarily because many exposure errors can be corrected during printing. Overexposure is tolerated by negative films much better than underexposure and leads to better prints, although the proper exposure will always produce superior results.
The shoulder of a characteristic curve is the region where the slope decreases and the curve tends to level off and become horizontal. The start of the shoulder region is the practical overexposure limit of a film.
The slope of the linear region, termed Gamma is a measure of the film's contrast and is adjustable through variation in developer chemistry and the amount of development time (and/or temperature). When development time is shortened or low-contrast developers are used to process film, gamma values are decreased. Alternatively, when development time is extended or high-contrast developers are used, the slope of the characteristic curve increases, as does the gamma value.
Reversal processed films produce better results when slightly underexposed, yielding more image detail than when they are overexposed. In general, high contrast films (such as Kodak Technical Pan) or films processed in high contrast developers will have the least exposure latitude, whereas lower contrast films will have the most. The selection of film developer and processing conditions is critical in the final outcome of contrast in negative films. Increased development time usually leads to a decrease in exposure latitude due to an increase in contrast.
Every image captured on film is composed of a range of light intensities with highlights positioned on the right side of a characteristic curve and shadows positioned on the left. When a photograph is overexposed, the intensity values all shift towards the right and overall density is increased. Alternatively, when a photograph is underexposed, intensity values shift to the left and resulting film density is decreased.
With extremely short or long exposures, the film's reciprocity relationship no longer holds, and the film requires additional exposure time to yield proper exposure. This phenomenon is called reciprocity failure, and it occurs in all color and black & white photographic emulsions regardless of film speed, dye composition, or silver halide concentration. The term "failure" only indicates that the linear relationship between exposure time and light intensity no longer holds and does not indicate a failure of the film emulsion in terms of performance. With classical film characteristic curves, the point of reciprocity failure can be determined by the position and shape of curved regions (the toe and shoulder) occurring to the left and right of the linear portion of the graph. Changes in film response to illumination levels are sometimes referred to as long-exposure effects and short-exposure effects. Under low-light conditions, exposure times must often be extended to the point of significant film speed loss. Extremely short exposures produce the same effect. Most films also display an increase in scattering of exposed silver halide grains, the formation of smaller latent-image centers, and a lower rate of development at the latent-image centers with exceedingly short exposure times.
The film manufacturers' data sheets and characteristic curves suggest how much additional time is needed for proper exposure. Each individual film emulsion has a response tuned to a particular range of illumination values, outside of which the film's response is compromised and the reciprocity law no longer holds. Reciprocity failure can often be compensated simply by an increase in exposure times or processing conditions for black & white films, but this is not always the case with color negative and transparency films. Most color films have three color-sensitive dye layers, each of which has a slightly different characteristic curve position and slope resulting in varied responses to the reciprocity effect with the potential to cause undesirable color shifts or casts. Often, both exposure times and color balance filters must be adjusted to compensate for very long or short exposures when using color films.