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Exposure  Latitude

 
  
 
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 g) 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: g  = where D D D log  H o r D1  -  D2 log  H1  -  log  H2 D (Delta) = Symbol for change or difference 2-13


   


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